Remanufacturing of electric vehicles: Challenges in production management

Due to the conceptual degrees of freedom in their product structure, electric vehicles offer high potential for remanufacturing-oriented product design. This potential is, however, not realized yet. Remanufacturing as one fundamental element of a circular economy is characterized by specific challenges caused by uncertain information about the condition and the timing of the returning product. By means of a case study within the remanufacturing industry, the effects of uncertainties on remanufacturing operations are examined and different approaches within the field of production management to meet these specific challenges are pointed out. Based on the result of the case study a production management framework outlining fields of action to deal with remanufacturing specific uncertainties is developed. In this context, the requirements for remanufacturing of electric vehicles are derived by analyzing similarities from other industry sectors. In conclusion, a solution approach for the implementation for electric vehicles is presented for strategic, tactical and operational procurement logistics and remanufacturing operations.

Motivation
The concept of a circular economy aims to decouple economic growth from the use of finite resources and the associated negative effects on the environment [1]. Remanufacturing corresponds to the objective of the circular economy since it enables the recovery of product components for further use in order to extend the useful life of products or their components.
The global remanufacturing industry has reached a total size of approximately 110 billion USD per year and creates about 450,000 jobs worldwide [2]. The automotive industry accounts for two-thirds of the remanufacturing market volume, even though remanufacturing is currently limited to vehicle components [3]. In other sectors, remanufacturing is executed on the level of the final product and is a central element of the manufacturer's business models.
The growing market share of electric vehicles offers new opportunities to implement remanufacturing at the vehicle level for passenger cars and light commercial vehicles. While vehicles with a powertrain based on an internal combustion engine have a predefined product and functional structure, conceptual degrees of freedom can be used in the design of electric vehicles to implement remanufacturing-oriented product design [4]. Additionally, the components of an electric powertrain are characterized by lower complexity and fewer mechanical interfaces to each other.
Several definitions of remanufacturing exist with slightly different foci. All definitions have in common that remanufacturing is described as an industrial process in which cores are brought into a condition that meets at least the quality and functional requirements of a new product [2,5,6]. Cores can also be upgraded to enhance product functionality. Remanufacturing includes re-using, processing or replacing of single components of the returned cores in order to enable a further product life cycle [7].
The generic remanufacturing process begins with the delivery of the cores and storage [8]. After optional cleaning, the product is disassembled. Components, which are technically or economically unsuitable for remanufacturing due to their condition are sorted out. All other cores may need detailed defect diagnosis before the actual reprocessing of the disassembled parts takes place. In reassembly, the processed components are reassembled, if necessary in combination with new parts.

Problem Statement
Remanufacturing is characterized by additional challenges in comparison to linear value chain production. As discussed in the literature and proven by different studies, the uncertainty about the condition, quantity and timing of returning cores is elementary for these challenges [9]. Varying failure patterns caused by different usage behaviours and multiple external factors lead to the unpredictable timing of product failure and different remanufacturing needs. The more components a product contains, the greater the deviations in causes and times of core returns are.
The combination of a fluctuating number of available cores and varying core conditions results in unreliable forecasts about the availability of remanufacturable cores and spare parts as well as the required production capacity [10]. Moreover, remanufacturers often have to deal with an asymmetry between core returns and market demand since the availability of cores increases during the saturation and degeneration phases of a product's market cycle while demand decreases [11]. Furthermore, depending on the condition of the cores required remanufacturing processes and process times may vary in order to achieve the desired condition [12]. Therefore, it is hardly possible to schedule the process reliably before disassembly and inspection of the cores. Subsequent changes in process and material requirements planning can also occur if further defects are detected during the remanufacturing process that were not detected during quality inspection.
These challenges are dealt with in numerous publications and a variety of solutions is presented. Remanufacturing research to date is characterized by an isolated consideration of the individual problem areas [13]. Due to the high planning complexity of remanufacturing, approaches are necessary that deal with the specific challenges in an integrated framework. Accordingly, only limited theoretical conclusions can be drawn for the implementation of remanufacturing in electric vehicle production. Rather, it is promising to investigate the production management of remanufacturing operations in other industries with regard to the applicability of remanufacturing at the vehicle level. Therefore, the following research question occurs: Which configuration options in product management for remanufacturing of electric vehicles can be derived from practice in order to meet uncertainty-related challenges?

Approach
In order to derive implications for the remanufacturing of electric vehicles, the industry practice is described and analyzed in five companies. The cases are derived in a three-step approach. First, publicly available information was examined in order to select suitable companies. Based on this, a comprehensive literature analysis was carried out with regard to the respective remanufacturing operations. Finally, experts from the production management department were interviewed using a standardized questionnaire.
The products examined in the study show a different level of complexity in order to consider a wide range of different approaches to solving the uncertainties discussed above. The products are becoming gradually more complex from brake callipers, starters, and generators, clutches, printers, combustion engines for heavy-duty trucks and construction machines to gas turbines. All companies, whose remanufacturing operations are examined in this study, are original equipment manufacturers (OEM) of the described products. This is important in order to ensure applicability to the production of electric vehicles since the complexity of remanufacturing products such as electric vehicles can only be controlled by its original manufacturer.
Firstly, as structuring means for the study, a framework is developed, depicting the fields of action for managing the production-related remanufacturing specific uncertainties. The fields of action are differentiated according to their strategic, tactical or operational character, depending on the planning horizon. Strategic tasks have a rough planning level and are typically carried out by the management for the long term. The tactical management level implements the planning requirements of the strategic level in the medium term and the operational level is responsible for detailed order management.
The management challenges described divide the fields of action associated with remanufacturing into two categories: procurement logistics and remanufacturing operations. One significant difference compared to conventional systems lies in procurement logistics, since remanufacturing is characterized by exogenous parts availability and therefore the coverage of dependent requirements cannot be completely controlled [12]. Additionally, depending on the core condition, the process steps to be carried out and the process times can vary considerably, which makes the planning of production operations complicated and requires an appropriate strategy.

Framework
Companies either purchase cores directly from the customer or access third-party service providers such as used parts dealers, scrap yards or core brokers. The multitude of supply sources increases the overall coordination effort and the complexity of procurement logistics. Since there is also a high degree of uncertainty regarding the return time of cores by the customer, close, cross-company cooperation with third-party service providers is necessary for supply chains that are directed backwards, as well as the active involvement of customers in the take-back process [14]. The configuration of the closed-loop supply chain represents the increased demands on cooperation with customers and third-party service providers and therefore represents the strategic management field of action of procurement logistics.
In a conventional forward supply chain, manufacturers sell their products directly to their customers or use retailers. Backward material flows typically only occur in connection with product returns, e.g. when the customer asserts warranty claims. In remanufacturing, more extensive requirements are placed on product take-back systems, as returned products are the raw material for production and thus indispensable for the value-adding process. Remanufacturing companies must create logistics systems that work efficiently despite the high number of material sources and the uncertainty regarding the availability of cores. The planning, implementation and control of efficient and cost-effective product flows from the end-user to the original manufacturer, with the purpose of generating added value by restoring the product, is called reverse logistics and is on the tactical level of procurement logistics [15].
The operational management task of procurement logistics is to ensure that a number of components tailored to the primary requirement are procured and made available for production. Remanufacturing differs from conventional logistics systems in terms of warehousing, procurement and distribution [16]. Therefore, a reliable forecast of how many reusable components will be available for remanufacturing and how many spare parts are needed cannot be precisely made. For these reasons, the dependent requirements supply needs to ensure the availability of cores to be remanufactured.
On the strategic management level of remanufacturing operations, the fundamental decision must be made for the question to what extent remanufacturing is in line with the corporate strategy. Numerous factors such as revenue potential, the consideration of marketing, the availability of specialists and legislative requirements can influence this decision [17]. Furthermore, a decision must be made about which products are technically and economically feasible for remanufacturing. In addition, a balance must be struck between the availability of cores and the market demand for reclaimed products. Therefore, at the strategic level, the remanufacturing scope is defined.
The design of efficient production processes is a success determining factor in the development of remanufacturing systems since both the costs and quality of the processed products as well as the performance of the system are influenced by the choice and design of the production processes [18]. Consequently, the tactical field of action for remanufacturing operations is defined as production design.
The operational level of remanufacturing operations is characterized by variable process times and production sequences due to the high inconsistency of the core condition, which is particularly challenging for inhouse-production planning and control [9]. Since not all cores have to undergo the same processes in order to be restored to a new condition, it is usually not possible to carry out precise process planning before certainty about the condition of the cores has been achieved. Processing times on the same workstation can also vary depending on the core's condition. Therefore, the scheduling of production processes is more complex than with conventional production systems.

Case 1: Remanufacturing of brake callipers, starters, and generators
The company in the first case is a multinational technology group with a strong focus on mechanical, mechatronic and electric automotive parts with activities in 60 countries. At the examined production site, remanufacturing of brake callipers, starters and generators is carried out for aftermarket activities. At the location, 300 employees remanufacture more than one million products yearly, handling over 3.000 different types of products. The cores are taken back via three channels. The first channel is the repurchase from independent workshops. Via agents, the cores are picked up and delivered to one of the regional sorting centres from a network of twelve such stations in Europe where the cores are identified, pre-sorted and evaluated. If the cores meet predefined criteria such as completeness and absence of obvious material eruptions and cracks, the value of the core is credited to the customer. The second option is the direct repurchase from major customers. Contracted workshops are advised to store and forward cores of certain product types. These major customers purchase an agreed quantity of remanufactured products at a reduced price. Lastly, the option of purchasing cores from independent core brokers is used.
The remanufacturing process begins with sorting and inspecting the quality of the delivered cores. If testing and sorting have already taken place at one of the sorting centres before delivery, the cores are stored without checking. The storage time can vary depending on the product type from a few days up to several years. When an order is placed, a corresponding number of cores is removed from stock. Since it cannot be assumed that all components of each product type can be remanufactured due to condition variations, forecasts approximate how many cores of a certain product type are required to meet the existing primary requirement for each product type.
The cores are disassembled non-destructively into their individual components and forwarded to the subsequent processes. Obviously defective components as well as wearing parts and small parts such as screws are directly sorted out and replaced. Components, which in any case have a limited life, are also excluded from reuse. Various cleaning processes like degreasing with technical alcohol and subsequent irradiation with blasting media are carried out depending on the type of component and the degree of contamination. The reusability of the individual components is assessed manually after cleaning. Depending on the test results, mechanical processing is carried out according to a predefined process for each component family. The reassembly follows immediately without intermediate storage. Non-recoverable components are replaced either by equivalent components of the same batch or by new parts. In the last step, a test of the finished remanufactured product as well as packaging and handover to shipping takes place.

Case 2: Remanufacturing of heavy-duty vehicles clutches
The company is a global technology group for automotive systems with activities in 40 countries including a network of fifteen remanufacturing locations in Europe, Asia, North and South America, and Africa. At the analysed plant, 200 employees remanufacture more than 200,000 units per year of a broad variety of different clutches.
The cores are procured via three channels. The most important channel is the direct repurchase from major customers who deliver used parts several times a week and in return, they purchase a certain quantity of remanufactured products. The second take-back channel is the purchase of cores from core brokers. Either they receive an order to procure a defined number of a product type or cores are acquired from the existing stock of the respective core broker. Furthermore, cores are supplied directly by workshops. Workshops collect the used parts resulting from the repair of customer vehicles and sell them.
The delivered cores are stored and remanufacturing is only conducted, once a primary purchase order has been received. In the first process step, the cores are categorized according to the extent of defects. The categorized cores are disassembled, which requires controlled destruction of the rivets or welded joints. The separated products are first disassembled and then cleaned of dirt and corrosion by means of a shot-blasting cleaning process. Cleaned components are inspected for defects and, if necessary, sorted out and sent for recycling. Quality control also determines which mechanical treatment processes are necessary to restore the component in mint condition. During the abrasive cleaning process, any identification numbers applied to the surface are removed. The subsequent sorting of the components according to their type requires highly qualified employees who can draw conclusions about the product type on the basis of the component geometries.
The subsequent mechanical processing implements the processing measures defined during quality control as well as adjustments that are necessary due to technical product upgrades. The mechanical processing to be carried out depends on the type of component and the state of wear and includes separating processes such as the removal of burrs and face turning of end faces as well as joining production processes such as the bonding of wear plates. In the last step, the reprocessed components are reassembled and a final functional test is carried out. The cores can go through the entire process several times.

Case 3: Remanufacturing of heavy commercial vehicles combustion engines
One of the world's largest manufacturers of trucks, buses, construction machinery and industrial engines with production sites in 18 countries including remanufacturing sites in Europe, USA and China for transmissions, engines, turbochargers, and hydraulic pumps remanufactures 200 different variants of truck engines with a total volume of about 3.000 units per year for the European and Asian markets at the analysed production site.
Cores are supplied by an external core broker, who supplies the plant once every two weeks. The required number of cores is forecasted from historical data and the order is placed with the core broker without direct order reference. The delivered cores are stored and, as soon as free production capacity is available, fed into the remanufacturing process.
The remanufacturing process starts with the disassembly of the engine. No initial testing of the engine, but a continuous quality test of the engine components during the entire remanufacturing process is executed. Engine components that prove to be defective during dismantling or in subsequent remanufacturing steps are disposed of or recycled accordingly. Among others, surface crack inspections, 3D coordinate measurements and dynamic testing methods are carried out. During disassembly, the components are sorted according to quality and the cleaning process required. Afterwards, the component cleaning is carried out, which is subject to highly varying process times, as the components must be soaked for a long time before the actual, abrasive cleaning process can take place. Cleaned components are then mechanically processed. The processing steps required for processing vary depending on the component group and condition of the respective component.
One major challenge for production planning and control is that the setup and processing times can differ by up to fifteen times depending on the type and condition of the component. Likewise, the recovery rate varies substantially between 10 % and 100 % depending on the component group. Consequently, a very high stock for some components is unavoidable, while other components have to be purchased externally to meet demand.
The components of an individual engine are not reassembled to the same engine but are at first stored. If all components necessary for the fulfilment of an order are in stock, reassembly can take place, otherwise, missing components must first be purchased externally. Finally, the end of line test is carried out, during which the remanufactured engine must meet the same quality standards as a newly manufactured engine.

Case 4: Remanufacturing of printers
The company in case four is a US manufacturer of office equipment including tabletop printers, multifunction devices, commercial production printing systems and printer cartridges. The company operates remanufacturing plants in North and South America, Europe, Australia and Japan.
Used products are mainly taken back via two channels. In particular, commercially used production printing systems and multifunctional systems are replaced by new equipment under lease agreements after a contractually agreed timespan. The second channel is to take back the device directly from the end customer, for example through exchange campaigns, in which the customer is granted a discount on the new device when returning an old product.
The returned devices are centrally checked for optical defects as well as for noise and heat generation and vibrations. According to their condition, the cores are sent to different plants. Devices without any signs of wear are centrally stored and prepared for direct reuse while faultlessly working cores with light signs of wear are sent to remanufacturing plants. Non-reusable devices are recycled. The products are completely disassembled and the components are sorted by type. The components are then cleaned using a minimalabrasive dry ice blasting process, which gently cleans the plastic, glass and precious metal components. Mechanical processing is not carried out. Only painting of the panel components is carried out. Component not suitable for reuse as well as wear parts like seals and bearings are replaced by a new component. Components for which technical upgrades exist are also replaced by the new components.
The tested and partially newly painted components are assembled in the next process step. New and remanufacturing products are assembled on the same line. For the assembly of new devices, no remanufactured components are used. Finally, the latest software is installed for all products and final function tests are carried out.

Case 5: Remanufacturing of constructions machine engines & gas turbines
A US manufacturer of mining machinery, diesel engines and industrial gas turbines and construction machinery remanufactures more than 6,000 of its products including engines, transmissions, hydraulic components and turbochargers at 17 production sites, employing over 4,000 people. In addition to processing its own product line, the company acts as a contract remanufacturer for third-party products and thus offers remanufacturing as a service. At the analysed remanufacturing plant diesel engines from construction machines, gas turbines and military equipment are remanufactured.
The company uses a deposit-or rebate-based take-back system in order to acquire cores. If a customer, or a licensed dealer, buys a remanufactured product, first, the same price as when buying an equivalent new product is paid. This price consists of the actual product price and a deposit for the return of the product. The deposit can be up to 50 % of the price paid and will be refunded to the customer or dealer as soon as the customer returns a similar type of product. If the cores meet all criteria like being complete, having no obvious cracks or break-outs and no independent repair measures have been carried out, the full deposit amount is refunded to the customer. Otherwise, the core will be rejected or the deposit will be repaid partially.
While the engines are returned directly from major customers or via contracted workshops in specifically provided transport boxes the gas turbines are transported to the plant by the customer. The single-item remanufacturing process is triggered by the arrival of a core and begins with an incoming quality check before disassembly. During disassembly, the components are sorted according to type and condition. The process steps are roughly defined for the respective components. The disassembled components lose their affiliation to the original product and are thus reused anonymously, so typically, components of many different products are used in a remanufacturing product. After cleaning and defect detection, all necessary mechanical processing steps are carried out, with the goal to not only restore the original condition but also upgrade the components to the current state of the art. Damaged components are treated with additive processes, to restore worn surfaces or repair microcracks. Ultimately defect components are replaced either by an equivalent remanufacturing part or by a new part. Finally, the engine or the gas turbine is assembled, the product is painted and a final function test is carried out.

Analysis
Based on the cases described, the following table summarizes the generic options currently available from the OEM remanufacturer to address specific reprocessing uncertainties. The management alternatives described within the six management tasks of the regulatory framework are often found in practice in combination.

Conclusion and outlook
Taking into account the technical, regulatory and market-related boundary conditions, the following management measures result within the six defined fields of action for the qualification of remanufacturing in electric vehicle production. In order to implement the proposed measures, however, further detailed investigations are necessary. Securing the supply of cores in a closed-loop supply chain can be achieved through close customer loyalty by means of circular economy business models. A combination of different options is an appropriate solution depending on the targeted customer groups such as private customers or commercial fleet operators. When leasing vehicles, the OEM remains the owner of the vehicle and can set a defined return date for the vehicle in the contract. Furthermore, it is possible to establish a deposit system in which the purchase price is increased with a premium, which is paid back in addition to the buy-back price after the vehicle is returned. Additionally, an OEM can secure a right of repurchase against the reduction of the purchase price in the purchase agreement. Finally, it is possible to offer buy-back prices above market prices on the free used-car market in order to secure returns.
The automotive industry typically works with a network of authorised dealers, through whom the reverse logistics of vehicles can be organized. In the case of affiliated contract workshops, it is possible to carry out the (preliminary) assessment of the condition on-site and to calculate the profitability of the order depending on the condition of the vehicle. The advantage is that vehicles can be sorted out directly when remanufacturing is economically or technically unfeasible to avoid expensive transport to the remanufacturing site.
When remanufacturing electric vehicles, it must be ensured that the dependent requirements of remanufacturing orders are covered by components of the original vehicle or by new parts. Therefore, components that are removed and intended for further use are assembled into the same original vehicle. In order to be able to carry out product recalls and warranty measures due to technical problems, the traceability of vehicle parts and the identifiability of their technical design must be ensured in the automotive industry [19]. Recalls typically relate to vehicle production periods. If components from different production periods do not match the production date of the vehicle, the definitive assignment of components to vehicles can only be guaranteed by a very complex data management system. Due to the number of components in a vehicle and the fact that vehicles are subject to constant design changes during ongoing production, the effort required to trace the production and usage history for electric vehicles is disproportionate. In addition, remanufacturing practice shows that customer acceptance of remanufacturing increases significantly when the product returns with the same parts. Consequently, spare parts must be supplied from the series production.
The remanufacturing scope results from the product structure of electric vehicles, which consists of core elements that are durable under normal conditions of use and components that wear and age. Consequently, disassembling the entire product and testing all components is likely to involve process steps that do not add value if no additional reprocessing needs are identified. In order to implement efficient production, it is important to design a functioning mechanism for reliable identification of defects and wear.
Electric vehicles have a fixed disassembly sequence, which must be reflected in the production design in the form of a sequential order of the stations. Consequently, the stations have a fixed assignment of the disassembly tasks. However, to ensure that only the order-related remanufacturing scope is actually taken into account during disassembly, it must be ensured that only the required stations can be approached. In contrast to disassembly at the vehicle level, removed components such as powertrain, battery pack or axles are completely disassembled, cleaned, inspected, processed if necessary and reassembled in accordance with industrial standards. The configuration of the reassembly corresponds to the disassembly in order to enable the same job-related route flexibility. Capacities are designed because of the defect and wear probabilities based on historic values.
In-house production planning and control is characterized by decisions under uncertainty. Only complex data processing concepts are suitable to guarantee the required efficiency within the described production design. First, a reliable identification of defects must be ensured. Machine learning algorithms that can process a large amount of influence data and independently identify complex dependencies are suitable for such a classification task. This information must then be processed within the scheduling process. The aim of scheduling is to utilize production resources as efficiently as possible and at the same time to react robustly to deviations due to extended process times or additional process steps. Simple planning heuristics do not meet these requirements, which is why the use of genetic algorithms to optimize order processing is expedient for the complexity of this task. To ensure the productivity of the remanufacturing system, the profitability of the single orders must be reassessed if there is a major deviation from the initial planning.


Halal Products : Not Restricted to Food and its Marketing Opportunity in the Muslim World

Halal refers to methods of producing goods & services in the manner allowed by Shariah or Islamic law, which invest besides food preparation and food products but also encompasses the production of pharmaceutical, Cosmetics, Garments & fashion, Tourism Practices and also services like tourism, finance & banking and many more. Halal not only a religious observance and obligation for Muslims but also has emerged as a powerful market force alike both for Muslims and non-Muslims, so gained an increasing acceptance among Muslims along with non-Muslim consumers. Halal is becoming a global symbol of a lifestyle choice and quality assurance choice has promoted values as economic and social justice, earth stewardship, social responsibility, and animal welfare have gained interest which is beyond religious compliance. The global halal market is 1.7 billion consumers and worth almost US$ 2.3 trillion. This paper has focused on items Muslims are using other than food.

narrated that now halal has become a global concept and stands for fair business as well as for social justice, sustainable environment, and animal welfare. Besides covering animal slaughtering for Muslims" eating according to Shariah it also encompasses products & services of the highest quality meeting the ever-increasing needs & awareness of Non-Muslim consumers. The data of halal oral used (food etc.) & other products are given in the following diagram: Source: https://azkahalal.files.wordpress.com/2014/05/t-l5-mian-riaz.pdf What is Halal? (2010); Samori et al. (2014); Rahim, Zainuddin, and Sariwati (2016), and Elasrag (2016) revealed that the Halal term derived from the Holy Quran which describes objects and actions. Yasid, Farhan, and Andriansyah (2016) also expressed that the halal word is absorbed from the Arabic language word alhalal giving the meaning of permissible. Yasid et al. (2016), Golnaz, Zainalabidin, and Mad Nasir (2010); Ab- Talib & Johan (2012); Rahim et al. (2016) and Elasrag (2016) expressed that halal is a way of life which is not restricted to types of food consumed by Muslims who are more knowledgeable & aware about their food for leading a meaningful life. Islam being a natural way of life encompasses an economic system that is based on human brotherhood, cooperation, consultation & dietary laws. Annabi, & Olajumoke (2017) mentioned that because of increasing market internalization many choices of market products consisting of new ingredients formulation are available. Based on Shariah some ingredients are not suitable for Muslim consumers because of their processing methods or sources. Qureshi et al. (2012); Hussain et al. (2014) and Krishnan et al. (2017) quoting various studies stated that the term "Halal" is designated for anything, entity, object, or action which according to Quran & Sunnah is permissible to engage in the purchase, implied to the body, or use. Islam & Chandrasekaran (2013) narrated that the "Noor index" measures consumer perceived Halalness of brands. Many multinational companies scored very well on Noor Index. There is a considerable overlapping between Vegetarian, Islamic & Green marketing leading to speculation that there is room to expand market potential & shape marketing strategy with halal certification.

Diagram Comparing Total Consumption (red color) and Halal Consumption (blue color)
Source: WIFE (2015) Halal Market Extent Wilkins et al. (2019) expressing the extent of the halal products market revealed that in the world Muslims" population is 1.6 billion representing 23 percent of the global population and faithful Muslims consume halal products permissible under Islamic law. The market of halal products is estimated to exceed the US $ 2.1 trillion in value whereas the annual market for only halal food is the US $ 632 billion representing 16 percent of global food consumption. Besides this Wilkins et al. (2019) further narrated that based in non-Muslim countries manufacturers & retailers of food such as some non-Muslim countries fast-food chains & International restaurants like McDonald"s, KFC, Subway, Pizza Express, & Nando"s now provide halal food. Abuznaid (2012) revealed that it is estimated in the world halal market is the fastest growing market at an annual rate of 15 %. Abuznaid (2012) and Elasrag (2016) quoting several studies revealed that it is estimated that the halal industry with an annual growth rate of 15 to 20 percent is one of the fastest-growing consumer's segments with a yearly value of US$ 560 billion & total worth of US $ 2.3 trillion with 1.8 billion consumers. Ab- Talib & Johan (2012) revealed quoting some studies that 70 percent of Muslim consumers follow halal standards so the halal food market comprises a population of 1.5 billion people giving a market worth more than US$ 600 billion. Sugita (2017) stated that the Halal industry including the food sector in recent years has significantly grown and gained much importance and the halal food sector by 2019 will grow in sales US$ 2.5 trillion becoming 21.2 percent of global food expenditure. Razak et al. (2015) expressed that for halal products consumer base in the global market is 1.7 billion consumers and worth almost US$ 2.3 trillion, making the halal market an important segment of the global market. The traditional markets lying in Asia and the Middle East but due to globalization effects, there is a significant development of halal market size in the USA & Europe. Yasid et al. (2016) expressed that the Muslim market reached the US $ 2001 billion, in 2013, or 12 percent of the global market. In 2013 for hall food Muslim Market increased to the US $ 1292 billion or 17.7 percent, of the total world food market. Bohari et al. (n.d) asserted that MIDA (Malaysian Investment Development Authority) in 2013 valued the global halal food industry between US$ 600 billion & US$ 2.3 trillion but it is not to focus only the halal food consumption but must also be concerned about all the supply chain process, & standard of food according to Shariah. Halal supply chain encompasses managing process of halal food products of different suppliers" points to different buyers" points located at various places especially who are simultaneously engaged in managing non-halal food items to satisfy their all types of customers. Especially in the situation when Muslim food consumers became more aware and not only concerned with the ingredients of the food for halal or non-halal aspects but also concern about entire activities in the supply chain to ensure that their consumable product is truly halal.  Wahab (2004) stated that halal products are not only limited to food but encompass all the things that come in contact in daily life especially used by Muslims. Islam and Chandrasekaran (2013) and Aisyah (2017) are of the view that traditionally Islamic marketing or Halal concept was understood to apply to food items, and financial & banking services only but now its consciousness is increasing and extended for all other consumable products like cosmetics & personal care products, pharmaceuticals, entertainment & leisure, education, fashion & toiletries, and tourism & hospitality services. Wilkins et al. (2019) asserted that the halal concept applies to all products including food, pharmaceuticals, fashion, banking, insurance, entertainment, and tourism. For most Muslims halal protocol for food is most important as compared to non-food products.
According to Razak et al. (2015) and Krishnan et al. (2017), Halal & Haram branding though for some products being in early stages is not a new concept and is not restricted to products comprising food, drinks & services only but encompasses all the matters of daily life & usage. The concept and usage of Halal make Muslims different from other religions. Hence while using pharmaceutical, cosmetics clothes it is to be ensured that their ingredients are halal, even care is to be taken for environment & jobs, etc. Therefore Elasrag (2016) asserted that Muslims must ensure that the source of the food they consume is halal. It means that not the ingredients themselves but the process as a whole is according to Islamic laws. Samori et al. (2014) in a study about Malaysia narrated that as a halal hub establishment is a worldwide collective effort of many industrialists in food, cosmetics, pharmaceuticals, packaging & logistics along with services. Murti (n.d.)  Ab-Talib and Johan (2012) narrated that halal is more than concerning about food. Halal besides food products encompasses products like pharmaceuticals, cosmetics & also services like tourism, finance & banking. Elasrag (2016) quoting several studies revealed that in recent years global halal industry is not restricted to items related to food only but has also encompassed many other items such as health products, pharmaceuticals, medical devices, toiletries, & cosmetics and service sector items electronic & print media, logistics, packaging, financing & branding, and as now affluent Muslims number has increased the halal industry further widened to lifestyle offering which also includes, fashion as well as hospitality services & travel. Ali et al. (2019) also expressed that Qur"an & Sunnah explain clearly haram & halal ingredients, in food, cosmetics, toiletries, pharmaceuticals, investment, finance, & services like the supply chain & tourism industry. Items expected to be avoided are, food, products & services containing haram elements of alcohol, animal remains particularly in cosmetics, & riba.
Demirel & Yasarsoy (n.d.) narrated that using food & drink, products of personal care, dressing equipment, cleaning, staying in accommodation establishments, having trips are the main & common, products & services. Many people exert efforts to consume healthy products & services. Nain et al. (2013) revealed that the pharmaceutical market per year is estimated to be worth 800 billion US Dollars globally and this market share of Muslims is estimated to be in hundreds of billion dollars.  Nain et al. (2013) revealed that the pharmaceutical market per year is estimated to be worth 800 billion US Dollars globally and this market share of Muslims is estimated to be in hundreds of billion dollars. Razak et al. (2015) believe that health care & personal hygiene products, cosmetics & toiletries are appraising halal options. Halal is recognized once as a lifestyle choice for halal products the options" range will increase exponentially. Elasrag (2016) stated that pharmaceutical & health products are a vast grown area in the world halal industry. In Muslim-majority countries, the estimates of demand for generic medical, pharmaceutical, healthcare products & wellness are about US$ 555 billion. For pharmaceuticals, the market growth worldwide increased by 4 percent in 2009 to a value exceeding the US $ 820 billion offer to halal pharmaceutical industry big potential opportunities to tap into. Increasing costs of healthcare providers a key differentiation factor to the halal market in generic pharmaceutical supply.
Source: https://azkahalal.files.wordpress.com/2014/05/t-l5-mian-riaz.pdf Islam & Chandrasekaran (2013) stated that the demand for halal pharmaceuticals for Muslims is growing. The long-standing issue is of the use of gelatin, which, is protein and can be obtained normally from two sources, from cow bone & hide (bovine) a halal source, and pig (porcine) a haram source. Alcohol (ethanol) is another avoidable ingredient in halal pharmaceuticals. Islam & Chandrasekaran and Elasrag (2016) Wahab (2004) stated that halal is not confined to food only but encompasses all the things coming in contact with daily life especially Muslims used products. Cosmetic products made from chemicals & plants are permissible except harmful for health products being poisonous. Cosmetics made from halal animal sources not containing filth are also halal and their use is permissible. Batu & Regenstein (2010) expressed that halal products certified responsibly are gaining acceptance as a benchmark for quality & safety assurance. Islam & Chandrasekaran (2013) asserted that the halal concept also affects non-food products; particularly, products touching the body in manners other than as food, e.g. pharmaceuticals, cosmetics, products of personal care, or even clothes. In cosmetics, the trend of halal choice is catching up with Muslim consumers. Cosmetics are only considered halal when all ingredients comply with halal & Shariah requirements and Haram ingredients such as material derived from pig, & alcohol are not used in their preparation. Moreover, all the contents must be manufactured, packaged, and stored, and delivered according to strictly followed halal standards. Halal cosmetics market sale is estimated to range yearly from the US $ 5 to 14 billion. Razak et al. (2015) believe that healthcare products & personal hygiene, cosmetics & toiletries all are appearing in halal options. Elasrag (2016) expressed that analysts are of the view that after the halal food industry, next in growth line is the cosmetics & personal care industry in the global halal market because halal cosmetics products have gained momentum in eco-ethical consumers ready to pay for products of organic, earthy & natural cosmetics matching their modern lifestyle. This is because of the high demand for halal beauty products & cosmetics stemming from demographics of religiously conscious, young & professionally dynamic population of Muslims. The concepts though very new in the scope of halal cosmetics spans to aspects of the production of cosmetics using halal ingredients and permissible substances required to be manufactured, packaged, stored & distributed according to Sharia demands. There is a growing demand for halal cosmetics & personal care products from conscientious customers becoming more selective in their choice so the Halal cosmetics market in Asia & the Middle East is booming while halal cosmetics across the Middle East registered annual growth reaching the value of US$ 12 billion & in Asia especially in Indonesia & Malaysia & in Europe, there is a surge of interest of halal cosmetics. Halal cosmetics in Malaysia contribute 10 percent to 20 percent of the cosmetics market.
Adiba (2019) and Elasrag (2016) asserted that halal cosmetics market growth is mirrored by augmentation of customers" knowledge about the products and awareness about ingredients used in it. They also asserted that currently halal cosmetic industry all over the world is estimated at 13 billion US Dollars that accounts for 11 percent of global industry and yearly growing at the rate of 12 percent. Halal cosmetic products by consumers are considered safe, clean & of high quality. Halal cosmetics products do not contain any ingredient derived from alcohol, pork or their derivatives so halal products are different from generally produced cosmetics. Elasrag (2016) expressed that many products & market of cosmetics is also plagued with the nonavailability of halal standards in the world. A collaboration of halal cosmetics products in the world market with interest groups such as ethical, organic, vegan & environmental rights may be helpful to strengthen the value of halal cosmetics products. Murti (n.d.) stated that Islam emphasizes its followers to see both their soul &body because the human body has the right to comfort & health. Therefore Muslims should lead a healthy life & take their body care with nutraceuticals & cosmetics and also use vaccines to prevent Meningitis while pilgrimage touring, & perfuming. So the USA, France & Germany respectively export to Muslim countries cosmetics of US $ 11.05, 16.64, and 11.17, billions. Ali et al. (2019) asserted that though halal cosmetics has a substantial share in the market because its demand has increased & gained substantial momentum due to the need for hygienic, safe human body-friendly cosmetics but there is scarcity accessibility & supply of these halal cosmetics & personal care products and it is a stirring concern for halal cosmetics industry development. One cause of halal cosmetics shortage is the continuous increase in consumers" awareness and knowledge of non-halal ingredients such as lard, gelatin, swine placenta, alcohol & collagen. Therefore because of consumers" knowledge & awareness about the ingredients of products, cosmetic manufacturers have to produce 100% halal products. Hence Muslims have no way but to consume non-halal cosmetics that are manufactured by non-Muslim producers, who cannot assure 100 percent halalness of products because of lack of religious knowledge. Cosmetic items manufactured by non-Muslims contain DNA, porcine & gelatine, lard & pork which render the product nonhalal. Sugibayashi, et al. (2019) mentioned that along with Muslims exhibiting high loyalty towards halal cosmetics products halal cosmetics also carry a wider demand from non-Muslim consumers, who attribute halal products and goods with better stringent standards of quality assurance & ethical consumerism. Suki and Suki (2018) revealed that consumers of halal cosmetics having a positive attitude towards products of cosmetics while buying look always for Halal label. They have a strong belief that the usage of halal cosmetics is important for them. Purchasers" attitude towards products of halal cosmetic is affected by some aspects such as religiosity & knowledge. Globally market of halal cosmetics is rising, and in the Middle East sale of cosmetics & related products is US $ 12 billion and demand is anticipated to rise until 2024 at the annual rate of 6.8 percent. The market of cosmetics in Europe, Malaysia & Indonesia is uprising. Sugibayashi, et al. (2019) asserted that halal cosmetics must not contain any ingredient derived from carrion, pig, human body parts, blood predatory animals, and insects & reptiles. Ingredients Halal cosmetics derived from halal animals must be slaughtered according to Shariah are considered halal. During manufacturing, preparation, processing, transporting, and storing of these halal products maintain pure and hygienic conditions must be ensured continuously.
Source: https://azkahalal.files.wordpress.com/2014/05/t-l5-mian-riaz.pdf Elasrag (2016) expressed that the world cosmetics industry is dominated and to some extent monopolized by companies of non-Muslims posing a challenge to halal ingredients issue in the manufacturing of cosmetic products such as alcohol which is haram in Islam & animal sources components such as gelatin & collagen deemed impure. Wahab (2004) stated that equipment & clothing made from leather, fur (except dogs & pigs), or plants whose slaughtering, preserving & tanning is according to Islamic principles are halal and their use is permissible. Murti (n.d.) stated that Muslim women buy halal fashion items including garments, hijab & its accessories, amounting to the US $ 44 billion, in 2015. As regards halal garments the raw material for fabricating coarse or fibrous clothing the raw material used is a polyunsaturated fatty acid from the transformation of lard. Muslims during praying are required to wear good quality clothing as a thankfulness & celebration of the blessing of God Almighty on them. However, swine hide to prepare jackets & hair for cosmetics or swine leather for shoes is strictly forbidden.

Tourism & Travel
Abuznaid (2012) Murti (n.d.) expressed that halal tourism also includes Shariah-compliant hotels easily achievable by poor Muslims, attracting Muslim travelers, and which also help to the growth of the economy. Comcem (2016) asserted that "Halal Tourism"& "Islamic Tourism" are equated while referring to products & services of tourism meeting the needs of Muslim travelers who do not compromise with their needs based on faith while traveling for permissible purposes. COMCEM (2016) also narrated that around the world hotel chains have started to accommodate Muslim travelers to enter into the profitable MFT market. However, in this area, there is a lack of certification & standardization has become very clear where every hotel is following its own adopted standards. Some hotels offer only halal food while others offer besides other facilities such as prayer rooms & gender-separated entertainment. Some hotels have obtained halal certification from certifying bodies. Many shopping, entertainment, & cultural venues have started providing halal food and offering spaces for prayers. Some recreational facilities like spas have started to provide Muslim travelers some tailored services as gender-separated "halal" spas. The number of Halal certified restaurants is increasing in many countries offering certified halal food. Islam and Chandrasekaran (2013) are of the view that traditionally Islamic marketing or Halal concept was understood to apply to food items and financial and banking services only. Ahmad (2014) believes that halal products along with Islamic banking are also considered Islamic products. Halal industry has a strong potential in international as well as in domestic markets & almost 70 percent of Muslims are following halal food standards. In 2007 the world halal market yearly stood for US$ 580 billion. Muslim population in 2014 was 1.8 billion & the halal global market was US $ 2.1 trillion. WEF (2015) reported that the Islamic finance industry globally was valued in the first half of 2014 at the US $ 1.9 trillion approximately and estimated to exceed the US $ 2 trillion by the end of 2014. Rachman and Syamsuddin (2019) asserted that the halal economy is the joint concept between the halal industry & Islamic finance. Islamic finance refers to financial transactions & banking transactions conducting methods according to Shariah law because both industries derive their principles & values from the same source that is Holy Qu"ran & Hadith. So the relationship between both is seen from the basic side of the principles of Shariah.

Islamic (Halal) Finance
WEF (2015) commented that now Shariah following Islamic finance is a US $ 2 trillion industry and in Islamic countries economic growth demand is creating the need for more bank services, creating more wealth to be protected through Islamic Insurance services or takaful and investment of funds in Sukuk called Islamic bonds &, infrastructure & other developments. Islamic finance is a significant beneficiary & key driver for both speedy growth and also increased intraregional trade in South America, Africa, Asia, & Middle East (SAME). WEF (2015) further expressed that the worldwide expansion of the Islamic economy increased by integration between global Islamic finance & halal industry, in providing funds as well as, in delivering the end products. Hayata et al. (2011) expressed that certification and standardization are very popular ways of reducing costs of transaction and there are economies of getting financial products certified as Islamic. Islamic law commands Muslims to invest in assets that are halal according to Islam. To identify halalness of investment in financial products a market has been created called the market for halal certification of financial products. However, it looks that the halal certification market has obtained smaller attention in analyzing economic institutions, which is not encouraging because halal certification is a fundamental feature that distinguishes & separates conventional finance from Islamic finance. Hayata et al. (2011) further expressed that the aim should be to fulfill the gap between conventional & Islamic finance, so firstly the market for halal certification must be described, and secondly inefficiencies in the process used for certification for transaction cost in empirical investigation & theory must be identified.

Packaging, Logistics & Supply Chain of Halal Food
Ab- Talib and Johan (2012) narrated that packaging of halal food products must also be focused on and be certified halal to have a wholesome halal product because the halal product does not matter only a consumption point, but must cover the whole supply chain aspect from the source of the product to final consumption. The status of halal packaging may be questionable because of packaging material sources from animals whose halal status is doubtful. It is also necessary during transportation to segregate sealed halal and non-halal products because if there is leakage such as of meat juice then segregation is a must. Some researchers are of the view that during packaging and shipping if any food is not according to Shariah law criteria it must be separated from halal food products considered being unclean. The images printed on the packaging must not violate Shariah law. Elasrag (2016) highlighted that a decade ago halal food products were provided in corner shops & with neighborhood butchers but now the halal market has shown a universal shift in the demand chain & supply chain of halal food items. Annabi, and Olajumoke (2017) asserted that because of growing market internalization, more choices of products in the market have become available consisting of new ingredients formulation. For Muslim consumers, some of these ingredients are unsuitable either because of their source or processing method which is not according to the law of Islam. So important on halal compliant products need is transparency in the supply chain of halal products. For this purpose, Halal Certifying Organizations (HCOs) have to play an important and crucial role, both as supply chain partners and as a link between halal consumers and halal product producers. Bohari et al. (n.d) asserted that in 2013 the global halal food industry was between US$ 600 billion to US$ 2.3 trillion. It is not to focus only the halal food consumption but must also be concerned about all the supply chain process, & standard of food according to Shariah. Halal supply chain encompasses managing process of halal food products of different suppliers" points to different buyers" points located at various places especially who are simultaneously engaged in managing nonhalal food items to satisfy their all types of customers. Especially in the situation when Muslim food consumers became more aware and concern about entire activities in the supply chain so ensure that their consumable product is truly halal. Wilkins et al. (2019) revealed that based in non-Muslim countries manufacturers & retailers of food because of international market saturation and global recession both foreign & in-home markets recognized the potential of faith-based marketing. Wilkins et al. (2019) quoted that in the United States halal food hit the US $ 20 billion on 7600 outlets, and in European countries such as UK, Belgium, the Netherlands, and France many fast-food chains & restaurants provide only halal food products at all or some outlets. Annabi, and Olajumoke (2017) asserted that the halal concept besides emerging as a Muslim brand identifier is transcending beyond a religious issue stick with Muslims, to the mainstream market as a synonymous global symbol of quality. It is believed that halal produce is safer, healthier, & tastier appealing to even Non-Muslims besides Muslims, and its demand is linked to this belief. Elasrag (2016) asserted that the cosmetic industry all over the world is dominated and to some extent monopolized by non-Muslim companies posing a serious challenge to the matter of halal ingredients in their manufactured cosmetic products. Sugibayashi, et al. (2019) mentioned that along with Muslims exhibiting high loyalty towards halal cosmetics products halal cosmetics also carry a wider demand from non-Muslim consumers, who attribute halal products and goods with better stringent standards of quality assurance & ethical consumerism. Batu and Regenstein (2010) expressed that halal products certified responsibly are gaining acceptance as a benchmark for quality and safety assurance. Annabi, and Olajumoke (2017) asserted that the halal concept along with emerging as a Muslim brand identifier is now transcending to be a global symbol from merely a religious issue to synonymous with quality in the general market of the mainstream. Now halal product demand is related to the belief of being safer, healthier, and tastier, attracting alike to both Muslims and non-Muslim. Because of growing market internalization, more choices of products in the market have become available consisting of new ingredients formulation.

Non-Muslims and Other Halal items
Ab-Talib and Johan (2012); Memon (2015) and Rahim et al. (2016) asserted referring to some researches that non-Muslims have also developed their likeness, tendency, and preference for Halal food products and prefer to purchase Halal food products due to their healthier, tastier, safe and clean characteristics. In the western countries, many non-Muslims also like halal food by developing their likeness for it and Golnaz et al. (2010) quoted an example of awareness of consumers and their understanding regarding the concept of halal food products in Moscow in Russia where the sale of halal stores increased from 45 million Dollars in 2004 to 70 Million Dollars in 2006. In 2008 Canadian Agri-Food Trade reported that in many non-Muslim countries there is a high demand for Halal products for both Muslim and non-Muslims consumers. Among non-Muslims buyers, the popularity of Halal food products is increasing because of Muslims" humane animal treatment, and because of their perception that halal food products are safer and healthier. Halal products undergo thorough inspection ensuring a hygienic and clean manufacturing process. To popularize Halal values among non-Muslims consumers awareness of society"s concerns on issues regarding, safety, health, and animal rights, social justice, safety, welfare, and the environment be increased. Russian non-Muslims buy from stores of Muslims due to their confidence that products are safe, fresh, and free from infections and also believe that Muslims would not cheat because are following their religious belief. Non-Muslims in Philippine also tend to prefer Halal logo-stamped food products for health reasons because they have become health conscious.

Conclusion
From the study of the above literature it is concluded that halal now has become a global concept and stands for fair business as well as for social justice, sustainable environment, and animal welfare therefore halal products market globally have 2 billion consumers growing 20 percent annually but the halal concept is not limited to food only rather encompasses all the things coming in contact with daily life especially to Muslims" used products, such as pharmaceuticals & cosmetics; media & communications, services like finance & banking, tourism & travel, garments & fashion, packaging, entertainment & recreation, and supply chain. The business of Halal products is very profitable in both Muslim countries as well as non-Muslim countries where demand for halal products showed significant growth e.g. Russia, Thailand, Japan, South Korea, Spain, & Mexico have already established halal industrial activities. Thomas Router forecasted that the global Muslim food market will be worth the US $: 1626 billion, pharmaceuticals 97 billion, cosmetics 39 billion.

Recommendations
According to Shariah halal business encompasses products & services of the highest quality, are hygienic and healthy so also demanded by non-Muslims because these are meeting the everincreasing needs & awareness of Non-Muslim consumers. Therefore Muslims should prefer to use halal products and also try to enter in halal business because still, a major share of the business is in the hands of non-Muslim companies and countries.


Cross-tier Interference Mitigation Considering Pilot Overhead for TDD MIMO Heterogeneous Networks

: In a two-tier heterogeneous network, cross-tier interference mitigation between macro-cell (MC) and small-cell (SC) is an impor-tant issue, where MC and SC share the same time/frequency resources. In this article, we propose a method to mitigate downlink cross-tier interference between MC and SC, where macro-cell base-station (MBS) and small-cell base-stations have multiple antenna arrays and their precoding weights are optimized to mitigate mutual cross-tier interference under a given pilot overhead. Simulation results show that the proposed scheme improves the downlink sum-rate compared with conventional scheme that coordinates cross-tier interference from MBS to small-cell users under the same pilot overhead condition.

Introduction
Two-tier heterogeneous network (HetNet) is a promising framework for nextgeneration wireless communication systems, where short-range small-cell (SC) base-stations are overlaid on a coverage area of macro-cell (MC) [1]. In particular, in two-tier HetNets, radio spectrum can be utilized more effectively if MC and SC share the same time/frequency resources [2]. However, cross-tier interference coordination between MC and SC is necessary to fully harvest potential performance of the HetNets [3]- [5].
Adaptive beam-forming using massive multi-input multi-output (mMIMO) technology [6] is an effective approach to mitigate inter-cell interference, e.g., when mMIMO is equipped at MC base station (MBS), the extra spatial degree of freedoms of mMIMO can be utilized to effectively mitigate cross-tier interference from MBS to SC users (SUs) if cross-channel state information (CrCSI) of the path between MC and SC is available. In [4], a locationaware pilot reuse scheme is proposed to coordinate uplink pilot interference. However, downlink cross-tier interference mitigation between MC and SCs is not considered. In [5], a downlink cross-tier interference mitigation scheme was proposed for HetNets with a time-division-duplex (TDD) mode, where CrCSI is estimated at MBS by overhearing uplink pilots from SUs and designing the precoding weight that cancels downlink cross-tier interference to SUs, where the pilot assignment to SUs is optimized to maximize SUs' sum-rate under a given pilot overhead. Although this scheme is effective in improving SU's rate while minimizing the required pilot overhead, cross-tier interference mitigation from SC base-stations (SBSs) to macro-users (MUs) is not considered.
In this article, based on works in [5], we propose an extended cross-tier interference mitigation scheme that coordinates not only cross-tier interference from MBS to SU but also the interference from SBS to MU in HetNets with TDD where mMIMO is equipped at MBS.

System Model
We consider a TDD downlink HetNet in Fig. 1 The received signal at the m-th MU is expressed as where P M and P S denote the transit powers of MBS and SBS, respectively. Similarly, the received signal at j-th SU in the k-th SC can be written as: Inter−user interference where n S k,j denotes the AWGN.
channel matrix estimated at MBS, where (a) T denotes transpose of a vector a, To mitigate inter-user interference, ZF precoding matrix at MBS, W ∈ C N M ×M , is given as where (·) H denotes conjugate transpose of a complex matrix. ∥A∥ 2 denotes the Frobenius norm of A ∈ C.
Similarly, let Hence, signal to interference plus noise ratio (SINR) at m-th MU and that at j-th SU in k-th SC, are given below, respectively.

Conventional scheme
This section explains a pilot-user allocation and cross-tier interference mitigation scheme in [5].
terference channel matrix, and h M,S 0,0,j denotes CSI of the path between MBS and j-th SU. f j ∈ C N M ×1 denotes the precoding vector to cancel cross-tier interference to j-th SU (j ≤ K O ). From (3) and (7), the concatenated ZF precoding matrix at MBS, W M ∈ C N M ×(K O +M ) , can be written as From (5) and (8), m-th MU's rate is lower-bounded as [5] R M γm = where, L is the number of symbols per frame. From (6) and (8), j-th SU rate that uses the orthogonal pilot is lower-bounded as [5] R S γ k,j = On the other hand, the other SU's rate affected by cross-tier interference from MBS is lower-bounded as [5]: It is clear from Eqs (9), (10), and (11) that pilot overhead (i.e., pilot length) increases as K O increases while SINR at each SU is improved by mitigating cross-tier interference. This implies that a trade-off relationship exists between pilot overhead and cross-tier interference mitigation capability. The downlink sum-rate can be given as where K O denotes a set of user indices who are allocated orthogonal pilots. The cardinality of K O is K O . The above sum-rate can be maximized by optimizing pilot allocation K O [5] 2 . This scheme mitigates cross-tier interference from MBS to SUs while minimizing the required pilot overhead. However, cross-tier interference mitigation from SBS to MUs are not considered.

Proposed scheme
Based on works in [5], we propose an extended cross-tier interference mitigation scheme that coordinates not only cross-tier interference from MBS to SU but also that from SBS to MUs, where SBS estimates CrCSI of the path between SBS and MUs by overhearing MU's pilots. The proposed scheme can mitigate cross-tier interference from SBS to MUs without changing the pilot allocation (i.e., without increasing the required pilot overhead).
Similarly to the conventional scheme in [5], we assume that all MUs and K O SUs are allocated the orthogonal pilots and the remaining SUs are allocated the same pilots. Since all MUs' pilots are orthogonal to SUs' ones, each SBS can distinguish MUs' and SUs' pilots. Thus, SBS can estimate cross-channel state information (CrCSI) between SBS and MUs by receiving orthogonal pilots from MUs. More concretely, let K IC denote the number of MUs who are strongly affected by cross-tier interference from SBS. By detecting K IC MUs' pilots, CrCSI between k-th SBS and K IC MUs can be estimated as where 0 ≤ K IC ≤ N S − M S . Hence, the precoding matrix at k-th SBS to cancel cross-tier interference to K IC MUs G k ∈ C N S ×K IC can be given as where g k,m ∈ C N S ×1 denotes the precoding vector of k-th SBS for m-th MU. From (4) and (14), the concatenated precoding matrix at k-th SBS, W S k ∈ C N S ×(M S +K IC ) , can be given as Assuming N S > M S , each SBS can cancel K IC (≤ N S − M S ) cross-tier interference at maximum at expense of decreasing the precoding gain to served SUs 3 . More concretely, MUs' sum-rate can be improved by increasing K IC but SU's rate is decreased due to the decreased precoding gain. Thus, it is necessary to find the optimum value of K IC that maximizes the total sumrate. In this article, each SBS mitigates cross-tier interference to the nearest MU.

Performance Evaluation
We consider a single MC with a radius of 1000 meters and there is no user within 100 meters of MC center area, while the radius of each SC is 30 meters and there is no user within 5 meters of SC center area [5]. The minimum distance between two SBSs is 120 meters [5]. Proposed method Conventional method [5] (a) SU's downlink sum rate versus KO Proposed method Conventional method [5] (b) MU's downlink sum rate versus KO  The received signal is affected by pathloss and independent equal-level two-path Rayleigh fading. Pathloss between MBS and MU /SU is 27.3 + 39.1 log 10(d), and pathloss between SBS and MU/SU is 36.8 + 36.7 log 10(d) [5]. We consider two scenarios in Fig. 1. In scenario 1 ( Fig. 1(a)), MUs are uniformly distributed in the MC, while scenario 2 ( Fig. 1(b)) assumes that one MU is located near each SC and the rest of MUs are uniformly distributed within the MC, where R edge is 5 meters. Figure 2 shows the downlink sum-rate versus K O for the proposed method (solid line) and the conventional method (dotted line) [5]. Here, the conventional method corresponds to K IC = 0, while the proposed scheme assumes K IC = 1. In Fig. 2(a), there is optimal K O that maximizes SUs' sum-rate. Since SBS uses spatial degree of freedom for mitigating cross-tier interference to MUs, SU's downlink rate slightly decreases in both scenarios. In Fig. 2(b), as K O increases, the downlink rate of MUs decreases monotonously due to the increase of pilot overhead. MU's downlink rate can be improved by applying the proposed scheme especially in scenario 2, because dominant cross-tier interference to MUs is mitigated. In Fig. 2(c), the downlink total sum-rate (i.e., MU rate + SU rate) can be improved by interference mitigation at SBS. This is because the effect of cross-tier interference mitigation to MUs is higher than the SUs' rate degradation. Figure 3 shows downlink sum-rate as a function of K IC , where K O = 4 is chosen as the best value to maximize the achieved sum-rate of the proposed method. In both scenarios, as K IC increases, MU's downlink sum rate is improved monotonously, while SU's sum rate is degraded slightly due to the reduced precoding gain at each SBS. The total sum-rate can be improved by choosing the proper K IC in the propose scheme especially when MU is located near SC.

Conclusion
In this article, we have proposed a downlink cross-tier interference between MC and SCs, The results show the effectiveness of the proposed approach in terms of total sum-rate in two-tier TDD-HetNets.