1. Field
The invention, in certain embodiments, relates to communication systems which may employ transmission of feedback information and responses thereto, on a wireless connection.
2. Description of the Related Art
Wireless data traffic is projected to grow significantly. However, innovations in cellular air-interface design, culminating in the third generation partnership project (3GPP) long term evolution (LTE), provide spectral efficiency performance that may not be able to improve at a corresponding rate. To meet the growing traffic demand, other approaches may be used, such as increasing the cellular capacity per square meter by either shrinking cell-sizes or acquiring additional spectrum. For example, smaller cells may be implemented through heterogeneous networks of picos and macros, such as for carrier frequencies below 6 GHz, for example LTE heterogeneous network (HetNet). Similarly, 500 MHz of more spectrum is being made available below 5 GHz, which may help to meet the growing demand. This added spectrum, however, may also eventually be outpaced by the demand. Moreover, the available spectrum below 6 GHz is limited and there may be practical limits to how small cells can shrink. Thus, resources in frequencies above 6 GHz may be used to meet this demand for future (for example beyond 4G) cellular systems.
Unlike traditional cellular systems, electromagnetic (EM) waves in for example the millimeters bands (for example, for frequencies above 6 GHz) do not benefit from diffraction and dispersion, making it difficult for them to propagate around obstacles. Moreover, such millimeter waves also suffer higher penetration loss in some materials. For example, penetration loss of concrete block is 10 times higher at millimeter bands as compared to microwave bands. As a result, millimeter transmissions may be much more likely to encounter shadowing effects than microwave transmission. Millimeter transmissions may also have less favorable link budgets due to lower power amplifier (PA) output powers and greater pathloss at these higher frequencies.
As a result, to provide sufficient coverage from each access point, for example, 100 meter radius, narrow directional antenna array beams may be used both at the access point (AP) and the user equipment (UE).
The smaller wavelengths may allow for fabrication of much larger antenna arrays in much smaller areas than is typical at microwave bands. For example, arrays with as many as 8 to 32 elements providing 18 to 30 dB in link budget gain may be implemented. Reliance on these array gains can complicate link acquisition and maintenance. Traditional cellular systems, such as 3G LTE, cannot simply be upbanded and expected to function in the millimeter bands.
For example, because of the large number of antennas, the beam created by the array may be fairly narrow. With narrow beams, the user equipment may lose radio connection to the access point in case of blockage or misalignment. This may be due to, for example, obstruction between the user equipment and access point by objects, such as humans, trees, cars, or the like. Misalignment may be due to misalignment of the antenna array beams caused by wind-induced vibrations at the access point, or beam misalignment due to changes in user orientation, for example, due to how a device is held.
Current cellular radio standards such as 3GPP LTE provide solutions for frequency bands below 6 GHz which have well known propagation characteristics. A LTE system which is simply upbanded to 70 GHz would not provide adequate coverage or economy. LTE relies on radio wave diffraction around obstacles and therefore an LTE millimeter wave system would not achieve a reasonable coverage reliability target, for example 90% coverage reliability. Similarly, the power efficiency of semiconductor devices is reduced at frequencies above 10 GHz. LTE, which employs OFDM modulation, conventionally requires a significant Power Amplifier (PA) backoff making the solution less desirable at 70 GHz.
Local area solutions such as IEEE 802.11ad and IEEE 802.15.c exist and define air interfaces for local area access. The solutions are typically targeted to indoor deployments or for personal area networks. For example, 10 meter ranges are typically sited as a solution.
For future (e.g., beyond 4G (B4G)) cellular system, one access architecture for deployment of cellular radio equipment may employ millimeter wave (mmWave) radio spectrum. Example requirements for B4G include peak data rate of 20-30 Gbps and latency of less than 1 ms. To allow this, several features may be required, very high bandwidth, very small subframe size, near line-of-sight with rapid site selection and collaboration, and narrow beam-width. There are two main issues related to latency, as discussed below.
In Rel-10 LTE, user equipment category 8 is capable of supporting a maximum transport block size (TBS) of 2998560 per 1 ms transmission time interval (TTI) which is equivalent to 3 Gbps (assuming 5 carriers are aggregated using 8×8 multiple input multiple output (MIMO)). The processing time requirement for this user equipment may be 3 ms. In B4G, if the subframe size is reduced to 0.1 ms and the peak data rate is 30 Gbps, then the user equipment may be required to processing the same maximum TBS as user equipment category 8 but for a 0.1 ms subframe. Using current technology, the processing time for this user equipment will remain 3 ms, which is significantly longer than the subframe length and therefore likely to introduced large latency when retransmission is required. Even with significant improvement in user equipment processing capability, the reduced processing time (e.g. 1 ms) may still be significantly larger than the subframe size and can lead to unnecessarily large latency.
Secondly, with mmWave and narrow beams, the user equipment may lose connection to the transmission point, for example, due to blockage or misalignment of the antenna array beams caused by human movement or by wind-induced vibrations at the access point, and may require rapid site selection. However, the transmission/reception point may require feedback from the user equipment that connection has been lost. Traditionally, the eNB may wait for HARQ feedback from the user equipment to determine that connection has been lost. However, this can take a long time due to user equipment processing requirement as discussed above.