3 G Long Term Evolution (LTE) is currently standardized in by the 3rd Generation Partnership Project (3 GPP) and features a downlink radio access that is based on Orthogonal Frequency Division Multiplex (OFDM) and an uplink radio access based on Single Carrier Frequency Division Multiple Access (SC-FDMA).
The scalability of the physical layer radio access scheme in order to fit various existing and future spectrum allocations is a key advantage of LTE. In other words, one and the same physical layer specification should be easily adaptable to various spectrum or bandwidth allocations throughout the world simply by using a different parametrization of key parameters (mainly the FFT size). This allows for economies of scale with respect to chip set design.
The OFDM-based downlink consists of a number of evenly spaced narrow-band sub-carriers that are allocated for data transmission (“used sub-carriers”) within a defined spectrum or bandwidth allocation.
In order to structure the physical layer resource handling, physical resource blocks are defined consisting each of a given number of, e.g., adjacent sub-carriers. Hence, the total number of subcarriers used within a given spectrum allocation is an integer multiple of the number of sub-carriers contained in one physical resource block. The current assumption in 3 GPP is that each sub-carrier is 15 kHz wide, and that one resource block contains 25 subcarriers. Hence, for a 5 MHz bandwidth allocation assuming 10% guard band on the edges, a total of 12 resource blocks containing a total of 300 sub-carriers is the current assumption in 3 GPP.
Regulatory requirements demand that the transmitted signals from radio equipment operating in 3 G/LTE spectrum allocations comply with specific spectrum masks that are, or will be defined for each existing or future spectrum allocation. As a consequence the amount of needed guard band—which is directly reducing the number of used sub-carriers—will vary depending on individual spectrum mask requirements and impairments on the radio front end. A simple and straight-forward measure to circumvent this problem would be, e.g., to reduce the number of sub-carriers in each resource block. For instance, applying 24 instead of 25 sub-carriers results in 288 used subcarriers (12 resource blocks each with 24 subcarriers) within 5 MHz. However, although such a reduction of subcarriers may be sufficient to fulfil the spectrum mask requirements in a 5 MHz spectrum allocation, it is far from clear that a different spectrum allocation can be efficiently utilized using an integer number of resource blocks with the same resource block size as for the 5 MHz spectrum allocation (due to reasons of the scalability). As a consequence, spectrum mask and impairments might, e.g., allow for 3.125 resource blocks (with 24 subcarriers each) to be used in a 1.25 MHz allocation, resulting in a waste of 0.125 resource blocks (i.e. 4% of the usable bandwidth). Even worse, if spectrum mask requirements and impairments require slightly more than 10% guardband and the resource block size of 25 subcarriers would be maintained, then slightly less than 3 resource blocks could be fitted into a 1.25 MHz spectrum allocation, leading to a waste of almost ⅓ of the usable bandwidth.
Furthermore, existing solutions of defining a number of fixed spectrum allocations, each with a certain number of resource blocks, are inflexible with respect to future spectrum allocations, for example when assuming spectrum allocations of size 1.25, 2.5, 5, 10, 15, and 20 MHz for LTE and basing the resource block sizes upon these number. However, future demands for other spectrum allocations, e.g. 7 MHz, would require revisions of the physical layer specification, which is disadvantageous.
Thus, a more flexible scheme would result in a generic description of the air interface and only the relevant specifications covering testing and RF requirements need to be updated when additional spectrum allocations are defined.
The present invention relates to techniques that allow for the definition of a generic physical layer definition with a resource block allocation scheme that supports various bandwidth allocations. It relates to a method and arrangement for allocating physical layer resource blocks whilst enabling unambiguous initial access procedures for radio cells.
Embodiments of the present invention thus allow support of a generic physical layer specification that makes both the standard and implementations easily extendable to various existing and future spectrum allocations. This is achieved by a method and arrangement that arranges resource blocks in such a way that a generic resource block size is maintained for all spectrum allocations, except for one or several fractional resource blocks, whose size and position is derived from the number of usable sub-carriers by means of clear rules defined in the standard or indicated to the user equipment via signalling. In addition, the invention introduces ways to inform the user equipment about the number of usable subcarriers. These include inter alia:                1) including the number of usable subcarriers as part of the system information in the broadcast channel (in addition to or instead of the system bandwidth);        2) defining rules that unambiguously relate the number of usable subcarriers to the system bandwidth that is included in the system information on the broadcast channel.        
Thus, the present invention allows specifying of one generic physical layer that is easily extendable to various existing and future bandwidth allocations, and further allows signalling support in order to inform a user equipment about the valid resource block allocation in a cell.