tailieunhanh - Mobile and Wireless Communications-Physical layer development and implementation 2012 Part 11

Tham khảo tài liệu 'mobile and wireless communications-physical layer development and implementation 2012 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Downlink Resource Scheduling in an LTE System 191 Time Fig. 1. LTE downlink time-frequency domain structure. reduce signalling overhead and retransmissions are dynamically scheduled so as to provide adaptability. The benefits of semi-persistent scheduling are described in Jiang et al. 2007 . An important constraint in LTE downlink scheduling is that all SBs for a given user need to use the same MCS in any given TTI1 Dahlman et al. 2008 . In the rest of this chapter we focus on the FD aspect of dynamic scheduling. Specifically the challenging problem of multiuser FD scheduling is formulated as an optimization problem taking into account this MCS restriction. Simpler sub-optimal solutions are also discussed. 2. System Model The system model we will use to study the resource allocation problem is now described2. An SB consists of a number Nsb of consecutive OFDM symbols. Let L be the total number of sub-carriers and Ld v L be the number of data-carrying sub-carriers for symbol V where V 1 2 . Nsb. Also let r c be the code rate associated with the MCS j G 1 2 . J Mj be the constellation size of the MCS j and Ts be the OFDM symbol duration. Then the bit rate rj that corresponds to a single SB is given by j - l 1 v . 1 T N b V 1 Let U be the number of simultaneous users and No be the total number of SBs that are available during each TTI. In addition let Ni be a subset of the No SBs whose Channel Quality 1 This applies in the non multiple-input-multiple-output MIMO configuration. For the MIMO configuration a maximum of two different MCSs can be used for data belonging to two different transport blocks. 2 The material in this section is based in part on Kwan et al. 2009b 192 Mobile and Wireless Communications Physical layer development and implementation Indicator CQI values are to be reported by user i the size of Ni is denoted by Ni. It is assumed that the Ni highest SB CQI values are fed back. Such a limited feedback scheme would necessitate a smaller bandwidth albeit