eral, spatial multiplexing schemes can be classified depending on whether the different multiplexed streams are jointly or separately coded. According to this criterion, there are single code-word* (SCW) and multiple code-word (MCW) Strictly speaking, a code word is defined as a coded transport block. However, the term code word will be used in a wide sense to refer an independently coded data stream hereinafter. © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer Code word Stream 1 Transport block Modulation Coding (QPSK, 16-QAM, 64-QAM) Stream 2 (a) Single code word (e.g., 2 streams) Code word 1 Stream 1 Modulation Transport Coding (QPSK, 16-QAM, block 1 64-QAM) Stream 2 Stream 3 Modulation Transport Coding block 2 (QPSK, 16-QAM, 64-QAM) Stream 4 Code word 2 (b) Multiple code words (e.g., 2 code words and 4 streams) Figure 3.22 Single code word and multiple code word multiplexing schemes. schemes. In the single code-word approach, one coded and modulated transport block is spatially multiplexed into several data streams (Figure 3.22a). In the mul- tiple code-word case (Figure 3.22b), different coding and modulation processes are implemented for each of the transport blocks that are simultaneously transmit- ted. In this case, it is possible to have a different coding or modulation rate for each code word (i.e., data stream independently coded). Consequently, an MCW approach allows for applying different FEC and HARQ processes to the different code words. In LTE, spatial multiplexing follows an MCW approach with two code words and a maximum of four transmitted data streams (Figure 3.22b). That is, two code words are simultaneously transmitted and each code word may be spatially multi- plexed into one or more data streams. Therefore, the number of multiplexed streams will be limited to R, and only two code words will be allowed. Different code words transmitted in the same time-frequency resource can be allocated either to the same user or to different users for achieving different pur- poses. The