Part IV MULTIPLE ACCESS AND ADVANCED TRANSCEIVER SCHEMES 363
19.11 Single-Carrier Modulation with Frequency
A special case of MC-CDMA occurs when the unitary transformation matrixPis chosen to be the FFT matrix. In such a case, multiplication by the spreading matrix and the IFFT inherent in the OFDM implementation cancel out. In other words, the transmit sequence that is transmitted over the channel is the original data sequence – plus a cyclic prefix that is just a prepending of a few data symbols at the beginning of each datablock.
This just seems like a rather contrived way of describing the single-carrier system that has already been discussed in Part III. However, the big difference here lies in the existence of the cyclic prefix, as well as in how the signal is processed at the receiver (see Figure 19.19). After stripping off the cyclic prefix, the signal is transformed by an FFT into the frequency domain. Due to the cyclic prefix, there are no residual effects of ISI or ICI. Then, the receiver performs equalization on each subcarrier (this can be zero-forcing or MMSE equalization), and finally transforms the signal back into the time domain via an IFFT (this is the despreading step of MC-CDMA). The receiver thus performs equalization in the frequency domain. Since FFTs or IFFTs can be implemented efficiently, the computational effort (per bit) for equalization goes only like log2(N ). This is a considerable
Orthogonal Frequency Division Multiplexing (OFDM) 443
Stripping
off CP S/P
1 Tap equalizer
1 Tap equa izer
IFFT FFT
1 Tap equa izer
ck, 0
~
ck, 1
~
c~k, N1
P/S conversion
Data sink
Figure 19.19 Block diagram of a single-carrier frequency domain equalization receiver.
advantage compared with the equalization techniques discussed in Chapter 16. On the downside, frequency domain equalization is a linear equalization scheme and therefore does not give optimum performance. Also, the extra overhead of a cyclic prefix has to be taken into account.
Further Reading
Several books describe multicarrier schemes, including OFDM and multicarrier CDMA, e.g., Hanzo et al. [2003] and Li and Stuber [2006]; another interesting description that also covers applications to various standardized systems is Bahai et al. [2004]. Review papers on that topic include Wang and Giannakis [2000], Jajszczyk and Wagrowski [2005], Hwang et al. [2009].
OFDM was invented by Chang [1966], and the cyclic prefix was proposed in Weinstein and Ebert [1971]. Optimization of a set of nonrectangular basis function that span the time-frequency plane according to different criteria (e.g., low sensitivity to time and frequency dispersion) can be found in Kozek and Molisch [1998]. Discussion of the cyclic prefix and its comparison to zero-padding can be found in Muquet et al. [2002]. The performance of a coded OFDM system in a multipath channel is analyzed, e.g., in Kim et al. [1999]. ICI is discussed in Cai and Giannakis [2003] and Choi et al. [2001]; the latter also discussed ICI mitigation techniques, as does a number of other papers (see, e.g., Schniter [2004] and Molisch et al. [2007a] and references therein.
Synchronization and channel estimation are very important topics. Some examples of time and frequency synchronization include Schmidl and Cox [1997], Speth et al. [1999], and van de Beek et al. [1999]. Channel estimation techniques are discussed in Edfors et al. [1998] and Li et al.
[1998]. Approximate DFT estimators were introduced in van de Beek et al. [1995] and later analyzed in detail in Edfors et al. [2000]. Filtering after eigen transformation was introduced in Li et al.
[1998] and extended to the transmit diversity case in Li et al. [1999]. Methods for PAR techniques are reviewed in May and Rohling [2000] and Jiang and Wu [2008]. Multi access considerations are described in Rohling [2005].
For adaptive modulation, the paper by Wong et al. [1999], and especially the overview paper by Keller and Hanzo [2000], gives a good account. Yang and Hanzo [2003] review MC-CDMA.
Falconer et al. [2003] and Benvenuto et al. [2010] discuss single-carrier frequency equalization schemes; a combination of this scheme with diversity is detailed in Clark [1998]. A unified analysis of OFDM and single-carrier schemes with cyclic prefix is given in Akino et al. [2009].
For updates and errata for this chapter, see wides.usc.edu/teaching/textbook
20
Multiantenna Systems
Since the 1990s, there has been enormous interest in multiantenna systems. As spectrum became a more and more precious resource, researchers investigated ways of improving the capacity of wireless systems without actually increasing the required spectrum. Multiantenna systems offer such a possibility.
When discussing multiantenna systems, we distinguish betweensmart antenna systems(systems with multiantenna elements at one link end only), and Multiple Input Multiple Output (MIMO) systems, which have multiantenna elements at both link ends. Both of these systems are discussed in this chapter.