20 **3.2. System model**

12 BookTitle

90 Contemporary Issues in Wireless Communications

20 per diversity branch.

35 provide satisfactory performance.

1 them. In traditional HS/MRC scheme, the strongest *L* signals are selected according to 2 signal-envelope amplitude [29–35]. However, some receiver implementations recover 3 directly the in-phase and quadrature components of the received branch signals. 4 Furthermore, the optimal maximum likelihood estimation (MLE) of the phase of a diversity 5 branch signal is implemented by first estimating the in-phase and quadrature branch signal 6 components and obtaining the signal phase as a derived quantity [36,37]. Other channel-7 estimation procedures also operate by first estimating the in-phase and quadrature branch 8 signal components [38–41]. Thus, rather than *N* available signals, there are 2*N* available 9 quadrature branch signal components for combining. In general, the largest 2*L* of these 2*N* 10 quadrature branch signal components will not be the same as the 2*L* quadrature branch

12 In this section, we investigate how much improvement in performance can be achieved 13 employing the GR with modified HS/MRC, namely, with the quadrature subbranch 14 HS/MRC and HS/MRC schemes, instead of the conventional HS/MRC combining scheme for 15 1-D signal modulations in multipath fading channel. At GR discussed in [42], the *N* 16 diversity branches are split into 2*N* in-phase and quadrature subbranches. Then the GR with 17 HS/MRC scheme is applied to these 2*N* subbranches. Obtained results show the better 18 performance is achieved by this quadrature subbranch HS/MRC scheme in comparison with 19 the traditional HS/MRC scheme for the same value of average signal-to-noise ratio (SNR)

21 Another problem discussed is the problem of partial cancellation factor (PCF) in DS-CDMA 22 wireless communication system with multipath fading channel. It is well known that the 23 multiple access interference (MAI) can be efficiently estimated by the partial parallel 24 interference cancellation (PPIC) procedure and then partially be cancelled out of the 25 received signal on a stage-by-stage basis for DS-CDMA wireless communication system 26 [43]. To ensure a high-quality performance, PCF for each PPIC stage needs to be chosen 27 appropriately, where the PCF should be increased as the reliability of the MAI estimates 28 improves. There are some papers on the selection of the PCF for a receiver based on the 29 PPIC. In [44–46], formulas for the optimal PCF were derived through straightforward 30 analysis based on soft decisions. In contrast, it is very difficult to obtain the optimal PCF for 31 a receiver based on PPIC with hard decisions owing to their nonlinear character. One 32 common approach to solve the nonlinear problem is to select an arbitrary PCF for the first 33 stage and then increase the value for each successive stage, since the MAI estimates may 34 become more reliable in later stages [43, 47, 48]. This approach is simple, but it might not

36 In this section, we use the Price's theorem [49, 50] to derive a range of the optimal PCF for 37 the first stage in PPIC of DS-CDMA wireless communication system with multipath fading 38 channel employing GR based on GASP [1–3], where the lower and upper boundary values 39 of the PCF can be explicitly calculated from the processing gain and the number of users of 40 DS-CDMA wireless communication system in the case of periodic code scenario. Computer

11 signal components of the *L* branch signals having the largest signal envelopes.
