C-SIC

Figure 4.13 shows the BER performance of C-SIC under the system load of K = 10 users. The performance of collaborative matched filter (C-MF) is also shown for the comparison. As expected, the performance of C-SIC is improved significantly compared with C-MF. It can be seen from the figure, the use of interference cancellation provides significant BER gain and approaches the performance of a single user system with Perfect Collaboration under higher ratios of inter-user channel SNR conditions as described earlier.

Figure 4.13: Performance of C-SIC in flat Rayleigh fading channels for K=10 and Gold sequences of N=31

Figure 4.14: Performance of C-SIC in flat Rayleigh fading channels for K=20 and Gold sequences of N=31

4.14. The BER of C-MF is shown for comparison. The performance C-SIC is significantly better compared with the C-MF as expected. The effect of system loading on degradation in BER for both systems is clearly seen from the figure. The performance gaps between the C-MF and C-SIC are still considerable, however it is noted both systems suffer for residual MAI and hence BER curves are not steep at high SNR region.

Figure 4.15: BER vs. number of users of C-SIC in flat Rayleigh fading channels with Eb/N0= 20dB, and

Gold sequences of N=31

The BER performance of C-SIC is shown in Figure 4.15 and compared with SIC without collaboration (Non Collaborative SIC) for 20 users under various degrees of user collaboration. Which are quantified by the relative SNR gains of inter-user channels to the transmit channels of

Figure 4.16: BER vs. number of users of C-SIC in flat Rayleigh fading channels with Eb/N0= 20dB and

Gold sequences of N=31

Figure 4.17: Performance of C-SIC and C-MF in flat Rayleigh fading channels with Eb/N0 = 20 dB and

the respective users. The C-SIC showed a superior BER performance giving several dB of gain for a given target BER. For example, with relative SNR gain of inter-user channels of only 0 dB, it shows gain of about 4 dB for the same target BER of 10−3. As the quality of inter-user channels improve (higher values of βk), the error performance of C-SIC improves rapidly. It is noted that

the relative inter-user channel gain of 10dB is sufficient to provide the near asymptotic gain from the user cooperation and the performance is very near to that of the system with inter-user SNR gain of 20 dB. The gap in BER performances between the Perfect Collaboration and C-SIC is due to imperfect interference generation and cancellation.

Figure 4.18: Performance of C-SIC in flat Rayleigh fading channels with near far ratio of 10 dB with Eb/N0of the weakest user=15dB and Gold sequences of N=31

In Figure 4.16 BER simulation results of C-SIC is plotted under different system loads in fading channel environments. As expected, it shows significant improvement in the error perfor- mance compared to the Non collaborative SIC as the inter-user channel gains βkincrease. Also,

the simulation performance results of C-SIC show that the BER performance under different user loading conditions does not degrade significantly to unacceptable level as the number of users in- crease. The performance of C-MF as shown in Figure 4.17 , is much worse than the C-SIC. Also, it is noted that the BER performance of C-MF do not improve much with higher SNR gains of inter-user channels under higher user loading conditions.

Figure 4.18 shows the performance of the C-SIC in fading channels and nearfar ratio of 10 dB. The desired user (weakest user) has unity power corresponding to the expected SNR at the receiver of 15 dB, while all other users have been assigned powers uniformly distributed between 0 and 10dB. It can be clearly seen from the figure that as βkincreases, the BER performance of

Figure 4.19: Performance of C-SIC in in flat Rayleigh fading channels with nearfar ratio of 10dB with Eb/N0of the weakest user=15dB and Gold sequences of N=31

As the number of users increases, the system BER performance is degraded as usual. Due to the collaboration, it is expected that the desired weak user benefits from the strong channel of its partner. From Figure 4.18, it can be noted that the performance of the desired user with C-SIC is indeed improved compared to the case of equal power users case as in Figure 4.16. This improved result can also be verified using the analytical approach and is left as the future study.

The comparison of collaborative schemes, C-SIC and C-MF under 10 dB nearfar conditions is shown in Figure 4.19. It is observed that use of C-MF does not improve BER with nearfar conditions as it did with C-SIC. The performance of C-MF based scheme is shown to be worse than in equal power case as in Figure 4.17. The main reason behind this is that, in nearfar conditions the estimates of users’ data becomes highly unreliable and hence diversity combining does not assist much to improve the performance of final decision.