Adaptive Multicarrier CDMA Space-Time Receivers - Adaptive Signal Processing in Wireless Communications - page 299

interface is also considered as a reference. In addition, in order to provide a more detailed
picture of the aggregate gain of the proposed MC-ISR receiver, we also compared its per-
formance with that of MC-MRC over the same two multicarrier CDMA-air interface
configurations, as well as with that of single-carrier MRC over current 3G DS-CDMA.
First, we derive the SNR req from link-level simulations. Then, we translate the link-level
results into system-level results using the procedure in Table 10.1 . In Table 10.4 , we pro-
vide the required SNR and the total throughput of DBPSK and D8PSK modulated data
for DS-CDMA, MT-CDMA, and MC-DS-CDMA. For DBPSK modulation, we observe
that we can improve the system performance by increasing the number of subcarriers.
Indeed, the total throughput continues to increase despite the increase in the number
of carriers. But a gain saturation is encountered as the number of subcarriers increases.
Note, however, that the throughput increase is more important with MC-ISR due to ICI
suppression. Table 10.4 also shows that MT-CDMA outperforms MC-DS-CDMA with
DBPSK modulation because it uses longer spreading sequences and exploits the sub-
carrier correlation. Moreover, due to the reduced subcarrier bandwidth, MC-DS-CDMA
has less frequency diversity, while MT-CDMA is better able to exploit path diversity,
and hence achieves better performance. Note also that MC-DS-CDMA is more robust
against ICI, but in applying MC-ISR, this advantage over MT-CDMA becomes obsolete
and the performance gap between MT-CDMA and MC-DS-CDMA increases.
Next, we compare different configurations with D8PSK modulation. We notice a
link-level deterioration for MT-CDMA as the number of subcarriers increases. Indeed,
higher-order modulation is more sensitive to the residual ICI. MC-DS-CDMA is much
less affected by this phenomenon because it is much more robust to ICI thanks to the
higher subcarrier spacing. Therefore, with high-order modulation, MC-DS-CDMA out-
performs MT-CDMA when the number of subcarriers is high enough. We notice also that
with the MC-MRC combiner, D8PSK MC-DS-CDMA outperforms D8PSK MT-CDMA
even with a small number of subcarriers. It is clear, however, that D8PSK is less efficient
than DBPSK modulation for all air interface configurations. In Table 10.4 we highlight
the most spectrum-efficient MC-ISR-air interface configuration for each modulation.
For both modulations MT-CDMA has the best link-level performance and the high-
est throughput (for a tested number of carriers less than or equal to 11). MT-CDMA
with nine subcarriers and DBPSK modulation outperforms all other configurations and
provides a throughput about 115% higher than that achievable with single-carrier MRC
over a 3G DS-CDMA-air interface. The net benefits due to the proposed MC-ISR com-
biner and to the potential migration to a next-generation MT-CDMA-air interface are
about 80 and 15%, respectively.

10.5 Conclusions

In this chapter we propose an adaptive multicarrier CDMA space-time receiver with
full interference suppression capabilities named MC-ISR. First, we derived a complete
model of the interference that takes into account MAI, ISI, and ICI in a multipath fading