On the performance of multicarrier DS-CDMA with imperfect power control and variable spreading factors

Multicarrier direct-sequence code-division multiple access (MC-DS-CDMA) becomes an attractive technique for the future fourth-generation (4G) wireless system because it can flexibly adapt transmission rates by changing both time and frequency spreading factors and possesses many physical-layer advantages in dispersive fading channels. However, power control errors (PCE) and the complete multiple access interference,(MAI) from all the intersubcarriers may significantly degrade the performance of the MC-DS-CDMA system. In this paper, we propose an analytical method to evaluate the joint effects of the PCE and the complete MAI on the multirate MC-DS-CDMA system. From analysis and simulation, we obtain some important insights into the performance issues of the MC-DS-CDMA system. First, the effect of PCE can exacerbate the impact of the complete MAI on the MC-DS-CDMA system, or vice versa. For BER = 10(-3) in a considered case, the joint effect of the complete MAI and PCE further degrades the performance by 2.1 dB compared with the sum of the degradation from the complete MAI and the PCE individually. Second, increasing frequency- or time-domain spreading gain can improve the performance of the MC-DS-CDMA system, but the system also becomes more sensitive to power control errors. Third, a larger PCE can possibly make the frequency-domain diversity diminish faster than the gain obtained from the time-domain spreading although an MC-DS-CDMA system with a larger frequency-domain spreading gain (M) is usually better than that with a larger time-domain spreading gain (G(o)). In our example, for the standard deviation of PCE (a,) equal to 0 dB, the BERs with (M, G(o)) = (4,16) and (16,4) are 9.3 X 10(-4) and 3.7 X 10(-5), respectively, while for sigma(e) = 4 dB, the BER performances of the two cases are all in the order of 10(-3).