Power and rate adaptation in CDMA communications

Abstract

In this thesis, we examine power and rate adaptation in CDMA communications over multipath fading channels. First, we propose a variable power and rate adaptation scheme, where the transmission power and the data rate are simultaneously adapted relative to channel variations. We analyze the average data rate that meets adequate QoS requirements, subject to an average transmission power constraint and a maximum transmission power limit. We also derive the power gains that the proposed scheme provides over power adaptation and rate adaptation. Our results show that higher power gain is attainable as we allocate more transmission power to better channels. The proposed adaptation scheme provides up to 3.7-5.4 and 1.5-3.2 dB of power gains over power adaptation and rate adaptation, respectively, when peak-to-average power ratio is 2 to 10, in Rayleigh fading channel with = 0.5, where is the decaying exponent of multipath intensity profile. The power gain over rate adaptation is more significant for channels with faster decaying multipath intensity profiles or weaker line-of-sight components. The power gain implies a power reduction at the transmitter, which translates into a reduction of interference to other users, leading to a capacity increase. We finally discuss the effect of maximum data rate, caused by a fixed bandwidth, and channel estimation error on the average data rate. Second, we consider power and rate adaptations in multicarrier (MC) DS/CDMA communications. We derive the optimal power allocation strategy in the frequency domain for single-user case, when the available power for a symbol transmission is fixed. The optimal power strategy in this case corresponds to selective transmission (ST), which allocates the transmission power to a sub-band with the largest channel gain. Then, we consider a dynamic allocation of transmission powers in the time domain, when ST scheme is employed in the MC system. We derive the optimal power adaptation policy in mini...