Abstract
Code Division Multiple Access (CDMA) and multiple input multiple output- (MIMO-) CDMA systems suffer from multiple access interference (MAI) which limits the spectral efficiency of these systems. By making these systems more power efficient, we can increase the overall spectral efficiency. This can be achieved through the use of improved modulation and coding techniques. Conventional MIMO-CDMA systems use fixed spreading code assignments. By strategically selecting the spreading codes as a function of the data to be transmitted, we can achieve coding gain and introduce additional degrees of freedom in the decision variables at the output of the matched filters. In this paper, we examine the bit error rate performance of parity bit-selected spreading and permutation spreading under different wireless channel conditions. A suboptimal detection technique based on maximum likelihood detection is proposed for these systems operating in frequency selective channels. Simulation results demonstrate that these code assignment techniques provide an improvement in performance in terms of bit error rate (BER) while providing increased spectral efficiency compared to the conventional system. Moreover, the proposed strategies are more robust to channel estimation errors as well as spatial correlation.
Highlights
The object of much research in wireless communications is to enable high and variable data rates to users to support the growing number of applications that involve the transfer of data [1]
We propose a suboptimal detection strategy based on the maximum likelihood detector for parity bitselected and permutation spreading strategies for MIMOCDMA systems operating in frequency selective channels without any antenna selection strategy
It is worth noting here that when the maximum channel delay is greater than the length of the spreading codes, the model is extended by incorporating new matrices to take into account the additional intersymbol interference (ISI) caused by symbols that are more than one signaling interval away from the desired signaling interval
Summary
The object of much research in wireless communications is to enable high and variable data rates to users to support the growing number of applications that involve the transfer of data [1]. Recent research has shown that combining DS-CDMA systems with Multiple Input Multiple Output (MIMO) techniques can achieve high gains in capacity, reliability, EURASIP Journal on Wireless Communications and Networking and data transmission speed [14,15,16,17,18,19,20,21,22] This is achievable by exploiting the spatial diversity made possible by multiple antennas at the transmitter and the receiver, allowing more degrees of freedom when the transfer functions between different transmit and receive antenna pairs are sufficiently uncorrelated. In conventional MIMO-CDMA systems, unique codes are assigned per user [16,17,18,19] or per antenna [20] The latter code assignment strategy requires the use of more spreading codes in the overall system, doing so improves the overall bit error rate (BER) performance.
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