Research On Key Techniques In MC-CDMA Systems

CDMA is one of key techniques for 3G mobile communication, which allows different users to share the same frequency band. In CDMA systems, users are separated by orthogonal spreading codes. CDMA systems are interference limited systems, i.e., capacity being controlled by the number of users, resulting in the needs of multiple access cancellation measures taken in receivers to obtain the improvement in performance. Orthogonal Frequency Division Multiplexing (OFDM) transmits high rate dada through parallel low rate sub-channel, by which ability resisting multi-path fading is increased. Besides, since the spectrums of neighbored sub-channels are overlapped, spectrum efficiency of OFDM is higher compared with that of traditional FDM. So OFDM is a good choice for reliable and high rate data transmission in complicated fading channels. It will be a key technique in future wireless communications. Multiple Input Multiple Output (MIMO) technique is an important technique for raising system capacity, performance and spectrum efficiency. Vertical Bell Laboratory Space Time Coding (V-BLAST) will be also the key method to raise dada rate and transmission reliability.In real environment, the channel is complicated and changeable. It is so necessary to transmit data with kinds of advanced techniques. Combining CDMA, OFDM and MIMO can bring great performance improvement, which is important for growing communication demands.Technique combined CDMA with OFDM is called multi-carrier CDMA. It has advantages from both CDMA and OFDM, such as strong ability in resisting interferences and multi-path fading, high confidentiality, soft handoff, high spectrum efficiency, and high data rate, and so on. However, just like common CDMA systems, multiple CDMA systems are interference limited, because the increasing interferences will affect signal detection when the number of users increases. In this paper, we focused on MC-CDMA techniques, in which spreading spectrum is in frequency domain. First, traditional detection methods for MC-CDMA systems are analyzed, and then multi-user detection based on QR decomposition is proposed. It is shown that complexity of the receiver can be greatly reduced by the use of proposed algorithm based on QR decomposition, in which computation of inversion of matrix and ordering is avoided. And performance of the algorithm is a little worse than the original one. Therefore, proposed detection algorithm based on QR decomposition can bring complexity reduction of the receiver without great performance degradation, which is very important.Secondly, a new scheme, which combines Trellis Coded Modulation (TCM) and MC-CDMA is investigated in this paper. In this method, each user is assigned a code set, which contains M orthogonal spreading codes, one of which is selected as spread code according to information bits. The proposed system can get performance gain without any band increasing because of application of TCM. But the number of users the system can contain is lowered to 1/M .Because each user uses M codes, while the number of codes is definite when length of the code remains the same. The problem is serious since there will be more and more users. Then multiple antennas aredeployed at the receiver. At the transmitter, every M₁ users share the same code set.At the receiver, signals from users with the same code set can be distinguished by multiple antennas just as in V-BLAST systems; while signals from users with different code set can be detected by orthogonal codes just as in common MC-CDMA systems. Bit error performance is analyzed in the paper, and a closed form is given finally. Simulation results show that, the proposed system has a great performance gain over traditional TCM-MC-CDMA systems, and meanwhile, the problem of limited number of users is overcome.