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One of the advantages of Multiple-Input Multiple-Output (MIMO) techniques is that they can provide link throughput, but the reliability of them remains to be improved. In order to improve the performance of bit error, based on the decomposed model of continuous phase modulation (CPM), combined with LDPC code and random interweaver, a scheme of joint coded modulation is introduced. In this paper, the model of LDPC-CPM iterative MIMO system is established firstly, then scheme of receiver by iterative detection realized and the decoding is proposed, finally the effect of main parameters on the performance of the system is studied, which can provide the reference basis for practical application. The research results show that the proposed scheme can improve channel capacity and BER performance in the background of extremely low SNR effectively, and give attention to both high-speed transmission and reliable transmission.

MIMO technology has been attractive in wireless communications [

It becomes particularly appealing when the MIMO scheme is concatenated with LDPC code. In the MIMO-LDPC system, based on the iterative detection decoding algorithm of Turbo code can be soft information exchange to further improve the receiver performance. There are many researches such as [

The transmitted baseband CPM signal is expressed as

where

where we assumed

The CPM modulation is said to be full- response if

where

It can be seen that CPM signals can be determined by the current input

continuous phase encoder (CPE) and a memoryless modulator (MM) [

We consider a generic transmitter block diagram of MIMO system is shown in

The channel is assumed to be invariant within a frame and change independently from frame to frame, and ideal power control is considered here result in the received signal power is equal for all antennas. Thus the received symbols during a frame can be written as

where channel matrix

The joint iterative CPM decoding and LDPC decoding algorithm can improve

the BER performance through exchange the soft information. We apply this algorithm to the MIMO system and get a scheme which is shown in

MIMO detector uses a Minimum Mean Square Error (MMSE) detection technique, so the problem is to determine weighting matrix

In other words, the function of the

where

According to the

which have increased the noise gain than before. We use the matched filter group to obtain the code word information probabilities

Contain LDPC code, interweaver and CPE as this inner loop, which CPM- SISO makes use of Log-MAP algorithm and LDPC-SISO adopts Log-FFT-BP iterative decoding algorithm.

For the last iteration

According to the previous sections, we have established a MMO system model at Rayleigh flat fading channel and assumed that perfectly known at the receiver. The length of each unencoded substream is equal to LDPC information bits, interweavers are chosen randomly. The other parameters are illustrated in figures below.

2 * 2 MIMO system easy to extend to more antennas. LDPC rate are employed 1/2. Modulation method is same as above OM2RC. As it may be seen, with the increase of code length, BER performance of the system is gradually improved. Therefore, the longer the LDPC code length, the better the performance of the system.

With the method of iterative decoding to optimize system performance, the performance impact of iteration times and must be considered.

the LDPC code iteration for 5 times. The same goes for CPM. It can be seen that with the increase of the number of iterations, and decrease of the system BER curve tends to convergence, and with the increase of signal-to-noise ratio, number of iterations the impact on the BER is more and more obvious.

In this paper, we analyze and derive the decomposition of CPM modulation signal, and applied a joint coded modulation with LDPC to MIMO architecture. Through simple MMSE-based detection and low-complexity iterative decoding with cancellation, both BER and channel capacity of improvements are achieved. We compared the effect of the number of antenna, code length and iteration times on the combined code modulation in the 2 * 2 MIMO system emphatically. The result shows that: The system has better performance for different code length and iteration times than SISO, which meets the requirements for real- time communication channel. At last, this algorithm is complete by software, so it costs lower equipment, and can be simple and practical.

This paper is funded by the International Exchange Program of Harbin Engineering University for Innovation-oriented Talents Cultivation, the Open Research Fund of State Key Laboratory of Tianjin Key Laboratory of Intelligent Information Processing in Remote Sensing (Grant No. 2016-ZW-KFJJ-01), the National Natural Science Foundation of China (Grant No. 61403093), the Assisted Project by Heilongjiang Province of China Postdoctoral Funds for Scientific Research Initiation (Grant No. LBH-Q14048), and the Fundamental Research Funds for the Central Universities (Grant No. HEUCF160813).

Xue, R., Lv, X. and Cao, Y. (2017) LDPC-CPM Scheme and Its Iterative Receiving for MIMO System. Int. J. Communications, Network and System Sciences, 10, 154-162. https://doi.org/10.4236/ijcns.2017.105B015