MMSE detection algorithms are employed as a benchmark, approximating optimal detection. Firstly, the bit error ratio (BER) of five algorithms, including GS, CPGS, PGS and JAGS iterative algorithms, is compared in uncorrelated and correlated channels, respectively. The MMSE scheme is utilized as a reference object. Its performance is not very different from that of the best detection system. Then, the initial values of the CJGS iterative algorithm are simulated and examined. Finally, a comparison of the computational difficulty for the above-mentioned methods has been made. Where i is the scheme’s number of iterations.
4.1. BER Performance
A comparison and analysis of the BER performance of different iterative algorithms with different antenna configurations is shown in
Figure 3. The channel matrix is independent and identically distributed, that is
,
. Signal modulation is 16-QAM.
In
Figure 3a, it is clear that all schemes have excellent BER performance after a modest quantity of rounds, and that the CJGS scheme has improved BER performance over the other repetition methods for the identical number of rounds. The BER of the CJGS and JAGS iterative algorithms is
after two iterations, but the gap between them is 8 dB. Therefore, the CJGS iterative method is better. From
Figure 3b, it can be noticed that the CJGS iterative scheme requires three iterations, while the other four iterative schemes require more than seven iteration cycles to achieve the ability of MMSE scheme when the MIMO antenna configuration is
. If the signal-to-noise ratio (S/N) is 20 dB, BER achievable by the CJGS iterative method is
after 3 iterations, and the BER achievable by GS, CPGS, JAGS and PGS after 7 iterations are
,
,
and
,respectively. From
Figure 3c, it can be shown that with the MIMO antenna configuration of
, CJGS, GS and JAGS can achieve better BER performance after 2 iterations, but the BER performance of CPGS and PGS is not very good.
Furthermore, by keeping
at a fixed value and increasing
, all algorithms can gain the identical BER in a fraction of the repetitions from
Figure 3a,b. Conversely, if
remains fixed and
increases, all algorithms need a higher signal-to-noise ratio to get the identical results, and the quantity of receptions required increases from
Figure 3b,c.
Figure 4 is a comparison and analysis of the BER behavior of different iterative algorithms. The antenna configurations are different. The channel matrix satisfies as
,
. Signal modulation is 16-QAM.
In
Figure 4a, with the MIMO antenna configuration of
, the performance of the other four iterative algorithms after running five iterations is still worse than that of the CJGS iterative algorithm in one iteration. The minimum BER achieved by MMSE detection is
, while the BER of the CJGS iteration is
after one iteration. From
Figure 4b, the CJGS iterative scheme provides much more superior performance when the MIMO antenna configuration is
, which converges faster than the remaining iterative methods From
Figure 4c, it can be found that the CJGS iterative algorithm after three iterations can achieve the identical effects as the MMSE detection scheme if the S/N is 16 dB.
A comparison and analysis of BER performance for different algorithms with different antenna configurations is shown in
Figure 5. The channel matrix satisfies as
,
. Signal modulation is 16-QAM.
The detection performance of the GS, CPGS, JAGS and PGS iterative algorithms is less satisfactory for an iteration number of one in
Figure 5, while the CJGS iterative algorithm achieves a desirable performance close to MMSE detection. All iterative algorithms become better in detection performance as the number of iterations increases. As shown in
Figure 5 and
Figure 3, all algorithms require more iterations to achieve optimal performance when the transmit-side antenna is correlated. If the MIMO antenna configured is set to
and the BER reaches
, the required S/N ratio for CJGS is 20 dB, while the S/N ratio requirement for PGS and JAGS iterations is 24 dB. Therefore, the proposed algorithms are more suitable for channel scenarios where correlation exists on the transmit side of large-scale MIMO systems.
In
Figure 6, the fully correlated channel was concerned, i.e.,
,
. The BER performance of the GS iterative algorithm, CPGS iterative method, JA-GS detection algorithm, PGS detection method and CJGS iterative algorithm are compared and analyzed for different antenna configurations with 16-QAM modulation.
As shown in
Figure 6, the detection capability of all the algorithms becomes better with more repetitions. From
Figure 6a, the designed algorithm is shown to perform as well as the performance of MMSE detection at one iteration when the MIMO antenna configuration is
, while all other iterations require more than five iterations to achieve the same results. If the S/N ratio is 16 dB, the BER achievable by the MMSE detection algorithm is
, while the BER of the CJGS iterative algorithm is
after five iterations. Thus, the CJGS iterative method can achieve a lower BER. From
Figure 6b, it can be observed that to achieve the performance of MMSE detection when the MIMO antenna configuration is
, the CJGS iterative algorithm needs to perform 3 iterations, while the other four iterative algorithms need more than 7 iteration cycles. From
Figure 6c, there has been evidence that all algorithms achieve lower BER after five iterations when the MIMO antenna configuration is
, but the suggested method has superior performance for equal iterations.
According to the previous description, near MMSE detection performance is achieved with proposed method with only a few iterations. The novel scheme outperforms the other four iterative algorithms. A reduction in the quantity of received side and increase in the sending side, then the ratio between them decreases. The proposed algorithm still shows a reliable performance, which can be well adapted to both channel-uncorrelated and channel-correlated scenarios.
As shown in
Figure 7, baseline1, baseline2 and baseline3 are the initial values of schemes 1, 2 and zero vector mentioned in Chapter 3, part B, respectively. When the Massive MIMO antenna configuration is
, the number of iterations is two and the baseband modulation method is 16-QAM, the CJGS iterative method has superior BER performance and faster convergence when initial values proposed in scheme 1 are used.
4.2. Complexity Analysis
In massive MIMO communications, computational cost is a critical metric for signal detection algorithms [
28]. The sophistication of the CJGS iterative scheme consists of three parts: initialization, Conjugate Gradient and Jacobi co-processing and GS iteration estimation, and only the number of true multiplicative operations of the method is considered in the analysis. The computation of
and
is only once required, which is the same as the MMSE detection algorithm. Therefore, this part of the calculation is ignored and only the complexity of the subsequent steps is analyzed. One real multiplication is noted as one real multiplication times, the multiplication of a complex number with a constant is noted as two real multiplication times, and the multiplication of two complex numbers is noted as four real multiplication times.
The complexity of : it involves multiplying a constant and a vector of with a complexity of .
The complexity of Conjugate Gradient and Jacobi co-processing:
The complexity of : it involves multiplying a matrix of and a vector of , with a complexity of .
The complexity of : it concerns multiplying a vector of and a vector of , with a complexity of . And it involves multiplying a matrix of and a vector of , with a complexity of . It concerns multiplying a vector of and a vector of , with a complexity of .
The complexity of : it involves multiplying a constant and a vector of , with a complexity of 2U. The complexity of is .
The complexity of the GS iteration: the calculation of involves multiplying twice a matrix of and a vector of with a complexity of .
Hence, total complexity of CJGS algorithm is
.Taken together, the computational complexity of the various algorithms is shown in
Table 1.
In
Figure 8, the complexity of the different algorithms for MIMO systems are compared. Base station antenna count is 128 and user-side antenna count ranges from 0 to 30 with three iterations. The suggested algorithm is much less complex than MMSE scheme whose complexity is
.