دانلود رایگان مقاله بهینه سازی توزیع سوپاپ بررسی عمل در سیستم های MIMO مقیاس بزرگ چند سلولی

Abstract

Salient characteristics of a wireless communication system deploying a great number of antennas at the base station (BS), namely a massive MIMO system, are investigated in this work. The asymptotic performance of the linear zero-forcing precoding scheme is found, both in terms of signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), and shown to be equivalent to the matched-filter beamforming performance. Furthermore, analysis of the massive MIMO system downlink is carried out from the viewpoint of uncoded BER performance, including some realistic adverse effects, such as interference from neighbouring cells, channel estimation errors due to background thermal noise, and pilot contamination. The latter has been shown to be the only impairment that remains in the MIMO multicell system with infinite number of BS antennas. For such scenario, we derive expressions for the asymptotic BER, i.e. in the limit of infinite number of antennas at BS. A quite simple and efficient method for optimizing the massive MIMO system performance under different optimization metrics is proposed, which consists of simply distributing the pilot sequences among the users of the cell in an efficient manner. As a result, a user rate gain of six times regarding the random strategy has been achieved for the downlink with unitary reuse factor, while the user rate increases twice for reuse factor of three. These benefits are achieved by only knowing the powers and the long-term fading coefficients of users in adjacent cells, for each pilot sequence.

6. Conclusion

In this work we have characterized the asymptotic performance of the massive MIMO system downlink under the point of view of the BER performance. We have demonstrated that both MF and ZF precoding schemes result in the same signal as N ! 1, and thus the results of [5] are also valid for the ZF beamforming. Then, we derived the exact asymptotic expression of the BER of a given user, based on the long-term fading coefficients and the power levels of other users. In the same way as the asymptotic SINR expression found in [5], the BER expression derived also depends only on the users in neighboring cells that reuse the same pilot sequence. Furthermore, we have proposed efficient forms of assigning these pilots to the users within the cell, by optimizing several per- formance metrics, including the asymptotic BER or alternatively the maximization of SINR found herein. The significant gains achieved by these pilot allocation techniques were demonstrated numerically. For instance, we have showed that a gain of 6 times regarding the random strategy can be achieved for the downlink rate with unitary reuse factor, while the data rate is increased from 4.79 Mbps to 11.15 Mbps for reuse factor of three. When combining the MaxminSINR technique with an appropriated reuse factor, we showed that the massive MIMO system are able to operate with a 95%-likely downlink rate of 11.15 Mbps, providing a communication free of errors for 98.78% of the users, guaranteeing the reliability of the system.