With many communication equipment manufacturers entering the millimeter-wave frequency band, massive MIMO is bound to set off a huge wave
1. Channel estimation
Channel estimation occupies an important position in a communication system. Whether the estimation is accurate or not will have a profound impact on the system performance, especially when the channel decoder cannot resist the influence of excessive channel estimation errors. First, the massive MIMO system has a large-scale antenna array, and the corresponding channel response conforms to a certain law of large numbers (LLN). Secondly, the current massive MIMO adopts the TDD technology. This technology is different from FDD technology and has channel reciprocity characteristics. Research on TDD technology is still challenging. Finally, the pilot contamination problem of massive MIMO systems remains unsolved. This problem arises when orthogonal pilot sequences are used in one cell, but the same set of pilot sequences is used between cells. The main reason for this problem is that when users use the same set of training sequences or non-orthogonal training sequences, the training sequences sent by users in adjacent cells are non-orthogonal. Therefore, the channel estimated by the base station is not the channel used between the local user and the base station, but a channel polluted by training sequences sent by users in other cells.
2. Channel modeling
In a massive MIMO system, the base station is equipped with a large number of antennas, which significantly improves the spatial resolution of MIMO transmission. The wireless transmission channel has new characteristics, and the channel model suitable for massive MIMO system needs to be discussed systematically. Under a given channel model and transmit power, the maximum transmission rate (ie, channel capacity) supported by the channel is accurately characterized, so as to reveal the influence of various channel characteristics on the channel capacity, and provide an important basis for the optimal design of the transmission system. As well as performance evaluations such as spectral efficiency and energy efficiency.
3. Signal detection
The signal detection technology in Massive MIMO system has a crucial impact on the overall performance of the system. Compared with the existing MIMO system, the base station in the massive MIMO system is equipped with a large number of antennas, so a large amount of data is generated, which puts forward higher requirements on the radio frequency and baseband processing algorithms. MIMO detection algorithms are expected to be practical, balancing low complexity and high parallelism, with hardware achievability and low power consumption
4. CSI acquisition
Under the requirements of 5G high reliability and low latency, the estimation of CSI must be real-time and accurate. CSI plays a supporting and guaranteeing role in future channel modeling and communication. If the CSI cannot be captured quickly and accurately, the transmission process will be severely disturbed and limited. According to the existing research results, if a fast fading module is introduced into a massive MIMO system, the CSI of the system will change slowly over time. In addition, the number of concurrent users served by the system has nothing to do with the number of base station antennas, and is limited by the system's ability to acquire CSI. Figure 5 shows the CSI framework in NR.
V. Design of large-scale antenna array devices
It is well known that the space between antennas is too small to cause mutual interference, so how to effectively deploy a large number of antennas in a limited space becomes a new challenge. The research on the above problems faces many challenges. With the deepening of research, researchers have high hopes for the application of massive MIMO technology in 5G. It is foreseeable that massive MIMO technology will become one of the core technologies that differentiate 5G from existing systems. Figure 6 shows an ADRES-based MIMO detector chip design architecture.
