MIMO and OFDM: Analysis of Wireless LAN core technologies (1)

The combination of MIMO and OFDM technologies is considered as the core technology of the next generation high-speed wireless LAN. This paper comprehensively analyzes the application of MIMO and OFDM technology in Wireless LAN, discusses the key technologies of MIMO and OFDM, and looks forward to its development prospects.

1. Introduction

As a new communication technology, wireless communication plays an increasingly important role in daily life. In recent years, the wireless LAN technology has developed rapidly, but the performance and speed of the wireless LAN are still some distance from that of Traditional Ethernet. Therefore, it is increasingly important to improve the performance and capacity of the wireless network.

At present, 802.11 has become the mainstream standard for wireless LAN. The establishment of the 1997 standard in 802.11 was a milestone in the development of wireless LAN. It was approved by a large number of LAN and computer experts. It defines a single MAC layer and a variety of physical layers, and has successively introduced 802.1lb, a and g physical layer standards. 802.1lb uses the KNN modulation technology to increase the data transmission rate, up to 11 Mbit/s. However, if the transmission rate exceeds 11 Mbit/s, it is very difficult to implement a more complex balance and modulation function to combat multi-path interference. Therefore, in order to promote the development of the wireless LAN, The 802.1l Working Group also introduced 0ofdm modulation technology. Recently, the 802.1lg standard officially approved uses the OFDM technology, and the same data transmission rate as 802.1la can reach 54 Mbit/s. In addition, 802.1la runs in the 5 GHz UNII band and uses the OFDM technology. However, it is not compatible with IEEE802.11b products. For the dominant IEEE802.11b in the current market, incompatibility means great difficulties in promotion. Secondly, due to the characteristics of radio wave transmission, the IEEE 802.1la running on 5 GHz has a relatively small coverage.

The IEEE802.11g works in the GHz band and can be connected to the 802.1lb Wi-Fi system. The same AP network exists, ensuring backward compatibility. In this way, the original WLAN system can smoothly transition to high-speed wireless LAN, prolong the service life of the 802.1lb product, and reduce users' investment. For multimedia services to be carried out in Wireless LAN in the future, the data rate of up to 54 Mbit/s is far from enough.

IEEE has set up an 802.1ln team to develop a new high-speed wireless LAN Standard 802.11n. 802.1ln uses the mim00ofdm technology and plans to increase the WLAN transmission rate from 54 Mbit/s of 802.11a and 802.1lg to over 108 Mbit/s, with a maximum rate of 320 Mbit/s, it became another major event after 802.1lb, 802.11a, and 802.11g.

2. mimo ofdm technology applied in Wireless LAN

2.1 OFDM technology

In fact, the OFDM technology is a kind of MCM (Multi-CarrierModulation, Multi-carrier modulation. The main idea is to divide the channel into many orthogonal subchannels and conduct narrowband modulation and transmission on each subchannel, thus reducing mutual interference between subchannels, at the same time, the spectrum utilization is improved. The signal bandwidth of each sub-channel is smaller than the relevant bandwidth of the Channel. Therefore, the frequency selective fading of each sub-channel is flat, greatly eliminating inter-symbol interference.

In each sub-channel, IFFT and FFT methods can be used for orthogonal modulation and decoding. With the development of large-scale integrated circuit technology and DSP technology, IFFT and FFT are very easy to implement. The introduction of Fast Fourier Transform (FFI) greatly reduces the implementation complexity of OFDM and improves the system performance, as shown in figure 2. Generally, wireless data services have asymmetry, that is, the amount of data transmitted in the downlink is much larger than the amount of data transmitted in the uplink. Therefore, both the user's high-speed data transmission needs and the wireless communication itself require the physical layer to support asymmetric high-speed data transmission, however, OFDM can easily achieve different transmission rates in the uplink and downlink by using different numbers of subchannels.

Currently, OFDM integrates space-time encoding, diversity, interference (including ISI and adjacent channel interference ICI) suppression, and smart antenna technology to maximize the reliability of the physical layer. For example, the performance can be further optimized by combining technologies such as adaptive modulation, adaptive encoding, dynamic sub-carrier allocation, and dynamic bit allocation algorithms.

In addition, compared with single-carrier systems, OFDM still has some disadvantages, which are susceptible to frequency deviation and have a high average peak power ratio (PAR ).

2.2 MIMO (multiple inputs and multiple outputs) Technology

Multi-input multi-output (MIMO) technology is a major breakthrough in the Smart Antenna Technology in the wireless communication field. MIMO technology can increase the capacity and spectrum utilization of the communication system exponentially without increasing the bandwidth. It is widely believed that MIMO will be a key technology required by the next generation of wireless communication systems.

In the room, the electromagnetic environment is more complex. The multi-pass effect, frequency selective fading, and the existence of other interference sources make it more difficult to achieve high-speed data transmission over wireless channels than wired channels. The multi-path effect may cause decline and thus is considered a harmful factor. However, the results show that for the MIM0 system, the multi-path effect can be used as a favorable factor. Generally, multi-path fading is considered a harmful factor. The MIMO system uses multiple antennas (OR array antennas) and multiple channels at the transmitting end and receiving end. Multi-input and multi-output MIMO is applicable to multi-path wireless channels. Figure 3 shows the schematic diagram of the MIMO system. Transmitting information stream S (k) is encoded to Form N information substreams Ci (k), I = l ,......, N. The N sub-streams are transmitted by N antennas, and received by M receiving antennas after passing through spatial channels. Multi-antenna receivers can separate and decode these data substreams using advanced space-time Encoding to achieve optimal processing.

In particular, the N substreams are sent to the channel at the same time, and each transmit signal occupies the same band, so the bandwidth is not increased. If the channel response between the transmitting and receiving antennas is independent, the MIMO system can create multiple parallel spatial channels. Through these parallel spatial channels, information is transmitted independently, and the data rate will inevitably increase.

MIMO considers multi-path wireless channels and transmitting and receiving as a whole for optimization, so as to achieve high communication capacity and spectrum utilization. This is a kind of diversity and Interference Cancellation processing that is close to the optimal joint in the airspace time domain.

System capacity is one of the most important indicators of the communication system, indicating the maximum transmission rate of the communication system. For a multi-input multi-output (MIMO) system with N as the number of transmitting antennas and M as the number of receiving antennas, assume that the channel is an independent channel with a large number of N and M, then the channel capacity C is approximately formula (1) C = [min (M, N)] Blog2 (p/2) (1)

Where B is the signal bandwidth, p is the average signal-to-noise ratio of the receiver, min (M, N) is M, and N is smaller. The above formula indicates that the maximum capacity or capacity ceiling of MIMO increases linearly with the increase of the minimum number of antennas when the power and bandwidth are fixed. Under the same conditions, the capacity of an ordinary smart antenna system that uses multiple antennas or antenna arrays at the receiving end or transmitting end increases only with the increase of the number of daily lines. Therefore, MIMO technology has great potential to improve the capacity of the wireless LAN.