WLAN Channel Capacity Investigation Based On MIMO_OFDM Technology

With the continuous development and progress of our society, network has coming into ten thousands of families, while the wireless network has been recognized as available to provide users with unprecedented flexibility, convenience, and can significantly improve the efficiency of networking. Wireless LAN is very simple, convenient and it can transmit information without the cable, so it has also been concerned by the growing number of people. However, a variety of wireless communication services and broadband data services develop in a rapid velocity, leads to the wireless resources, particularly spectrum resources is becoming increasingly tense, the severe shortage of spectrum resources has increasingly become the cause of curbing the development of wireless communications, how to make full use of these limited wireless communication resources more efficiently, is becoming the focus of development new technologies. In the wireless local area network (WLAN) environment, the electromagnetic environment is more complex, multi-path effects, frequency selective fading to enable the realization of high-speed data transmission more difficult than cable transmission. Therefore, in the wireless LAN environment, to seek more effective wireless communication technology has become the global communication-related people's common topics.Multiple Input Multiple Output (MIMO) technology can make full use of space resources, without increasing the system bandwidth and antennas'total transmitting power, effectively against the channel fading, greatly improve the spectrum efficiency and channel capacity, it is one of the necessary technologies of achieving high-speed data transmission.Orthogonal Frequency Division Multiplexing (OFDM) technology, can convert frequency-selective channel to a number of narrowband flat fading channels, so as to effectively improve the spectrum efficiency. Its ability to anti-multipath interference and anti-fading is very strong, combining the MIMO technology and OFDM technology to form a MIMO_OFDM system, complement each other, so the next generation of wireless communication systems is bound to take these two key techniques.In this paper, the following aspects have been studied:(1)Analysis the characteristics of mobile wireless channel, and establish wireless channel reference model. State the diversity technology of wireless communication systems, and the combining technology impacting on the transmission performance. through simulation to compare the OSC(Optimal Selection Combining),EGC(Equal Gain Combining) and MRC(Maximum Ratio Combing), then select which technology has the strong impact on the error rate. After system simulation, confirm that the superiority of maximal ratio combining. However, due to maximal ratio combining with the more complicated way, and the performance is similar with the equal gain combining, so in fact the EGC is the usual method to combine the received signals. OSC can be realized in a simple way, so it is often used in the case of low signal to noise ratio.(2)Research the basic principles and information theory of MIMO wireless communication technology, and analysis MIMO channel capacity, introduce the formula of MIMO channel capacity under the case of average power distribution and optimal power distribution, and then state the concept of average channel capacity and outage capacity, compare channel capacity of various systems (including SISO, SIMO, MISO, MIMO), a great advantage can be seen in transmit and receive diversity (MIMO) system than the transmit diversity system or receive diversity system. On this basis, analysis MIMO systems in the WLAN environment, in view of Ricean factor on the impact of channel capacity, the conclusion is that the bigger Ricean K factor, the greater loss of channel capacity would be.When the transmitter can get access to channel state information (CSI) by means of channel estimation, the adaptive power allocation can be achieved. Allocating higher transmission power to the channel with a better transmission performance, the channel can make full use of its advantage, while if the sub-channel has the lower signal to noise ratio, poor transmission characteristics,then distribute lower transmission power and even no distribution of transmission power, to avoid loss of data, resulting in waste of resources. The distribution way that the fittest survived and the worst abandoned make full use of the channel resource of MIMO system, and greatly improve the system overall performance, especially in low signal to noise ratio situation, the more obvious advantages can be seen.(3)Study the broadband communication technology: MIMO_OFDM. Introduce the characteristics of OFDM technology also means the basic principles, as well as the necessary of combining MIMO with OFDM technologies. Describe MIMO_OFDM systematic framework and structure, then derive the MIMO_OFDM broadband system channel capacity. Discuss on the number of subcarriers, the angular extension, as well as factors such as antenna spacing on the impact of channel capacity. Towards indoor wireless transmission environment, the main factors that influence channel capacity including the Ricean factor, and the relevance of inter-antenna of the mobile terminal. Typical indoor channel is the Ricean fading channel, so the fading channel will be broken down into two components: Line of sight and scattering components. Basing on the above analysis, discuss the impact factors of channel capacity.Analysised the simulation results, we can see that the number of subcarriers of OFDM modulation technique would have a greater impact on the channel capacity. With the increasing of the number of subcarriers, channel capacity will gradually increase. When the number of subcarriers up to 128, the increasing rate of channel capacity tends to slow; the angular extension will also have significant impacts on channel capacity. The trend is that the bigger of dert value, the greater channel capacity increases; When changing the number of transmitting antennas and receiving antennas, the channel capacity change in a large scope. The more transmitting and receiving antennas, the greater transmission rate would be; Antenna array elements'spacing and the number of scatters increasing is proportional to the channel capacity.Simulations discussed above are based on the uniform linear array, this paper also do a brief analysis and simulation on the square antenna array, by analyzing the simulation results, we can see that the channel capacity of square antenna array is smaller than the uniform linear array's capacity.