Читать книгу Spatial Multidimensional Cooperative Transmission Theories And Key Technologies - Lin Bai - Страница 38

Table 2.1. Characteristic parameters of Taylor single-parameter pattern synthesis.

Note: u₃ denotes half-power beamwidth and ηb denotes beam efficiency.

In a conventional wireless communication system, the transmitting end and the receiving end usually use one antenna each. This single-antenna system is also called a single-input single-output (SISO) system. For such a system, Shannon¹ proposed the channel capacity formula in 1948 as follows: C = B lb(1 + S/N), where B represents the channel bandwidth and S/N represents the signal-to-noise ratio at the receiving end. It determines the upper limit rate for reliable communication in noisy channels. No matter what channel coding method and modulation method is used, it can only be accessed little by little but cannot be surpassed. This seems to be a recognized and insurmountable boundary and becomes a bottleneck in the development of wireless communications. According to Shannon’s channel capacity formula, increasing the SNR can improve the efficiency of the spectrum. For every 3-dB increase in SNR, the channel capacity increases by 1 bit/Hz/s. However, in the actual communication system, it is not recommended to increase the transmission power of the transmitting end in consideration of the actual conditions such as electromagnetic pollution, performance of radio frequency circuit, and the interferences among users. Diversity technique is another way to increase the spectrum usage efficiency. If a single antenna is used at the transmitting end and multiple antennas are used at the receiving end, this diversity is often called diversity reception which is also known as the single-input multiple-output (SIMO) system. The use of optimal combined diversity reception techniques generally improves the SNR at the receiving end, thereby increasing the channel capacity and the spectrum usage efficiency. If multiple antennas are used at the transmitting end and a single antenna is used at the receiving end, this diversity is often called the transmit diversity, which is also known as the multiple-input single-output (MISO) system. However, if the state information of the channel is not known at the transmitting end, beamforming technology and adaptive allocation cannot be used in the multi-transmitting antenna for transmitting power, and thus, the channel capacity cannot be improved much. The development and integration of SIMO and MISO technologies have evolved into MIMO technology, which is an effective method to break through the SISO channel capacity bottleneck. The core idea of the system is to synthesize the signals at both ends of the spatial sampling by way of generating effective multi-parallel spatial data channels (increasing the data traffic), so as to greatly improve the channel capacity, or by way of increasing the diversity to improve communication (reduce bit error rate).