The 802.16e protocol supports two different multi-antenna implementations of MIMO (Multiple Input Multiple Output) and AAS (Adaptive Antenna System). This article is based on the introduction of MIMO and AAS principles. Their characteristics and performance were analyzed and compared.
1 IntroductionThe 802.16e protocol supports two different multi-antenna implementations: MIMO (mulTIple input mulTIple output) and adaptive antenna system (adapTIve antenna, system, AAS). MIMO is an optional technology that can be supported on both the uplink and downlink. The supported MIMO modes are divided into three types: spatial diversity (SD) mode, spatial multiplexing (SM) mode, and diversity. A hybrid mode combined with multiplexing, namely adaptive MIMO. Spatial diversity can obtain additional diversity gain and coding gain, but can not increase the data rate; although spatial multiplexing can maximize the average transmission rate of MIMO systems, only limited diversity gain can be obtained; adaptive MIMO can provide diversity gain System capacity can be increased, resulting in a good compromise between high spectral efficiency and transmission quality, but is more complex to process than using diversity or multiplexing alone.
AAS is an optional technology that can be optionally supported in both the uplink and downlink. The use of AAS technology can increase system capacity, expand coverage, and improve communication reliability. AAS technology can be implemented in multiple beam selection or adaptive mode.
The principle and characteristics of the two multi-antenna technologies are detailed below, and the similarities and differences between the two are compared on this basis.
2. Principle and performance of MIMOMIMO, also known as multiple transmit multiple receive antenna (MTMRA) technology, is a major breakthrough in antenna technology in the field of wireless mobile communications, because it is theoretically possible to multiply the system without increasing time and frequency resources. Capacity and spectrum efficiency. The concept of MIMO technology is very simple. Any wireless communication system, as long as multiple antennas are used at both the transmitting end and the receiving end, constitute a wireless MIMO system. Depending on the number of antennas at both ends of the transceiver, MIMO can also include a single input multiple output (SIMO) system and multiple input and single output (SOM) systems compared to a single single input single output (SISO) system. Multiple input single output, MISO) system.
MIMO has two functional forms, namely spatial diversity and spatial multiplexing. The spatial multiplexing technology can greatly improve the channel capacity by utilizing the spatial multiplexing gain provided by the MIMO channel. The spatial diversity using the spatial diversity gain provided by the MIMO channel can improve the channel reliability and reduce the channel error rate.
The integrated antenna architecture (number of transmitting antennas × number of receiving antennas) and functional form, the MIMO technology supported by 802.16e is as follows.
â— Supports downlink transmit diversity of BS (base station) with 2 antennas, 3 antennas, and 4 antennas.
â— Uplink transmit diversity of MS (mobile station) supporting 2 antennas.
â— Supports downlink transmission of BS spatial multiplexing of 2 antennas, 3 antennas, and 4 antennas.
â— Supports uplink transmission of MS antenna multiplexing of 2 antennas.
2.1 Spatial Diversity
Propagation of wireless signals over complex wireless channels produces Rayleigh fading with varying fading characteristics at different spatial locations. If the positional spacing of the two antennas is greater than the correlation distance (usually separated by more than 10 signal wavelengths), it is considered that the signals at the two locations are completely uncorrelated, then the multiple transmission paths provided by the multiple antennas at the transmitting end or the receiving end can be used. Selecting or combining received signals with relatively small fading phenomena from multiple independent transmission paths to maintain stable link quality, so that signal space diversity can be achieved against the effects of fading. Spatial diversity is divided into two types: receive diversity and transmit diversity. It can be considered that the SIMO system is the receive diversity, and the MISO system is the transmit diversity. It should be noted that the spatial diversity is suitable for multi-scatterer multi-path applications, and the antenna spacing should be appropriately opened to ensure that the transmitted and received signals are independent of each other. Sexuality, forming an independent transmission path to make full use of the multipath (also called full multipath) caused by multi-scatterers, antenna placement and multipath channels must meet certain conditions (in multipath propagation environments, increase array elements) Spacing and angular expansion, combined with space-time processing, facilitate capture, separation, and merging of multipaths.
Space-time coding is often used to achieve spatial diversity. Space-time coding can be designed in two different ways.
(1) Space-time coding
The space-time trellis code (STTC) considers the modulation and diversity of the code as a whole. First, the information bit stream to be transmitted is sent to the channel encoder of the forward error correction (as shown in Figure 1), and it is Mapping in certain signal constellations, such as QPSK, MPSK, MQAM and other constellations. The Trellis encoder determines a branch in the Trellis diagram as the output of the encoder based on the input symbol and the state of the encoder. The n symbols on one branch are sent to the branches of the n transmit antennas. These n symbols are respectively pulsed and modulated and sent directly to n antennas. STTC can obtain full set gain and relatively high coding gain, but cannot achieve full rate transmission, and searching for Trellis codes conforming to design criteria is very complicated, and the decoding complexity is high, which is not covered in 802.16e.
(2) Space-time block/packet coding
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