Wireless Antenna
MIMO (Multiple Input Multiple Output) wireless system uses multiple antennas at the transmitter and receiver to transmit and receive multiple data streams to achieve a higher rate of data and improves the performance of radio systems to increase spectral efficiency. MIMO techniques are widely used in IEEE 802.11n WLAN and cellular networks such as 4G WiMAX and LTE (Long Term Evolution) systems. A MIMO system uses spatially separated antennas to obtain spatial diversity in an environment of multipath. MIMO system can provide either to fight against the increasing diversity fading or provide a capacity gain of data rate increased. Three different types MIMO techniques. The first type is to maximize spatial diversity by improving the efficiency of power transmission. The second type is spatial multiplexing where multiple independent signals Data are transmitted by antennas to increase data throughput. The third type uses the knowledge of the channel to the issuer for profits building, also known as beamforming.
Spatial diversity can be implemented using SIMO (Single Input Multiple Output) or MISO (Multiple Input Single Output) systems. SIMO transmission antennas using single and multiple receive antennas to provide diversity reception, while MISO uses multiple antennas transmit and receive only antenna to provide Transmit Diversity. Signal strength in a wireless channel is in flux constant. The signal is disappearing diversity when the signal is considerably reduced and is used to fight against fading. In aerials SIMO dishes to see the faded versions of the same transmitted signal the receiver combines these signals to produce a variation of the amplitude of the signal result, which has fewer and more easily decoded. A simple diagram of the transmitter diversity is implemented with space-time block code (STBC), also known as the Alamouti code. Two sequences of symbols of the signal (both on the same data is encoded differently, but using STBC) are transmitted simultaneously from two separate antennas to provide diversity in space-time domain.
Similar to STBC, spatial frequency code block (CFB) May also be used, unless the coding is done in the antenna / frequency domain regime and frequency domain transmission such as OFDM (Orthogonal Frequency Division Multiplexing) is used. The goal of both STBC and SFBC schemes is to increase system capacity, but to improve the S / N (Signal to Noise) and reduce the BER (Bit Error Rate) system transmitting coded information. With increasing channel SNR, it is possible to use a modulation scheme of the above order and receive data at distances greater (range). Because each symbol has more bits of data at the highest modulation scheme, indirectly results in the ability of the system increased.
MIMO spatial multiplexing in the system provides a linear increase of transmission rate proportional the number of transmit-receive pair of antennas, without increasing the bandwidth or transmission power. A system of 2 x 2 MIMO uses two transmit and two receive antennas. The flow bits to transmit an average of two streams in the multiplexed, modulated and transmitted simultaneously from each transmit antenna. For two different symbols are transmitted from two antennas in one symbol period, the transmission rate is double compared to the SISO system. In terms of Channel favorable, the receiver can distinguish between two co-channel signals, extracts of the two signals and combines them to obtain the original bit stream. Multiplexing Space is especially useful in deployments in urban areas, where being on time is less important than the high rate of data in the device termination.
Training beam MIMO is a technique that uses multiple transmit antennas and forms the beam of the antenna in the general direction of the target receptor. Beamforming is used when knowledge of the channel can be used to improve system performance is available at the transmitter. Knowledge refers to conditions channel Channel property or instant statistics as to transmit and receive correlation properties of the antenna. In general, the transmitter beamforming can increase the intensity of the signal at the receiver up to a factor T (number of transmit antennas). A pre-encoding vector is derived from knowledge of the Channel and transmission signal is multiplied by the pre-coding, which can be interpreted as a beamformer.
Improved beamforming SNR for higher throughput and link robustness while reducing interference. When receiving a signal, beamforming can increase gain in the direction of wanted signals and decrease the gain in the direction of interference and noise. Beamforming can provide the range and increase capacity to a wireless network.
Paul Ngai, P.E. is an engineering consultant specialized in Telecommunications Network and Systems. He is also the principal of Network Systems Technologies LLC (http://www.nstecs.net), a telecommunications consulting firm provides planning, analysis, design, testing and operation support services.
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