A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
A comparison of wi-fi and wimax with case studies - Florida State ...
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4.3.1.2.2. AAS (Advanced Antenna Systems)<br />
Multiantena is a good way to improve system performance. However it includes some<br />
problems such as transmit diversity, beamforming, <strong>and</strong> spatial multiplexing.. The AAS is a<br />
solution for them. [50][113][114]<br />
Transmit diversity<br />
Transmit diversity requires at least two transmitting antennas <strong>and</strong> one receiving antenna. For<br />
this type <strong>of</strong> transmission, WiMax de<strong>fi</strong>ned STBC scheme, for example the 2x1 antenna system<br />
<strong>wi</strong>th Alamouti codes. The advantage <strong>of</strong> it is the same as <strong>wi</strong>th MIMO. The MS or end users <strong>wi</strong>ll<br />
not have a complex set up <strong>and</strong> the cost is lower.<br />
Beamforming<br />
Beamforming has great ability to quard against interference <strong>and</strong> improve the coverage range,<br />
capacity, reliability, <strong>and</strong> received SINR. The idea is to use multiple antennas to transmit the same<br />
signal in the direction <strong>of</strong> the receiver. To operate using this method, the signal must be weighted<br />
properly for each antenna <strong>and</strong> the transmitter has to have accurate knowledge <strong>of</strong> the channel.<br />
WiMax supports beamforming for both uplink <strong>and</strong> downlink.<br />
Spatial multiplexing<br />
Spatial multiplexing means using multiple antennas to transmit multiple independent<br />
streams. When both receiver <strong>and</strong> transmitter have more than one antenna, by performing the<br />
STBC scheme, the streams can be split. The purpose <strong>of</strong> this method is to improve the data rate or<br />
capacity <strong>of</strong> the system. The increasing ratio is linear <strong>wi</strong>th the number <strong>of</strong> antennas. For example, a<br />
2x2 MIMO system <strong>wi</strong>ll double the capacity. If the MS only has one antenna, the spatial<br />
multiplexing is still supported. However, spatial multiplexing only works under good SINR<br />
conditions.<br />
4.3.1.2.3. QoS<br />
QoS (Quality <strong>of</strong> Service) is a basic function <strong>of</strong> the WiMax MAC-layer. By using<br />
connection-oriented MAC structure, the QoS control is robust. All the uplink <strong>and</strong> downlink<br />
connections are controlled by the BS. Before transmission, a BS <strong>and</strong> a MS set up a unidirectional<br />
logical link called “connection”. This link is between two MAC layers <strong>and</strong> every connection is<br />
veri<strong>fi</strong>ed by a 16-bit connection identi<strong>fi</strong>er (CID). [50]<br />
To ensure the QoS, WiMax de<strong>fi</strong>nes a scheme named “service flow”. It is a one way flow <strong>of</strong><br />
packets that includes a special set <strong>of</strong> QoS parameters <strong>and</strong> is veri<strong>fi</strong>ed by a 32-bit service flow<br />
identi<strong>fi</strong>er (SFID). These parameters are used to support different kind <strong>of</strong> QoS <strong>of</strong> different tasks.<br />
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