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Optimizing and Testing WLANs phần 10

và giải thích kết quả. Thứ nhất, các vị trí kiểm tra các điểm truy cập đáng kể ảnh hưởng đến chất lượng của các kết quả. Như đã lưu ý, vị trí ban đầu được dựa trên phỏng đoán, kinh nghiệm và bản năng cài đặt. Lặp đi lặp lại cuộc khảo sát trang web cho các vị trí AP thử nghiệm khác nhau có thể rất nặng nề, | Testing MIMO Systems The various MCS indices are assigned to different combinations of the above parameters. These combinations produce PHY data rates ranging from 6.5 to 600 Mb s and everything in between. In general for a 20 MHz bandwidth and 1 spatial stream 1 TX and 1 RX antenna the maximum PHY data rate achievable is 72.2 Mb s for 2 spatial streams 144.4 MHz for 3 spatial streams 216.7 Mb s and for the full 4 spatial streams 4 X 4 MIMO 288.9 Mb s. The PHY data rates more than double when the 40 MHz bandwidth is used to a maximum of 600 Mb s. Note that the number of IEEE 802.11n options does not end with the above combinations. In addition to the normal convolutional codes an optional Low-Density Parity Check LDPC coding is also possible in cases where a stronger FEC is required. Also in addition to the standard spatial multiplexing i.e. one stream per subchannel STBC is also supported as an option as well as transmit beamforming TBF . 9.2.4 Channel Estimation To allow the receiver to properly decode the data in each frame it is necessary to obtain an estimate of the channel properties between the transmitter and receiver so that the matrix operations required to extract the data streams transported by each mode of the channel can be performed. Accurate channel estimation must be done frequently preferably prior to every frame because the indoor channel is time varying particularly in the case of mobile 802.11n stations. The 802.11n PHY achieves this by transmitting a special predefined sequence of signals referred to as the high-throughput Long Training Field HT-LTF symbols in the preamble of each frame before the actual medium access control MAC data is transmitted. As the HT-LTF is a known pattern the receiver can use this to calculate and refine its channel estimates. The receiver effectively sets up a candidate channel matrix and then modifies it to cause the expected signal the HT-LTF pattern to match the signal actually received. The number of .