16QAM vs QPSK -> Now with the 16 possible points in the constellation diagram we have 16 possible symbols. For this 16 symbols we need 4 bits for coding . Compared to the QPSK modulation now we have doubled the transfer rate when using the same symbol compared to the QPSK, points are closer to each other and so the allowed noise circle radius is decreased. Noise can be interpreted as a vector which turns around the points of the constellation diagram producing a circle with a noise amplitude dependend radius. If we have normalized constellation (. max distance is 1 – . distance between two outer edges of circle of outer constellation points), then:
required distance QAM16 -> 1/(SQRT(QAM res.)) = 1/SQRT(16) =
required distance QPSK -> 1/(SQRT(QPSK res.)) = 1/SQRT(4) =
So we get a ratio of QPSK/QAM distance required to get no overlap of the points of: 20 * log(/) = 6dB.
So to have the same S/N ratio with the same noise level the signal for 16QAM has to be 6 dB stronger than QPSK. The same case is between 64QAM and 16 QAM(6db difference), 256QAM and 64QAM(6db difference) ….
We see that with each bit we add to the symbol rate we need a 3 dB better S/N of the received signal. So we see the tradeoff we have to do between the increasing transfer rate and the required S/N ratio.
Of course another way to increase the transfer rate is a higher symbol clock with the same symbol width.
(PS All this I found in http:///acrobat_download/applicationnotes/AN97047_ )
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Raleigh subsequently founded Clarity Wireless in 1996 and Airgo Networks in 2001 to commercialize the technology. Clarity developed specifications in the Broadband Wireless Internet Forum (BWIF) that led to the IEEE (commercialized as WiMAX ) and LTE standards, both of which support MIMO. Airgo designed and shipped the first MIMO-OFDM chipsets for what became the IEEE standard. MIMO-OFDM is also used in the standard and is expected to play a major role in and fifth generation ( 5G ) mobile phone systems.