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Supported Bit Rates

The number of bits per second that can pass over a wireless connection is affected by many factors, including antenna selection, interference, distance and other factors. However, the two main factors that drive the potential bit rate of a link are the bandwidth of the signal (measured in MHz) and the modulation scheme.

On the basis of raw speed, wider channel slots (i.e. more MHz of signal bandwidth used for a connection) drive higher bit rates. In many frequency bands, particularly ones that are subject to licensing requirements, the width of each channel is regulated. In other bands, there are fewer restrictions, so wider channel widths can be used to support higher bit rates.

Modern modulation technologies can pack more bits into a given amount of channel bandwidth. Changing from a modulation technique that uses two bits per symbol (such as QPSK) to one that uses four bits per symbol (such as 16QAM) will double the bit rate on a wireless link without changing the channel bandwidth. There is, of course, a cost in doing this, with more processing power needed on both ends of the connection to generate and detect these more complex signals. Plus, there is another penalty associated with the more complex modulation schemes: they are more sensitive to noise and interference. This is why Wi-Fi signals, among others, will automatically adjust their modulation (and consequently bit rate) between more simple and more complex schemes to adapt to changing RF channel conditions.

One of the most technically advanced modulation schemes available is OFDM (Orthogonal Frequency Division Multiplexing) and its close relation COFDM (Coded Orthogonal Frequency Division Multiplexing). These technologies use hundreds or thousands of individual RF carriers within the channel bandwidth, each of which carries a low speed data signal. This technique makes it easier for the receiver to handle multi-path distortion caused by signal reflections, and also makes it possible to ignore certain types of interference. Of course, this complexity requires powerful signal processing chips. With COFDM, It also becomes possible for several devices to share a common RF channel, provided that each device is synchronized and controlled by a central base station. Because of these advantages, COFDM technology is widely used in 4G LTE cellular applications and some dedicated wireless video systems.

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