Wireless Transmission Methods

Wireless LANs use a method of transmission called Spread Spectrum. The alternative method is narrow band as used by radio stations. Narrowband uses a single frequency but a very high power level. Spread Spectrum works in an opposite way, this is because it is likely in the unlicensed wireless space that other devices will cause interference. The signal is spread across a number of frequencies and the receiver is set to listen on the same frequencies, this reduces the interference degradation and reduces the power required to send the signal.

There are two types of Spread Spectrum:
Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS)

Encoding is the process of transforming a single digit into a sequence of symbols to be transmitted so that if part of the sequence is lost it can still be understood at the receiving end. Modulation is a method through which symbols are represented on the wave.

WLANs today use DSSS, or a version of it. FHSS is used by some cordless phones and Bluetooth. FHSS is not preferred and causes interference with WLANs because it utilizes 75 channels in the spectrum and hops between them, it must use all 75 channels, which is just for 300-400ms. FHSS uses GFSK to encode data (2 level GFSK uses 2 frequencies, 4 level uses 4). FHSS uses little power and mitigates interference by "dancing" around it. DSSS gets around interference by sending a redundant bit pattern (Sequence) for each bit sent, so that if any of the bit pattern is lost we've got a good chance of recovering it, this does of course amount to large overhead. Chips or PN (Pseudorandom Noise) codes is the redundant information coded into each signal. There is 11 bits per bit to be sent. This is really basic 802.11 to achieve 1 and 2mbps.

802.11a and g uses Orthogonal Frequency Division Multiplexing (OFDM). OFDM resists multipath problems by carrying data in 52 sub carriers within the 20MHz radio channel. 48 are for data and 4 are pilot for monitoring interference and path shifts. The carrier is 20MHz and can be bigger, which just means more throughput. Each carrier (or tone) is considered independent to other tones and interference will only degrade that tone, meaning less redundant chips.