Orthogonal Frequency Division Multiplexing for Wireless

Transmitting reliable, error-free communication over a wireless network can be a challenge. Collision, signal propagation, signal fading and interference are only a few obstacles that can be overcome with signal modulation and multiplexing, or sharing a common medium. A low-frequency signal stands no chance to reach the receiver without distortion and fading because it's modulated to a much higher frequency range using a radio carrier. Moreover, the many wireless senders don't have to compete for bandwidth if you use the right multiplexing scheme.     

Multiplexing

Multiplexing is an age-old communication technique. The idea is to give all senders and receivers a chance to use the common medium or "highway." Multiplexing takes messages from several sending devices at a high radio-carrier frequency to the receiver. The low-speed messages are modulated to travel on a high carrier frequency to an intermediate device on the receiving side that de-multiplexes the combined unit of signals and routes them to their appropriate destinations. Each signal wave requires a header with sender and receiver identifiers for correct posting.

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Wireless Multiplexing Schemes

Physical or geographical space, time, frequency and code are aspects of wireless signals that can be used for multiplexing. Hence, wireless protocols use one or more techniques such as space division, time division, frequency division and code division multiplexing. Frequency division multiplexing schemes divide the electromagnetic spectrum, assigning a specific frequency range to each wireless sender-receiver pair so that two neighboring signals don't collide and disperse. For example, a radio station called FM1920 most likely transmits its programs at a frequency of 1920 kHz. Others should steer clear of this range.

Orthogonal Frequency Division Multiplexing

In case of Orthogonal FDM, orthogonal frequencies are superimposed on the original signals. The maximum of one subcarrier frequency appears exactly at a frequency at which all other subcarriers equal zero. This makes the one subcarrier stand out with no competitor. OFDM's advantage lies in the fact that frequency selective fading influences only some subcarriers, not the entire signal. OFDM is a special case of Multiple Carrier Modulation schemes, in which several carrier frequencies drive the signals in an intact state to the other end. MCM also keeps guard spaces between the subcarriers, providing extra protection from inter-symbol interference.

Coded Orthogonal Frequency Division Multiplexing

If further partitioning is desired, OFDM is extended to coded OFDM; in addition to multiple radio carriers, codes are used to identify each signal. The receiver computes a checksum to verify the code associated with the signal.

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