Why the switch from W-CDMA to OFDMA

The basic problem that service providers needed to have solved is how to get more data to users, quicker and cheaper.  However, W-CDMA just won't allow more data to be transmitted"”or pushed onto a high-speed data stream"”and sent to the user.  OFDM resolves the problem by splitting the high-speed data stream into several lower speed data streams and sending the lower speed streams on individual frequency channels (subcarriers) to the user.  The user's device, then recombines the lower speed streams and effectively sees the information as if transmitted on a single, very high-speed data stream.

 

An OFDMA handset must be capable of receiving all 2,048 subcarriers defined in the LTE standard that a base station could transmit.  But a base station need only support transmitting 72 subcarriers.  So depending on a number of environmental factors, user data requirements, QoS, etc., a base station determines how many subcarriers are assigned to a user for a particular call.

 

The 'O' in OFDMA stands for orthogonal.  By using orthogonal carrier waves, the 2,048 subcarrier frequencies can be spaced"”in fact overlapped to maximize efficiency"”without interfering with each other.  The orthogonal carrier waves (just like the spreading codes in CDMA) must be carefully calculated to cancel each other out as far as possible.  This requires very precisely tuned antennas and sophisticated processing, which prevented OFDMA from becoming available in portable devices before now.

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