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Orthogonal Frequency Division Multiplexing - General Purpose Processor Approach

The NTRG is working on a re-configurable radio system for fixed and mobile wireless nodes using a General Purpose Processor platform. This system will be capable of using multiple modulation schemes such as AM,FM,FSK,M-aryPSK and multiple-carrier schemes such as OFDM and COFDM (Coded-OFDM). One of the main objectives is to integrate previous work carried out on Automatic Modulation Scheme Recognition techniques resulting in an adaptive wireless transceiver system capable of automatically selecting schemes based on the channel Signal to Noise Radio (SNR) and user requirements in order to maximize channel capacity usage

Once the band of interest is digitized, General Purpose Processors are used for the tasks normally taken care of by hardware demodulation techniques eg., FM radio, walkie-talkie, repeaters, etc. This allows the designer to implement creative and versatile radio systems that would be extremely complicated and time-consuming if realised in hardware. Apart from a hardware front-end and a suitable digital to analog converter, multiple radio systems can operate in the software domain without an increase in the amount of hardware required.

 
Fig. 1: Basic Block Diagram of a Re-Configurable Radio System

 
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation method. It is used for high data rate systems such as Digital Audio and Digital Video Broadcasting (DAB, DVB-T) and is ideally suited for mobile wireless nodes for the following reasons:
  • Resilent to multipath fading channel effects
    Data are distributed equally between sub-carriers; this frequency diversity technique dramatically reduces the effects of multi-path fading channels compared to single-carrier systems. Destructive interference (common fading effects) results from a number of reflected transmitted signals arriving out of phase with the direct path signal at the receiver possibly resulting in a null signal. Data are also interleaved prior to transmission resulting in an even greater degree of resilency against fading and multi-channel distortion.

 
Fig. 2: Direct path and reflected signals due to buildings, etc., arriving at the receiver result in multi-path fading effects

  • High Spectral Efficiency
    Orthogonal signals are sub-carriers spaced such that there is no interference between any individual sub-carrier and the rest of the sub-carrier set. If the sub-carrier spacing is 1/Ts, where Ts is the symbol period, then the spectrum of each sub-carrier is null at the centre frequency of each other sub-carrier. The sub-carriers are spaced as close as theoretically possible resulting in very high spectral efficency

  • Very High Data Rates Possible
    By spreading the transmitted data over a number of sub-carriers, choosing a suitable modulation scheme such as 64-QAM, and coding techniques (puncture codes), it is possible to achieve very high data rates eg. IEEE 802.11a - capable of 54MB\s.

  • Common Examples
    IEEE 802.11a uses 52 sub-carriers; 4 of these are used for pilot signals
    Digital Video Broadcasting (DVB) uses up 6817 sub-carriers!

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