Although it may appear to be a particularly complicated form of modulation, it is in fact well suited to digital signal processing techniques. OFDM is a form of transmission that uses a large number of closely spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference.
This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution.
The major difference between the downlink and uplink transmission scheme is that each subcarrier in the uplink carries information about each transmitted modulation symbol as shown in figure below, whereas in downlink each subcarrier only carries information related to one specific modulation symbol.
Google serves cookies to analyze traffic to this site and for serving personalized ads. Learn more. Skip to content. Last updated: 12th Feb ' OFDM Fundamental OFDM was selected for the downlink because it can Improved spectral efficiency Reduce ISI effect by multipath Provide better Protection against frequency selective fading OFDM is a scheme that offers good resistance to multipath and is now widely recognized as the method of choice for mitigating multipath for broadband wireless.
The impact of this is minimal for systems with large number of sub-carriers. The CP duration needed is just dependent on the channel delay spread, so it remains constant with increasing sub-carriers. The symbol duration increases with larger N. Thus the percentage of bandwidth wasted due to CP transmission goes down with increasing sub-carrier count. The lecture starts with an example of a fixed profile WiMax with sub-carriers.
Each sub-carrier has a bandwidth of This is much less than the coherence bandwidth of KHz. Thus a broadband communication channel has been converted into parallel narrowband channels. In the example, the OFDM symbol duration is 64 microseconds and the cyclic prefix is 8 microseconds. The lecture looks at the matrix representation of the MIMO. Zero forcing receiver and the MMSE receiver are also covered. Calculation of the channel matrices across all sub-carrier is discussed.
The lecture concludes with an introductory discussion on the impact of a frequency offset in an OFDM system. OFDM inter-carrier orthogonality is lost if the receiver synchronization leaves a uncorrected frequency offset.
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