trev_278-stott

Trev_278-stott - TUTORIAL COFDM The how and why of COFDM J.H Stott BBC Research and Development Coded Orthogonal Frequency Division

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TUTORIAL COFDM EBU Technical Review - Winter 1998 1 J.H. Stott The how and why of COFDM J.H. Stott BBC Research and Development Coded Orthogonal Frequency Division Multiplexing (COFDM) is a form of modulation which is particularly well-suited to the needs of the terrestrial broadcasting channel. COFDM can cope with high levels of multipath propagation, with a wide spread of delays between the received signals. This leads to the concept of single-frequency networks in which many transmitters send the same signal on the same frequency, generating “artificial multipath”. COFDM also copes well with co-channel narrowband interference, as may be caused by the carriers of existing analogue services. COFDM has therefore been chosen for two recent new standards for broadcasting – DAB and DVB-T, both of which have been optimized for their respective applications and have options to suit particular needs. The special performance of COFDM in respect of multipath and interference is only achieved by a careful choice of parameters and with attention to detail in the way in which the forward error-correction coding is applied. 1. Introduction Digital techniques have been used for many years by broadcasters in the production, distribu- tion and storage of their programme material. They have also been used in “supporting roles” in broadcasting itself, with the introduction of Teletext and digital sound (NICAM)for television, and the Radio Data System (RDS)toaccompany FM sound broadcasts. These have all used relatively conventional forms of digital modulation. Sound and television terrestrial broadcasting is now entering a new age in which the main audio and video signals will themselves be broadcast in digital form. Systems for DAB [1] and DVB-T [2] have been standardized by ETSI for use in Europe and elsewhere in the world. These systems have been designed in recognition of the circumstances in which they will be used: Original language: English Manuscript received: 22/1/99.
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TUTORIAL COFDM EBU Technical Review - Winter 1998 2 J.H. Stott DAB (unlike its AM and FM predecessors) was especially designed to cope with the rig- ours of reception in moving cars – especially the problem of multipath reception which, in this case, is time-varying; For DVB-T, a higher capacity than DAB was essential, mobile reception was not a priority, but multi-path tolerance was still important because of the widespread use of set-top TV antennas. A new form of modulation – COFDM – was chosen for both systems, albeit with differences in detail, and with appropriate changes of parameters, to suit the different requirements of DAB and DVB-T. Both systems include a degree of flexibility. COFDM involves modulating the data onto a large number of carriers using the FDM tech- nique. The key features which make it work, in a manner that is so well suited to terrestrial channels, include: orthogonality (the “O”of COFDM); the addition of a guard interval ; the use of error coding (the “C”of COFDM), interleaving and
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This note was uploaded on 01/22/2012 for the course EE 4414 taught by Professor Wang during the Fall '07 term at NYU Poly.

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Trev_278-stott - TUTORIAL COFDM The how and why of COFDM J.H Stott BBC Research and Development Coded Orthogonal Frequency Division

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