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Date: 28 January 1999
From: hvv@VILLAGE.UUNET.BE
Subject: Re: Questions related to DAB

Since nobody seems to be eager to reply to the questions from Tracey
Daley, I'll try to formulate an answer without mathematics.

1) FDM

In a conventional frequency division multiplexing the coded
information into a large number of closely spaced carriers, each with
individual low bit rate. The frequency band is divided into N non-
overlapping frequency subchannels.Each subchannel is modulated with a
separate symbol and, then, the N subchannels are frequency
multiplexed. Since the spectra of the different subchannels do not
overlap, there must be a sufficient guard space between the adjacent
subchannels to isolate them at the receiver using conventional
filters. The weak point of FDM is therefore its low spectrum
efficiency which makes it rather unsuitable for broadcast use.

2) OFDM

A more efficient use of bandwidth (i.e. a better spectrum efficiency)
can be obtained with a FDM system if the spectra of the individual
subchannels are permitted to overlap thanks to the "orthogonal"
arranging of the carriers. The spectrum of each carrier follows a
sinx/x function. There are periodic nulls in the spectrum. By
orthogonality is meant that these nulls correspond exactly to the
peak of adjacent carriers. It is thanks to this orthogonal
arrangement that the inter-carrier distance is equal to the inverse
of the symbol duration of a single carrier and that it is possible to
retrieve the transmitted information relatively easily (with FFT in
the receiver) without mutual interference.

3) COFDM

Though OFDM resolves the problem of transmission channel selectivity,
it does not suppress the frequency selective fading i.e. some of the
carriers may be enhance by constructive interference, while others
may suffer from destructive interference. To suppress this frequency
selective fading the data stream is coded using convolutional coding
together with time-and frequency interleaving, in conjunction with a
maximum likelihood soft decision Viterbi decoding in the receiver.
The convolutional coding accomplishes re-distribution of the elements
of the digital bit stream so that successive data bits are
transmitted at distant locations in the time-frequency domain.

Conclusion: orthogonality means that the spectra of the individual
carriers in a OFDM system have nulls at intervals of 1/ts so that no
carrier data has energy at the frequency of any other carrier data.

And by the way, since in a Single Frequency Network all transmitters
are operating on exactly the same carrier frequency, it is obvious
that SFN increases spectrum efficiency isn't it.

Also please have a look at:
http://www.bbc.co.uk/rd/pubs/papers/paper_15/paper_15.htm.
for the (very good) article "Explaining some of the magic of COFDM"
from J.H.Stott (BBC)

Herman Van Velthoven
Pioneer

<<MODERATOR'S NOTE: As you can see from the volume of messages today,
RADIO-L members were contributing to the discussion late last week.
I was the bottleneck in that I was unable to post the replies last
Thursday and Friday. I was unexpectedly away, and hadn't set up the
"new" hard drive for remote access. Contributions such as these
are always helpful, and thanks for the information. Ted Ledingham>>

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