Wavecom W61PC V7.5.0 - Manual (Page 110)

100  Fundamentals of Radio Data Transmission WAVECOM Decoder W61PC/LAN Manual V7.5
In simplex systems there is of course no return channel, so the IRS cannot request repetitions. Therefore
the codes used must be very robust and be able to correct errors at the receiving end - Forward Error
Correction (FEC) is used.
Intensive research has led to the development of a large number of block codes with superior error detec-
tion and correction features, e.g. Hamming, BCH, Golay and Reed-Solomon codes.
One of the worst enemies of simplex links is burst noise, which may corrupt many successive bits. To
combat this type of noise, bit spreading or bit interleaving is used. The bits of successive code words
are spread in time. In this way burst errors will only influence a few bits of each codeword, and the error
correcting code may have a decent chance to correct the errors. The HNG-FEC and RUM-FEC channel
codes use this method.
Another method is codeword repetition, in which a code word is repeated several characters later in the
transmission. To improve error detection and correction, the repeated character may be bit-inverted. The
original character and the repeated character are then compared at the IRS. SI-FEC and SITOR-B are ex-
amples of this type of code.
One code type has been successful in particular; that is the convolutional code, in which the value of the
parity bits depends of the values of a number of preceding data bits. The data bits are shifted through a
shift-register with taps. The output of the taps is EXORed to form the value of the parity bits. After convo-
lution the bits are interleaved to further improve noise immunity. FEC-A uses this method. Convolutional
coding and the related Viterbi decoding have found widespread use in satellite communication.
Modern communication systems often utilize a combination of coding schemes to obtain higher invulnera-
bility against errors at a lower complexity level. Thus in satellite communication, codes are concatenat-
ed; that is to say, a convolutional inner code towards the modulator is concatenated with a Reed-
Solomon outer code, with an interleaver inserted in between. Recent research has refined the concate-
nated coding and added an iterative decoding algorithm to achieve what is called turbo coding. Further
research has led to the discovery of so called Low Density Parity Codes (LDPC), the use of which has
enabled communicators to come very close to the theoretical bandwidth limit of a communication channel.
Modern technology has enabled more sophisticated approaches to combat the challenges of radio commu-
nication. Inter-symbol interference is one such challenge. Due to the time-varying nature of fading chan-
nels one symbol may be delayed in such a way that it interferes with the next symbol at the receiving end.
By shaping the digital pulses in certain patterns, the level of ISI may be reduced. The raised cosine pulse
shape is the shape most frequently used in conjunction with equalization.
Equalization
To combat the perturbations introduced during the transmission via time-varying communication chan-
nels, modern digital communication systems employ a technique called equalization. Preset equaliza-
tion transmits a training sequence which is compared to a similar sequence at the receiving end. The
difference between the received and local sequences is used to adjust a special filter. Adaptive equaliza-
tion continually measures the characteristics of the channel from the data received and dynamically ad-
justs the filter. To take advantage of the powerful features of equalization, good channel error perfor-
mance is necessary.
The equalizer sits between the demodulator and the detector.
Synchronization
To enable the receiving end of a data or telegraph link to interpret the received code words in a meaning-
ful way, the receiver must first be synchronized to the incoming bit stream by finding the bit level transi-
tions, and next achieve code word phase. Basically the receiver will search for a certain bit pattern in the
bit stream and when found transmitter and receiver are synchronized.
Before the widespread use of electronic circuits, all telegraph devices were of electro-mechanical nature
and therefore prone to mechanical wear and tear. This in turn necessitated comparatively large tolerances
and made stable synchronization over even short periods difficult. To overcome this serious problem, the
ITA-2 alphabet adopted what is known as start-stop or asynchronous operation, which achieves syn-
chronism for each codeword.
In start-stop systems a codeword is wrapped into an "envelope" consisting of a leading start bit (logical
'0') and one or more trailing stop bits (logical '1') - for ITA-2 the code words are 1 + 5 + 1.5 = 7.5 bits
long. Bit synchronization is then achieved by detection of the start element. The stop element(s) serve(s)
the purpose of telling the receiver to reset its detection mechanisms and wait for the next start bit. To en-