Interleaving

6.6.1 INTERLEAVING PRINCIPLE

The efficiency of the convolutional coding described above is based on an assumption that
errors will be randomly distributed. However, radio transmission paths tend to be prone to
frequency-dependent ‘bursty’ type errors due primarily to fading.
Therefore, convolutional coding alone may not be able to compensate for a large number of
consecutive errors on a single channel frequency

Block Interleaving

To overcome this, the data bursts are not sent in their natural order, but are interleaved
among a set of timeslots within the multiframe.
Interleaving is applied after error coding and removed at the receiver before the decoding.
Thus the coding algorithm has a more random distribution of errors to deal with.

In the example above, 8 data blocks for each of channels 1-3 are transmitted consecutively. If

a noise burst occurs at the point and for the duration shown, 6 data blocks will be affected, 1
block of channel 1 and 5 blocks of channel 2. This is probably insignificant for channel 1 but
could have significant implications for channel 2.

Interleaving - Effects of ‘Burst’ Noise

If the channel data blocks are interleaved as shown in the lower illustration, 6 blocks will still
be affected but in this case only two from each channel. Therefore the significance of the noise
burst (per channel) has been reduced.

Interleaving

In addition, existing error correction algorithms function most efficiently when errors are
random in nature. They would therefore have a greater probability of correcting all errors on
each of the three channels in the second case above but would be unlikely to correct all the
errors on channel 2 in the first case above.

6.6.2 GSM INTERLEAVING IMPLEMENTATION

GSM employs interleaving to reduce the effects of burst noise over the air interface. This
takes place after channel coding but before converting the coded bit stream into data bursts.
The degree of interleaving used in GSM depends on the type of traffic carried. One 456-bit
block of data is spread over:
· 8 timeslots for full-rate speech (as shown above)
· 4 timeslots for most control channels
· up to 19 timeslots for data
Each timeslot referred to above equates to a burst of data over the air interface. The following
section describes the radio burst multiplexing of data blocks over the air interface and the
different types of bursts used by GSM