If the voice signal is modulated
and transmitted directly after channel coding, due to the condition changes in
mobile communication channel, a deep of the fading will influence a successive
string of bits and cause high bit error rate.
If the bits of a successive string
are interfered or lost, the other end of the communication can not decode the
interfered or lost bits. To solve this problem, some technique or method to separate the
successive bits are required. Thus the successive bits in a message can be
transmitted dispersedly so that the bit error should be discrete. In this way,
even if errors occur, the errors are only concerned with a single or very short
bit stream, which will not lead to that the whole burst or the whole message
block cannot be decoded. In this case, the channel coding will take effect and
recover the bit errors. This method is called interleaving technique. The
interleaving method is the most effective coding method for dispersion of bit
errors.
The key point of interleaving is to
disperse some bits( suppose there are
“b” bits) of the code into some ( suppose to be “n” bursts) burst
sequences so as to change the adjacent relationship between bits. The higher
the value of “n” is, the better the transmitting works. However, the
transmission delay is higher too. Therefore, a balanced consideration is
needed, the interleaving is related to the purpose of the channel. In the GSM
system, the second interleaving is applied.
After channel coding, the extracted
456 bits are distributed into 8 groups with each group containing 57 bits. That
is the first interleaving, also called internal interleaving as shown in the
above diagram. Through the first interleaving, the successive messages in the
groups are dispersed. One burst carries two segments of voice information
composed of 57 bits. Obviously, if the two groups of 57 bits information from
the first interleaving of a successive 20ms voice blocks are inserted to the
same burst sequence, the loss of the burst will lead to loss 25% bits in the
20ms voice block. Therefore, one more interleaving is needed between two voice
blocks, which is called the inter-block interleaving or second interleaving.
Suppose that voice block B is divided into 8 groups:
perform inter-block interleaving to the first four groups (B0, B1, B2 and B3)
of block B and the last four groups (A4, A5, A6 and A7) of the previous voice
block A ; thus, four bursts are constituted: (B0, A4), (B1, A5), (B2, A6) and
(B3, A7); to break the adjacency relationship between successive bits, bits of
block A occupy the even position of the burst while bits of block B occupy the
odd position of the burst. For example, B0 occupies the odd bit of the burst
while A4 occupies the even bit. Similarly, perform interleaving to the last
four groups of block B and the first four groups of the next block C. After the
second interleaving, a 20ms voice block is inserted into 8 different burst
sequences respectively and then transmitted one by one. Even if a whole burst
is lost during transmission process, only 12.5% of each voice block is
influenced and the errors can be corrected through channel coding at the other
end. In addition, the second interleaving for the control information is
different. The interleaving mode is (B0, B4), (B1, B5), (B2, B6) and (B3, B7).
nicely explained.
ReplyDeleteCan you explain diagonal and rectangular interleaving as well?
Thank You! Well explained.
ReplyDeletethats really good but i cant understand your statistic inf. please explain them more exactly whats that 20ms voice? voice burst is only 0.557ms.
ReplyDeleteNot bad and LTE coming soon worldwide and already have many countries.
ReplyDelete