GSM Hopping Sequence Generator
==============================


This is an implementation of the GSM Hopping Sequence Generator,
contributed to the GSM Project in 2008. 
It is here for my own reference.


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From: Alfonso De Gregorio <adg@crypto.lo.gy>
Date: May 7, 2008 6:46:52 AM PDT
To: gsm@lists.segfault.net
Subject: [gsm] Hopping Sequence Generator
Reply-To: gsm@lists.segfault.net


Hi,

whatever solution we'll come up with to channel hopping (ie, frequency
hopping or wideband sampling), we'll need to know where to tune next.
The hopping sequence generation  is specified in sect. 6.2.3 of  3GPP
TS 05.02, where the standard describes how logical channels gets
mapped onto physical channels. A copy of the document is available at:
<http://ucesp.ws.binghamton.edu/xli/eece542/std/3gpp/GSM0502-8a0.doc>.

The pseudo-code is as follows (please, find the implementation in 
attach):

if HSN == 0
     MAI := (FN + MAIO) mod N
  else
     M   := (T2 + RNTABLE[HSN XOR ((T1 mod 64) + T3)])
     M'  := (M mod 2^(INT(log2(N)+1)))
     T'  := (T3 mod 2^(INT(log2(N)+1)))
     if M' < N
         S := M'
     else
         S := (M' + T') mod N
     MAI := (MAIO + S) mod N

  RFCHN := MA[MAI]

Where:
- The Frame Number is specified in terms of T1, T2 and T3 as received
in the synchronization burst from the synchrnonization channel (SCH),
FN :=  51 * ((T3-T2) mod 26) + T3 + 51 * 26 * T1
- The Hopping Sequence Number (HSN) is a control parameter. With HSN =
0 we have a cyclic hopping pattern; pseudo-random patterns are
produced with 1<=HSN<=63.

Cheers