Service Manual

GSM-204-323
CTU frequency hopping
31st Oct 01
Tech. 5–22
Technical Description: Horizon macro outdoor
CONTROLLED INTRODUCTION
68P02902W02-A
Baseband
frequency
hopping (BBH)
BBH requires all eight timeslots of the CTU Tx (downlink) at the same frequency.
In the Rx (uplink) direction, the frequency agility of the CTU is used to change
timeslot frequencies on a timeslot basis. The BCCH frequency is always
transmitted at maximum cell site power.
BBH can use either Tx blocks or CCB Tx combining equipment. The main
reason for using BBH instead of SFH is to enable frequency hopping when
using CCBs, because the mechanical tuning of CCBs is too slow for SFH.
The number of CTUs required to support BBH is equal to the number of
frequencies used.
NOTE
BBH is not supported on GSM850 or PCS1900 BTSs.
BBH example
In Figure 5-11 MSs A, B and C are using TS 5 of CTUs 0, 1 and 2 respectively.
Figure 5-11 Diagram of BBH example using three CTUs
If the MSs are using cyclic hopping across ARFCNs 10, 20, 30 (an example
using EGSM900), each MS must transmit a burst of information each TDMA
frame (4.615 ms) on a different frequency. The data for the burst is received by
each CTU in turn (ARFCN 10, 20, 30), as shown in Table 5-3:
Table 5-3 BBH sequence example (EGSM900)
Burst Sequence
Steps
CTU 0
Tx Rx
CTU 1
Tx Rx
CTU 2
Tx Rx
1 A
10
A
10
B
20
B
20
C
30
C
30
2 C
10
A
20
A
20
B
30
B
30
C
10
3 B
10
A
30
C
20
B
10
A
30
C
20
4 (same as 1) A
10
A
10
B
20
B
20
C
30
C
30
5 (same as 2) C
10
A
20
A
20
B
30
B
30
C
10
6 (same as 3) B
10
A
30
C
20
B
10
A
30
C
20
In the uplink direction, the controlling CTU (for example CTU 0 for MS A in
Figure 5-11) tunes TS 5 in accordance with the frequency expected from the MS
for that particular burst.