User Manual

ArchitectureNortel Networks Confidential 3–45
S8000/S8002/S8006 BTS Reference Manual
The value of the propagation delay is sent to the DRX by means of the OML link
of the private PCM. From these two data, each DRX makes the necessary
corrections and regenerates the GSM TIME bus.
If, for any reason, the GSM time is not distributed on the BDT unit, the BDT unit
locally maintains the GSM TIME bus signals and continues to provide the GSM
time to the DRX units.
The BDT unit is made up of a logic block and a calculation block.
Digital block
The BDT unit receives a 26 MHz clock signal derived from the radio unit clock. This
clock signal has the same stability properties as the 4Fbit clock signal provided by
the BCF synchronization board and is more stable in the short term. The digital
block generates the following signals:
H4M (4.096 MHz)
STRTM (recurrent pulse at 577 microseconds)
TIME_DATA (containing T1, T2, T3 and TN)
Calculation block
The calculation block synchronizes the H4M and STRTM signals with the
synchronization unit signals of the BCF. In addition, it updates the values T1, T2,
T3 and TN.
The synchronization principle consists of forcing a divider–by–24 counter to divide
by 23 (if the BDT is slow) or by 25 (if it is fast). This way, every 23 or 25 periods
of 26 MHz (depending on whether the slow BDT is accelerated or the fast BDT is
slowed down), the BDT corrects a period of 26 MHz.
3.4.1.4 TX logic unit
The main role of the TX logic unit is to control the radio subassembly in real time.
It receives the BCF configuration commands from the AMNU. It carries out the
processing and sends back reports.
Once configured, the TX logic unit reads, on each time slot, the data present on the
FH bus. Then it calculates the frequency code and the power code to be used with
the radio interface.
Transmission power
In general, radio power is determined by two inputs. One controls the maximum
static power and the other gives the dynamic attenuation at each time slot.
The static power is given by the BCF or CBCF in the CONFIG message. The TX
calculates attenuation to compensate for cable loss between the TX–driver and the
power amplifier.