9
Uplink and Downlink use different bands (45MHz apart for GSM900)
In the previous example we can see that the timeslots are offset by 3 between the downlink and the
uplink. We receive information in timeslot two in the downlink we have two timeslots in which to switch to
the uplink frequency and be ready to transmit information. Then, we have to get ready to receive our next
time slot of information in the next frame.
7
）：
Power Control
：
As the mobile moves around the cell, its transmitter power needs to be varied. When it's close to the base
station, power levels are set low to reduce the interference to other users. When the mobile is further from the
base station, its power level needs to increase to overcome the increased path loss. However, if too much
power is used, the user’s battery will run down too quickly. All GSM mobiles are able to control their output
power in 2dB steps. The base station commands the mobile to a particular MS TX Level (power level) by
watching the power level of the received signal at the BS.
There may be many users in the same cell. If every
Mobile phone has the same emission power,
ones closer to Base Station can block the ones which are farther;
O
n the other hand, because if closer ones
emit more power
The table above shows the maximum and minimum power levels on the mobiles in different systems. The final
row shows the power steps, which are all numbered, and how they relate to the max and min powers.
8
）：
Timing Control
：
If the burst arrives at the correct time, it will fit into its physical channel and not disturb any other burst that may
follow it in the next timeslot. However, if it is delayed, due to a long distance to travel, it may arrive late and
impact the following burst from another user. In this case the mobile is instructed by the BS to burst earlier
which will correctly align the burst in the timeslot. The message sent by the BS is called the Timing Advance.
The base station monitors the burst to see when it arrives at the base station. If it arrives late or early, the base
station will note how many times it has changed since the last Timing Advance adjustment, and if there have
been more than 4 x ¼ bit periods change in one direction, the adjustment will be made again.
Main parameter of GSM is showed as follows:
PGSM
EGSM
DCS
PCS
ARFCN Range
1~124 975~1023,
0,
1~124
512~885 512~810
Uplink Band
Range
890MHz~915MHz 880MHz~915MHz 1710MHz~1785MH
z
1850MHz~1910MH
z
Downlink Band
Range
935MHz~960MHz 925MHz~960MHz 1805MHz~1880MH
z
1930MHz~1990MH
z
Bandwidth
200kHz 200kHz 200kHz 200kHz
duplex
Interval
Frequency
45MHz 45MHz 95MHz 85MHz
TDMA User
Number
8 8 8 8
Modulation Mode
0.3GMSK 0.3GMSK 0.3GMSK 0.3GMSK
Summary of Contents for W420
Page 1: ...Voxtel Repair Manual Item W420 ...
Page 23: ...23 3 LDO circuit ...
