Half Rate and extending the 35km from a tower
coverage limit
In any digital wireless communication system the analogue voice that we speak has to
be converted to digital so it can be transmitted to one person to another. In GSM there is
to main processes. Conversion of the voice to digital, then packing of these data packets
to be sent to a tower.
The conversion of the voice to digital packets can be done 3 ways, using Half Rate
coding (HR), Full Rate coding (FR) or Enhanced Full Rate coding.
After this is done the data packets are then placed into Time Division Multiple
Access (TDMA) created slots, or Time Slots. This is done buy dividing a GSM voice
channel up in to 8 parts (0-7), Each part is called a Time Slot (TS) and has the same
amount of time per frame (1 frame = 8 time slots).
FR and EFR but use a full TS, were HR only uses half a TS. This gives HR two main
usages in a GSM network.
1. To double the capacity of a tower.
Because the TS are only half used, the other half can be used for another voice data
packet. Which doubles the capacity of a GSM voice channel.
2. To double the range that a mobile phone can talk to a tower.
This is what I am going to concentrate in this article.
However there is one main disadvantage. Bad (half the quality) voice quality.
Before I start I must talk about one other part of the GSM network. When a signal is
sent from a mobile, it takes a small amount of time to get to the tower. This amount of
time is small, but nevertheless it must be taken into account as because GSM uses Time
Diversion Multiple Access (TDMA). Which means that each Time Slot (TS) must be received by
the tower at the same time or the time slots will get out of sync and start to over lap
each other.
As the mobile phone get further away from the tower the amount of time it takes for
the signal to reach the tower increases, this can be measured and worked out buy the
tower. Because of this, the tower is able to tell the mobile phone to send its data
packets earlier (or in advance). The amount that the mobile has to send the data packets
in advance is call Time Advance or TA. The TA is able to work because each data packet in
positioned in the middle of each TS, with space in front and behind it. This space is
called the Guard Bit. It can be increased (To increase the amount of time it takes the
data packet to reach the tower) or decreased ( To decrease the amount of time it takes for
the data packet to reach the tower).
The TA is a measure of how much the guard bit has been reduced. 00 No reduction (close
to the tower), to 63 (Max distance from a tower). Each TA equals approximately 550m from
the tower your mobile phone is talking to. This is were the 35KM limit comes in. 64 Time
Advances x (approx) .550 KM = 35KM.
Now that we understand all that (Bit confusing isn't it? :-), We can now talk about
how HR can double this 35 KM limit.
Because HR only uses half the space on a Time Slot, The size of the Guard Bit can be
increased (Doubled actually). This allows for a doubling of the Time Advance from 64 to
128. With 128 steps we are now able to increase the distance from 35 KM (64x550m) to 70KM
(128x550m). This will now enable us to use our mobile phone beyond the 35KM limit.
Unfortunately there are a few restrictions.
1. Your network and mobile phone must support HR
2. You must have a carkit and external antenna with a gain of 5dB or higher.
3. The geography of the land between you and the tower must be quite flat with out any
obstacles (Hills, High buildings, Lots of high trees, etc.. etc..)
4. You must be receive enough signal to commutate with the tower.
Unfortunately no network in Australia supports HR, so I have never tested going beyond
the 35KM limit. But for most people the 35KM limit will do.
For more info, have a look at my Aust CarKits and Antennas page.