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Signal Questions and Problems

Q. Why can't I use my phone more the 35 km's from a base station?

A. Basically once the Base Station is more then 35 km's away the TDMA signalling fails because of the time required to reply.

To allow maximum number of users access, each band is subdivided into 124 carrier frequencies spaced 200KHz apart, using FDMA techniques. By applying TDMA techniques, each of these carrier frequencies is further subdivided into time slots which provide each user with the carrier frequency for approximately 0.577ms. This equates to approx 217 jumps per second, but amongst a very small frequency range so encryption is a must for proper security of calls. In fact it is not exactly that, it is hopping 13 times every 60 ms, which gives 13/0.06 per second. 0.577ms = 13 frames/60 ms /8 time slots

As soon as you get more then 35km from the base station, the phone cannot respond in time (eg in that time slot) so the base station starts handling another call.

Q. My Phone shows Full Strength but I can't make a call, why?

A. There are two possible explinations here.
1) That you are outside the 35 km limit of the base station.
2) That you are picking up a weaker signal that's bouncing off something else. At 900MHz the signal will bounce off many objects and can cause problems similar to the ghosting of a TV set. The main signal is too weak to make a call and results in error's on the handset, like Network Error and a few others.

Q. I can use my Phone up to 120 km's away from a base station, why?

A. Alcatel and Telstra Australia have been playing with a system whereapon by halving the number of time slots (thus halving Base Station capacity) they can double the distance up to 70 km's. I have heard of problems with this mainly to do with the Handset's not working properly and do not know if it's working yet. From what I understand it will not work.

This is the method that will work...

With one timeslot you get:

|guard|slot|guard|

Where the guard bits are to allow the slot to arrive at slightly the wrong time. The Timing Advance can adjust timing to get the phone within the slot+guard if the phone is within 35km. Further away and you need to use every other slot:

|guard|slot|guard|guard|slot2|guard|

Now you effectively have 2guard+slot2/2 timing lee-way instead of just guard. The timing advance is still set to 35km, but the phone hits the neighbour timeslot instead of its own.

The mobile can advance its timing up to 70km, which is 70000m, which at 3E8m/s is .23ms. Each timeslot is .577ms, which means that one timeslot gives 70km, and two timeslots gives 0.807ms which is 242.1km, or 121.05km radius, not 70km. This is not to say that the radio would be capable of 121km!

The mobile has to advance its timing by the round-trip because its time is 35km off true (due to the outbound propagation delay) then it has to allow another 35km for the way back.