Method and apparatus for temperature compensation in a time division multiplex receiver

In a receiver for signals set in time slots within fixed length time frames, noise is supplied as an input signal and the received signal value is stored with measured temperature values. Temperature corrections for temperature are determined and stored to be applied subsequently.

The invention relates to temperature correction in a telecommunications 
receiver, in particular in a receiver for signals sent in time slots 
within fixed length time frames. 
The present invention is defined in the claims to which reference should 
now be made. 
According to one aspect, the present invention preferably consists in a 
method of correcting the output of the receiver in a time division 
multiplex broadcast unit in accordance with ambient temperature of the 
receiver, comprising providing an input reference noise signal to the 
receiver in a predetermined time slot of a time division multiplex channel 
of the unit and processing the corresponding output noise signal from the 
receiver to determine a correction factor which is related to a measured 
value of the current ambient temperature, this factor being used 
subsequently to correct output signals from the receiver at said ambient 
temperature. 
The invention preferably provides a time division multiplex broadcast unit 
comprising a receiver, a noise source, temperature measurement means and 
processing means, the noise source selectively providing a noise signal as 
an input to the receiver in a predetermined time slot of the time division 
multiplex channel so that the receiver provides an output signal dependent 
on the magnitude of the input noise signal and at least one temperature as 
measured by the temperature measurement means, and the processing means 
serving to determine an input signal correction factor from the signal and 
storing this correction factor against the associated temperature. 
The receiver can be a transmitter/receiver ("transceiver"). The broadcast 
unit can be a base station for time division multiplex/time division 
multiple access (TDM/TDMA) communications with a plurality of subscriber 
units which can be at fixed locations. The receiver can receive TDMA. 
Preferably, correction factors for different associated temperatures are 
determined, and stored for use in temperature compensation of received 
signals. The temperature measuring means can measure a first temperature 
value at a first location of the broadcast unit, and a second temperature 
at a second location. The first location can be adjacent to an antenna. 
The antenna can be exposed, for example, at the top of a mast head. The 
second location can be adjacent processing circuitry, particularly in the 
body of the broadcast unit. 
Each correction factor can be determined dependent on a plurality of 
temperatures and stored together with data representing those 
temperatures, for example, as a look-up table in microprocessor memory. In 
particular, a correction value can be associated with a temperature 
measured at the first position and a temperature measured at the second 
position. 
The stored correction factors can be updated intermittently or 
continuously, according to a predetermined rule. In particular, correction 
update or re-calibration can occur after a predetermined period of time 
has elapsed or whenever a novel combination of temperatures is 
encountered. Re-calibration can also be initiated under microprocessor 
control, whenever a fault condition is suspected. 
In essence, the look-up table of correction factors can be filled in "as 
you go". No separate initial calibration procedure is required. 
With repeated calibration, changes in correction factors and/or 
temperatures can be monitored, allowing detection of fault conditions. 
Furthermore, changes in the apparent noise received at a site can be 
monitored.

The drawing shows a base station comprising a mast which supports an aerial 
1 at the top and a radio unit 2 near the aerial with an input-output 
connection to the aerial. This connection comprises a diplexer unit 3 
through which a transmitter 4 and a receiver 5 are connected to the 
aerial. The radio unit 2 also incorporates a reference noise source 6 
which can be selectively connected to the input of the receiver 5 under 
the control of switch means 7 for determining temperature correction data, 
as described hereinafter. 
The radio signal received by the receiver 5 is amplified and converted to 
an IF signal, and supplied via a cable 8 to a converter and demodulator 
unit 9 at the base of the mast. 
A downconvert stage 10 receives the IF signal, and after conversion, passes 
the signal to a demodulator stage 11 which produces an output data signal 
to other equipment 12 of the communications system at the base station. 
The downconvert stage 10 has a variable gain which is controlled by a 
controller 13 in accordance with the temperature T1 of the receiver 5, as 
determined by an electronic thermometer 14, and the temperature T2 of the 
downconvert stage, as determined by an electronic thermometer 15. A 
look-up table stored in a memory 16 holds appropriate gain values for 
predetermined combinations of temperature ranges, and these are retrieved 
by the T1, T2 controller 13 in accordance with the values of the two 
temperatures measured. 
The gain values stored in the memory 16 are determined from a calibration 
procedure which is carried out while the communication system is in 
operation receiving radio signals, and which involves the use of the 
reference noise source 6 to provide a reference input to the receiver 5 in 
a predetermined channel of the TDM input. The switch means 7 is closed to 
initiate calibration, and this causes the noise signal to be sent to the 
receiver 5 in a predetermined TDM time slot in each TDM frame. The 
corresponding output of the receiver 5 is monitored, and by comparison 
with the known reference noise signal, the system is able to determine a 
correction factor or gain value for the particular temperatures, T1, T2 
that prevail at that time. 
The controller 13 may operate to receive the output signal from the 
receiver as well as data of temperatures T1 and T2. The controller 
calculates correction factor/gain values and updates the values stored in 
memory 16 for the various combinations of temperature bands. The values 
are kept in, for example, a 10.times.10 matrix of temperature bands, which 
correspond to temperatures T1 and T2. In other embodiments, different 
sized matrixes can be used. 
The controller 13 may be the central management entity of the base station. 
The noise source 6 preferably generates a noise signal at a fixed level 
above the system reference level. This reference level is the minimum 
level at which reception with a satisfactory Bit Error Rate will occur. In 
the preferred embodiment the system reference level is -101 dBm and the 
Bit Error Rate is no more than one in 10.sup.3. 
The noise signal is preferably applied to the transceiver during one 
selected TDM time slot in a multi-slot TDM frame preferably having 10 
slots. This slot is one which is not currently being used for data 
transfer. TDM frames are sent in TDM channels. In practice, the circuitry 
associated with three channels are in proximity at a base station. 
Applying noise on one channel only might cause cross-channel interference 
to the other channels. To overcome this, the noise signal is provided 
simultaneously on all three channels in simultaneous time slots, none of 
which carry data. The temperature corrections are then determined for all 
three channels. 
Calibrations or re-calibrations may be triggered by a predetermined time 
lapse, for example, half-an-hour, or whenever the temperatures T1 and T2 
are in a combination of ranges which has not been previously determined, 
or whenever a fault condition is suspected. 
Repeated re-calibration ensures that variations in receiver sensitivity are 
compensated for accurately over time. Furthermore, trends in corrections 
and temperature values can be followed, giving rise to early detection of 
fault conditions. Repeated calibrations also provide an early warning 
should the apparent level of noise at a base site change significantly. 
This effect could be due to a faulty antenna or other equipment, or could 
be due to a genuine increase in noise at the site. Both conditions would 
require investigation so are usefully monitored. The calibration data in 
the matrix may be read by a remote processor for monitoring performance of 
the system and providing early warning of problems.