GB2369967A

GB2369967A - Received signal measurement using a diversity antenna

Info

Publication number: GB2369967A
Application number: GB0119839A
Authority: GB
Inventors: Martin Popple
Original assignee: Simoco International Ltd
Current assignee: Simoco International Ltd
Priority date: 2000-08-14
Filing date: 2001-08-14
Publication date: 2002-06-12
Also published as: WO2002015412A2; WO2002015412A3; GB0019993D0; GB0119839D0

Abstract

A mobile unit of a digital mobile communications system includes a diverse antenna receiver arrangement comprising a main antenna branch 1, a second diverse antenna branch 2, and a diverse antenna switch 3 for switching between the two antenna branches. A channel acquisition controller 11 runs a channel acquisition process which causes the mobile unit to scan plural frequencies of the radio system in turn and take successive signal samples on each frequency, in which scan two signal samples are taken on each carrier frequency, one on the main antenna branch 1, and then one from the diverse antenna branch 2, before the receiver retunes to the next frequency to be sampled. After all the frequencies have been dual-sampled in this way, the process is redone a number of times and after a selected overall sampling period the mean signal strength of each frequency is determined from the samples taken. The mobile unit then, for example, selects the frequency with the greatest signal strength as the frequency (radio channel) to use.

Description

- 1 Siqnal Measurement in Radio Communications Systems 5 The present

invention relates to the measurement of signals in communications systems and in particular to the measurement of received signal quality (e.g. strength) in mobile radio communications systems, such as the TETRA (Terrestrial Trunked RAdio) system.

10 As is known in the art, in a cellular mobile radio communications system, the area to be covered by the radio system is typically split up into a number of cells, with each cell having its own radio base station.

The total number of radio frequencies or channels 15 available to the system operator is split up into a number of channel sets and the channels allocated to the cells, with one channel set per cell in a regular pattern across the overall radio system network. Each channel (frequency) set is typically reused many times 20 within the network, and the allocation of the channel sets is such that nearby cells do not use the same channels (i.e. frequencies), which could otherwise lead to interference.

It is important that a mobile radio unit in a 25 cellular radio system uses radio channels (i.e. frequencies) of the cell that it is currently located in, as if a mobile unit uses a radio channel of a cell other than the cell it is located in, then interference could be generated for cells employing the same 30 frequency elsewhere. In such a situation, the mobile unit would also most likely be operating at a greater than necessary range, thereby requiring more power and thus shortening the life of its battery.

Thus it is important that a mobile radio unit uses 35 the correct radio channels. This can be relatively easily achieved when the mobile unit is already in contact with the radio system, for example, by

appropriate control by the radio system or by the mobile unit, as it is already connected to the system, already having a suitable history of, e. g., signal measurements, to allow it to make a good channel selection decision.

5 However, in some circumstances a mobile unit may need to decide which radio channel to use of its own accord, for example when it is initially turned on after a period out of contact with the radio system, or when the control process to select a new radio cell to use as 10 the mobile radio unit moves through the radio system (e.g. a cell reselection algorithm) has failed. In these circumstances the mobile unit has to select an initial radio channel, e.g. control channel, to use, without any knowledge of where it is in relation to the 15 base stations and cells of the radio network.

To assist in such cell and channel selection, the mobile radio unit typically measures the signal strength (RSSI - received signal strength indication in TETRA) of signals on radio channels (frequencies) that it knows 20 are assigned to the radio network (which channels and frequencies will typically be predetermined and can therefore be stored in the mobile radio unit) and uses those signal strength measurements to select the channel and frequency to use, e.g. by selecting the channel 25 offering the highest signal strength (which is therefore likely to be from the closest base station, and thus serving the cell that the mobile radio unit is currently located in).

In such an arrangement it is important that the 30 signal strength measurements are as accurate as possible, so as to improve the accuracy of the channel and cell re-seleetion process. In order to increase the measurement accuracy therefore, typically a number of signal strength samples on a given frequency (channel) 35 are sampled in a particular time period and averaged to give the signal strength measurement that is used in the selection process.

- 3 Thus, in a typical present radio channel acquisition process, a single signal strength sample on each of the radio frequencies known to be used by the radio system is taken in turn, in a set order and after 5 all frequencies have been sampled once, the sampling of the frequencies is repeated a number of times up to a selected total time period allowed for the signal strength sampling. This helps to ensure that each sample on a given frequency is as far apart as possible, 10 so as to minimise sample correlation. After the total time period for the signal strength sampling has elapsed, the mean signal strength of each available radio frequency is determined, and the frequency with the signal having greatest signal strength (therefore 15 being potentially most likely to provide good service) is selected as the radio frequency (i.e. channel) to use. In such an arrangement, the signal strength measurement accuracy is basically dependent on the 20 number of de-correlated (i.e. independent) individual samples measured for each signal (i.e. frequency).

A more accurate signal strength measurement will be obtained by taking more, and more widely spaced (in time), i.e. more de-correlated, samples for each signal.

25 However, taking more samples and having greater spacing between the samples means that the signal strength measurements take longer, and therefore that there will be a greater delay before the mobile unit selects a radio channel to use (and thus acquires service and is 30 ready for use), which delay may be undesirable to the user. There is therefore a compromise between the accuracy of the measurements (i.e. the number of de-correlated samples) and the time taken to obtain them, i.e. the radio channel acquisition time.

35 In known radio channel acquisition processes, the period over which signal samples are taken is typically chosen to be around 5 seconds, as a compromise between

trying to ensure that a slow moving mobile unit will have moved out of a fade (which would affect considerably the signal strength measurement), and that a fast moving mobile unit will not have moved too great 5 a distance, while having a relatively short delay before the mobile unit acquires service. In the TETRA specification, a "notional" slow moving mobile unit is

taken to be moving at 3 km/in (so a 5 second period is equivalent to a distance of 6 wavelengths), and a 10 ''notional" fast moving mobile unit is taken to be moving at 200 km/in (and so in 5 seconds moves approximately 280m). It will typically take a mobile radio unit 5 ms to tune its receiver to each carrier frequency and then 15 around 3 ms to take the signal sample measurement. This gives a total time per signal sample of 8 ms, assuming that an individual sample is taken on each carrier frequency in turn (as would typically be the case, since two immediately successive samples taken on the same 20 carrier frequency could reduce significantly the probability of those samples being decorrelated and therefore their usefulness). Thus in an overall sampling period of 5 seconds, the number of signal samples per individual radio frequency carrier, for 25 different overall numbers of radio frequency carriers covered by the signal sampling search, would be as follows: Table 1:

30 Number of different Number of samples per | radio frequency individual carrier in 5s.

carriers sampled.

_ 10 62 30 20 35 50 12

100 6

The number of samples per individual carrier and thus the measurement accuracy could be improved by increasing the overall sampling period to be greater 5 than 5 seconds, but as discussed above that would increase the service acquisition delay, and may therefore be undesirable.

In many modern radio systems, to improve the down-link (i.e. base station to mobile station) 10 reception, particularly at low mobile speeds and for stationary mobile units, mobile radio units are often equipped with a multi-antenna diversity reception system. As is known in the art, in such a diversity reception scheme, the mobile unit switches between 15 reception on the different antennas so as to obtain the best signal, as the signal strength of a signal received by each antenna will differ due to the physical spacing of the antennas. Typically, a mobile unit will be equipped with two antennas and one receiver as a 20 compromise between performance and cost, although more antennas can be used if desired. An antenna selection algorithm switches between reception on the different antennas when, for example, the received signal level on the current antenna drops below a given threshold.

25 (See, for example, "Microwave Mobile Communications", W.C. Jakes, IEEE Press, ISBN 0-7803-1069-1.) Such a diversity reception system can help to keep the signal quality high even when, for example, the mobile unit is moving slowly through a fading environment.

30 The Applicants have recognized that such diversity antenna systems can also be used advantageously when there is a need to take plural signal strength measurements on a plurality of different radio frequencies in succession.

35 Thus, according to a first aspect of the present invention, there is provided a method of operating a radio unit having a diverse antenna arrangement having

at least two antenna branches, the method comprising: the radio unit taking plural signal samples on each of a plurality of different radio frequencies, and then using the signal samples to estimate the strength of the 5 signal on each frequency; wherein the signal samples for at least one of the frequencies include samples taken on at least two different antenna branches of the diverse antenna arrangement. lo According to a second aspect of the present invention, there is provided a radio unit comprising: a diverse antenna arrangement having at least two antenna branches; means for controlling the radio unit to take plural 15 signal samples on each of a plurality of different radio frequencies, and for controlling the radio unit to take the samples such that the signal samples for at least one of the frequencies include samples taken on at least two different antenna branches of the diverse antenna 20 arrangement; and means for using the signal samples to estimate the strength of the signal on each frequency.

In the signal measuring method and apparatus of the present invention, the radio unit takes plural signal 25 samples on each frequency, but takes samples on at least one frequency on different antenna branches of its diverse antenna arrangement, rather than always taking the samples on that frequency from the same antenna.

The Applicants have recognized that where a diverse 30 antenna arrangement is used it is advantageous to take signal samples from different antenna branches, because the signal samples on the different diverse antenna branches should be de-correlated (unlike in the case of a static single antenna), since the antennas of a 35 diverse system are usually arranged to maximise decorrelation. Thus, it is advantageous to take signal samples from different antenna branches, because samples

taken from different diverse antenna branches should be more assured of being de-correlated, which means that a set of such samples should provide a more accurate signal strength assessment for the frequency for a given 5 number of signal samples.

Furthermore, as the de-correlation of the signal samples on the frequency or frequencies for which samples from different antenna branches are taken is more certain and so the signal assessment more accurate, 10 the present invention allows the overall number of samples taken for each such frequency to be reduced (thereby reducing the overall sampling period and thus service acquisition time) without a commensurate decrease in the signal strength measurement accuracy.

15 Thus the present invention provides, for example, a system whereby the signal strength measurement accuracy can be improved when using the same overall sampling time as in known arrangements, or the overall measurement and sampling time (and thus the service 20 acquisition time) can be reduced while still achieving the same accuracy as in known arrangements.

Although it is not necessary to take samples on different antenna branches on each frequency, in a particularly preferred embodiment of the present 25 invention, the signal samples for each frequency include samples taken on at least two different antenna branches of the diverse antenna arrangement. This allows the advantages of the present invention to be realized for each frequency assessment.

30 The plural signal samples on each frequency can be taken as desired. They are preferably spaced in time, so as to increase the probability of them being decorrelated. Thus in a particularly preferred embodiment, a signal sample or signal samples are taken 35 on each frequency in turn. The sequence can then be repeated as desired to build up a set of plural samples for each frequency.

- 8 Where the samples are to be taken on different antenna branches for a given frequency, those samples could, for example, be taken on different, spaced, "visits" to that frequency. In such an arrangement, the 5 radio unit could, for example, take samples in the following sequence: frequency 1, antenna branch 1; frequency 2, antenna branch 1; frequency 3, antenna branch 1; frequency 4, antenna branch 1; frequency 1, antenna branch 2; frequency 2, antenna branch 2; 10 frequency 3, antenna branch 2; frequency 4, antenna branch 2; frequency 1, antenna branch 1, etc., and so on. However, in a particularly preferred embodiment where samples are to be taken on different antenna 15 branches for a given frequency, the radio unit takes at least two signal samples when it tunes to the frequency, and most preferably whenever it tunes to the frequency, during the sampling process, each sample being taken from a different antenna branch of the diverse antenna 20 arrangement. The radio unit correspondingly preferably includes means for controlling the radio unit to take a signal sample from each of at least two different antenna branches of the diverse antenna arrangement when it tunes to a given frequency, and preferably whenever 25 it tunes to a given frequency, during the sampling process. In other words, in this embodiment of the present invention, the radio unit takes two signal samples but on different antenna branches of the diverse antenna 30 arrangement, when tuned to a given frequency, rather than a single sample on each frequency as in typical prior art techniques. Thus, the radio unit tunes to the

frequency, samples the signal received on one antenna branch of its diverse antenna arrangement, and then 35 samples the signal received on a second different antenna branch of its diverse antenna system, before retuning to a different frequency, rather than simply

- 9 - taking a single signal sample on a given frequency before retuning to a new frequency.

Thus in this embodiment, where samples on different antenna branches are taken on each frequency, the radio 5 unit would, for example, take signal samples in the following sequence: frequency 1, antenna branch 1; frequency 1, antenna branch 2; frequency 2, antenna branch 1; frequency 2, antenna branch 2; frequency 3, antenna branch 1; frequency 3, antenna branch 2; 10 frequency 4, antenna branch 1; frequency 4, antenna branch 2; frequency 1, antenna branch 1; frequency 1, antenna branch 2, etc.; and so on.

With regard to this preferred embodiment, the Applicants have recognized that where a diverse antenna 15 arrangement is used, then as signal samples on the different diverse antenna branches should be de-correlated (since the antennas of a diverse system are arranged to maximise decorrelation), then immediately successive signal samples on a given radio 20 frequency but taken from different antenna branches should be sufficiently de-correlated for the samples still to be useful in a signal strength measurement calculation (unlike in a single antenna system, where immediately successive samples on the same frequency 25 could typically be highly correlated, particularly for a slow moving mobile unit).

To take the signal samples on different antenna branches but on the same carrier frequency, there is no need for the radio unit to re-tune its synthesizer to a 30 new frequency between the two samples, rather all that needs to be done is to switch antennas (which might typically take around 100 As). Thus assuming as above that it takes 5 ms to tune to each carrier frequency and 3 ms to take each sample, where two diverse antenna 35 branches are sampled, two at least reasonably de-correlated signal samples on a given carrier can be obtained in around 11 ms, i.e. effectively 5.5 ms per

- 10 sample for a given frequency. Thus this embodiment further allows more useful (i.e. de-correlated) signal samples to be taken in a given time period.

This can be seen, for example, from a comparison 5 with the number of signal samples per carrier for a known signal strength sampling process given in Table 1 above. In an overall sampling period of 5 seconds, the number of signal samples per individual radio frequency carrier for a radio unit operating in accordance with 10 this embodiment of the present invention and taking two signal samples per carrier (and assuming it takes 11 me to obtain the two samples), would be as follows: Table 2:

15 Number of different Number of samples per radio frequency carriers individual carrier in 5 s.

sampled. 10 90 30 30 20 50 18

100 9 As can be seen, in this embodiment, more signal samples are available for processing after the 5 second 25 overall sampling period.

Thus this embodiment of the present invention allows the same number of signal samples to be obtained in a shorter overall sampling time period, thereby reducing the service acquisition time even further, or a 30 larger number of signal samples (and thus even more accurate signal strength measurements) to be obtained in the same overall sampling time period.

In the present invention, signal samples are taken on at least two branches of the diversity antenna 35 arrangement. Where the antenna arrangement has more than two branches, then the radio unit could still only

- 11 take signal samples on two of the antenna branches for a given frequency, or alternatively signal samples from more than two different antenna branches, e.g. up to the number of different antenna branches, could be taken.

5 This latter arrangement would, for example, where the signal samples on the different branches are taken successively while tuned to a given frequency before retuning, further reduce the effective time required for obtaining each individual de-correlated signal sample.

10 The radio unit should determine from each signal sample a signal strength measurement. This can be done in any suitable manner known in the art. The overall signal strength on a given frequency can equally be estimated from the individual signal samples on that 15 frequency as desired and in any suitable manner known in the art. Thus, for example, the individual signal strength measurements could be averaged to give a mean signal strength for the frequency, as is done in known techniques. 20 The present invention is as will be appreciated from the above, particularly applicable to the operation of mobile communications units of a mobile communications system. Thus the radio unit will typically be such a mobile communications unit.

25 The present invention can be used wherever signal strength measurements for plural frequencies are required. In a particularly preferred embodiment it is used to obtain signal measurements for channel acquisition algorithms as discussed above, and/or for 30 handover control, such as cell reselection algorithms used to identify suitable target cells when a mobile radio unit needs to be handed over from its serving radio cell to a new cell.

Typically in such arrangements, the radio unit 35 would, as is known in the art, cycle repeatedly around a predetermined list of frequencies taking an appropriate signal sample or samples on each frequency before

- 12 retuning to the next frequency in the list, until the overall sampling period is finished, and then use the signal samples to estimate the signal strength on each frequency (e.g. by averaging the appropriate samples).

5 Thus, according to a third aspect of the present invention, there is provided a method of operating a mobile unit of a mobile communications system, in which system plural frequencies are used for communication, and which mobile unit stores a list of frequencies used 10 by the communications system and has a diverse antenna arrangement comprising at least two antenna branches, the method comprising: the mobile unit taking plural signal samples on each frequency in the stored list, and then using the 15 signal samples to estimate the strength of the signal on each frequency; wherein the signal samples for at least one of the frequencies include samples taken on at least two different antenna branches of the diverse antenna 20 arrangement.

According to a fourth aspect of the present invention, there is provided a mobile unit of a mobile communications system, in which system plural frequencies are used for communication, the mobile unit 25 comprising: means for storing a list of frequencies used by the communications system; a diverse antenna arrangement having at least two antenna branches; 30 means for controlling the radio unit to take plural signal samples on each frequency in the stored list, and for controlling the radio unit to take the samples such that the signal samples for at least one of the frequencies include samples taken on at least two 35 different antenna branches of the diverse antenna arrangement; and means for using the signal samples to estimate the

strength of the signal on each frequency.

The above aspects of the present invention can include any one or more of the preferred features of the invention discussed above. Thus, for example, in a 5 particularly preferred embodiment of these aspects of the invention, the mobile unit tunes to each frequency on the list in turn, and while tuned to each given frequency, samples the signal on that frequency as received from a first antenna branch of the diverse 10 antenna arrangement, and samples the signal on the frequency as received via a second, different antenna branch of the diverse antenna arrangement, before retuning to the next frequency on the list. Similarly, the mobile unit preferably comprises means for tuning to 15 each frequency on the list in turn; and means for, while tuned to each given frequency, sampling the signal on that frequency as received from a first antenna branch of the diverse antenna arrangement, and for sampling the signal on the frequency as received via a second, 20 different antenna branch of the diverse antenna arrangement, before retuning to the next frequency on the list.

In the above aspects of the present invention, the frequency list to be scanned could be a list of all 25 frequencies used by the radio system, or it could be a reduced list, for example of only those frequencies known to be in use in a particular area or in the appropriate communications system cells, as is known in the art.

30 The mobile unit should scan all frequencies in the list at least once. It would typically, as is known in the art, repeat the sampling for all the frequencies once a given sampling scan is completed, and so on, such that the list is run through a number of times, e.g. 35 until a predetermined overall sampling time period has expired. Thus, the mobile unit preferably continually repeats the sampling process once all frequencies on the

- 14 list have been tuned to, until a predetermined overall sampling time period has expired. The signal samples would then be used to estimate the signal strength on each sampled frequency (e.g. by taking the mean of the 5 appropriate signal samples).

Although the invention has been described above with reference to the measurement of signal strength, it is, as will be appreciated by those skilled in the art, applicable to the taking of samples of radio signals 10 generally and not just signal strength measurements, wherever plural de-correlated samples, particularly on different frequencies, of a radio signal or signals are required. Thus according to a fifth aspect of the present 15 invention, there is provided a method of operating a radio unit having a diverse antenna arrangement having at least two antenna branches, the method comprising: the radio unit taking plural signal samples on each of a plurality of different radio frequencies; 20 wherein the signal samples for at least one of the frequencies include samples taken on at least two different antenna branches of the diverse antenna arrangement. According to a sixth aspect of the present 25 invention, there is provided a radio unit, comprising: a diverse antenna arrangement having at least two antenna branches; and means for controlling the radio unit to take plural signal samples on each of a plurality of different radio 30 frequencies, and for controlling the radio unit to take the samples such that the signal samples for at least one of the frequencies include samples taken on at least two different antenna branches of the diverse antenna arrangement. 35 These aspects of the invention again preferably include any one or more of the preferred features discussed above. Thus in a particularly preferred

- 15 embodiment of these aspects of the invention, the radio unit takes signal samples on each of a plurality of different radio frequencies in turn, and takes at least two signal samples when tuned to a given frequency, each 5 sample being taken from a different antenna branch of the diverse antenna arrangement. Similarly, the radio unit preferably comprises means for controlling the radio unit to take signal samples on each of a plurality of different radio frequencies in turn, and for 10 controlling the radio unit to take a signal sample from each of at least two different antenna branches of the diverse antenna arrangement when tuned to a given frequency. In these aspects of the invention, the signal 15 samples could be used, for example, to estimate a suitable signal parameter, such as, for example, signal strength, as discussed above, and/or any other suitable signal parameter or characteristic, such as for example, more sophisticated signal quality measures, such as bit 20 error rate, or message error rate, or whether the signal carries particular, e. g. modulation, characteristics.

The methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen 25 that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on data processing means, and a computer program element comprising computer software code 30 portions for performing the methods hereinabove described when the program element is run on data processing means. The invention also extends to a computer software carrier comprising such software which when used to operate a radio unit comprising a digital 35 computer causes in conjunction with said computer said radio unit to carry out the steps of the method of the present invention. Such a computer software carrier

- 16 could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

5 It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier 10 for carrying out at least one of the steps of the methods set out hereinabove.

A preferred embodiment of the present invention will now be described by way of example only and with reference to the accompanying Figure l which shows 15 schematically a mobile unit of a digital mobile communications system incorporating a typical diverse antenna receiver arrangement and that can be operated in accordance with the present invention.

The mobile unit shown schematically in Figure 1 20 includes a diverseantenna receiver arrangement comprising a main antenna branch 1, a second diverse antenna branch 2, and a diverse antenna switch 3 for switching between the two antenna branches. Such a multi-antenna diversity arrangement is used, as is known 25 in the art, to improve reception by the mobile unit and in particular to improve performance at low mobile speeds. In use, the diverse antenna switch 3 is controlled by a switch controller 8 via a control line 13 to select 30 one of the antenna branches for use and the signal from that branch is then provided via the switch 3 to a single branch "analogue" receiver 4 which processes the received signal in the normal way and provides it to an A/D converter 5.

35 The A/D converter provides the signal appropriately to a sample memory 6. The signal samples in the sample memory 6 are then used for various purposes. For

- 17 example, they can be used for call control via a call controller 7 which provides the received signal to a speaker 9 for replay to the user.

The signal samples from the memory 6 are also 5 provided to a number of other controllers which process them appropriately and also control the switch 3 via the switch controller 8 both to switch antenna branches on the basis of the sampled signal and/or to obtain samples from the different antenna branches in accordance with 10 the present invention.

The first such controller is a selection diversity controller 10 which monitors the received signal and indicates to the switch controller when a switch to the alternative antenna branch is required to improve 15 the signal reception. Typically, the received signal level for the current antenna branch is monitored and if that signal level drops below a predefined threshold, the controller 10 causes the switch 3 to select the other antenna branch. Because the diverse antenna 20 arrangement is deliberately arranged such that there should be low correlation between the different antenna branches, the new antenna is unlikely to be in the same fade as the antenna for which the signal level had dropped, and thus the signal quality should be higher 25 from the new antenna branch.

The second such controller is a channel allocation or acquisition controller 11 which operates in accordance with the present invention to exploit the diverse antenna arrangement so as to provide an improved 30 method of estimating the signal strength of signals on different frequencies available to the mobile unit when the mobile unit needs to acquire a communications channel. The channel acquisition controller 11 runs a 35 channel acquisition process which causes the mobile unit to scan plural frequencies of the communications system in turn and take successive signal samples on each

- 18 frequency, in which scan two signal samples are taken on each carrier frequency, one on the main antenna branch 1, and then one from the diverse antenna branch 2, before the receiver retunes to the next frequency to be 5 sampled. In other words, the mobile unit is controlled to tune to each frequency in turn and while tuned to each given frequency, to sample the signal on that frequency as received from the main antenna branch 1 of the diverse antenna arrangement, to then switch to the 10 second antenna branch 2 of the diverse antenna arrangement and sample the signal on the frequency as received via that antenna branch, or vice- versa, before retuning to the next frequency.

This operation is carried out under the control of 15 channel allocation controller 11, which takes samples from the sample memory 6 and also controls the switch controller 8 to control the diverse antenna selection switch 3 to switch between the main and diverse antennas appropriately when the mobile unit is sampling each 20 frequency.

After all the carrier frequencies have been dual-sampled in this way, the process is redone a number of times and after a selected overall sampling period the mean signal strength of each carrier frequency is 25 determined from the samples taken.

Once the signal strength of each carrier frequency has been estimated, for example by taking the mean of each signal strength sample, the mobile unit will then, for example, select the carrier with the greatest signal 30 strength (and therefore being the most likely to provide good service) as the carrier frequency (communications channel) to use.

The operation of a mobile unit operating in this manner was simulated and compared to a simulation of a 35 mobile unit using the known channel acquisition technique discussed above. For both the simulations, an overall sampling period of 5 seconds was assumed, with

- 19 it being assumed that for the already known technique each sample on each frequency took 8 ms to obtain, whereas for the method operating in accordance with the present embodiment, two de-correlated samples on a given 5 frequency would be obtained in 11 me. (The diverse antenna arrangement was assumed to provide a low (approximately zero) correlation between antenna branches.) Each simulation was run many times for a mobile 10 unit moving at a given speed and assessing a given overall number of network carriers. The variance of all of the mean signal strengths estimated over all of the runs for a given set of conditions (i. e. sampling method, mobile unit speed, and number of network 15 carriers) was then determined. This was repeated for different sets of operation conditions (i.e. mobile unit speed and number of network carriers sampled) .

The variances in mean signal level obtained for each method are shown below for different overall 20 numbers of radio network carriers and mobile unit speeds. Table 3. Variance Gains for 100 Sampled Network Carriers. 25 Speed Range (km/lh) Variance in mean signal strength estimates Improvement in (dB) variance (dB) Known Method Method of the present embodiment 3-200 52 3.8 1.4

05-3 66 43 2.3

0.1-05 170 8.7 8.3

- 20 Table 4. Variance Gains for 50 Sampled Network Carriers.

_ Speed flange (km/h) Variance in mean signal strength estimates Improvement in (dB) variance (dI3) I(nown Method Method of the present embodiment .. _ _

3-200 2.7 1.8 0.9

0.5-3 4.7 2.5 2.0

_ 5 0.1-0.5 14.9 7.4 7.2

Table 5. Variance Gains for lO Sampled Network Carriers.

Speed Range (km/h) Variance in mean signal strength estimates Improvement in (dB) variance (dB) Known Method Method of the present embodiment 3-200 O.S 0.4 0.4

10 0.5-3 4.1.. 2.0 2.1

0.1-0.5 14.3 7.1 7.2

(It was found that the signal variance of both methods was essentially constant in the speed range of 15 3-200 km/in (i.e. the TETRA specification). This is

believed to be because the total period over which signals are sampled is always sufficiently large to get reasonable de-correlation at those speeds. Thus simulations for mobile unit speeds in this range were 20 all considered together. The signal level variances at lower speed ranges varied more noticeably, and were therefore considered in two speed ranges, 0.1-0.5 km/in and 0.5-3 km/in. (Although these speed ranges are, for example, strictly speaking outside the TETRA 25 specification, they are still of practical importance to

the mobile user.)) It can be seen from the above Tables that although even for low speeds (3 km/in) only a small improvement, typically less than 1.5 dB, is obtained with the method 30 of the present embodiment, for very low speeds (closer

to stationary), large measurement improvements are obtained. Such an improvement at low speeds is likely to give a significant practical improvement to the operation of a radio system, for example if a mobile 5 unit is stationary when needing to reacquire service that has temporarily been lost.

Further improvements in the signal strength measurements could be obtained where more than two antenna branches are provided by taking samples from 10 each different antenna branch in turn while tuned to a given frequency, since that would further reduce the effective time taken to obtain a de-correlated sample on a given carrier. This is independent of which diversity combining algorithm is employed "in call". (As is known 15 in the art, typically two antenna branches and a single receiver as shown in Figure 1 are employed as a compromise between performance and cost, but more antenna branches can be used if desired.) As will be appreciated by those skilled in the art, 20 the present invention is applicable wherever improved signal strength measurements are required and therefore not just in a channel acquisition process. It could also be used for example in cell re-selection processes used in hand-over processes. This could be provided 25 under the control of a suitable cell re-selection controller 12 as shown in Figure 1 which would in a similar way to the channel allocation controller 11 take samples from the sample memory 6 and control the switch controller '3 to switch the diverse switch 3 30 appropriately as the signal samples are obtained.

As can be seen from the above, the present invention provides an improved method and apparatus for determining signal strength measurements, which can either improve the measurement accuracy or reduce the 35 time in which a sufficiently accurate measurement is obtained, and which is particularly useful at very low mobile unit speeds.

- 22 It will also be appreciated by those skilled in the art that the present invention relates to the operation of (and improvements in the operation of) the receiving communications unit and is independent of the operation 5 of the transmitting communications unit, e.g. base station, which transmitting communications unit can therefore continue to operate in its standard manner (and as desired).

Claims

- 23 CLAIMS

1. A method of operating a radio unit having a diverse antenna arrangement having at least two antenna 5 branches, the method comprising: the radio unit taking plural signal samples on each of a plurality of different radio frequencies, and then using the signal samples to estimate the strength of the signal on each frequency; 10 wherein the signal samples for at least one of the frequencies include samples taken from at least two different antenna branches of the diverse antenna arrangement. 15 2. The method of claim 1, wherein the signal samples for each frequency include samples taken from at least two different antenna branches of the diverse antenna arrangement. 20 3. The method of claim 1 or 2, wherein the plural signal samples on each frequency are spaced in time.

4. The method of claim 1, 2 or 3, wherein the radio unit takes at least two signal samples when it tunes to a frequency, each sample being taken from a different antenna branch of the diverse antenna arrangement.

5. The method of any one of the preceding claims, wherein the radio unit tunes to a frequency, samples the 30 signal received on one antenna branch of its diverse antenna arrangement, and then samples the signal received on a second different antenna branch of its diverse antenna system, before retuning to a different frequency. 6. The method of any one of the preceding claims, wherein the radio unit cycles repeatedly around a

- 24 predetermined list of frequencies taking an appropriate signal sample or samples on each frequency before retuning to the next frequency in the list.

5 7. A method of operating a mobile unit of a mobile communications system, in which system plural frequencies are used for communication, and which mobile unit stores a list of frequencies used by the communications system and has a diverse antenna 10 arrangement comprising at least two antenna branches, the method comprising: the mobile unit taking plural signal samples on each frequency in the stored list, and then using the signal samples to estimate the strength of the signal on 15 each frequency; wherein the signal samples for at least one of the frequencies include samples taken from at least two different antenna branches of the diverse antenna arrangement. 8. The method of claim 7, wherein the mobile unit tunes to each frequency on the list in turn, and while tuned to each given frequency, samples the signal on that frequency as received from a first antenna branch 25 of the diverse antenna arrangement, and samples the signal on the frequency as received via a second, different antenna branch of the diverse antenna arrangement, before retuning to the next frequency on the list.

9. The method of claim 7 or 8, wherein the mobile unit continually repeats the sampling process once all frequencies on the list have been tuned to, until a predetermined overall sampling time period has expired.

10. A method of operating a radio unit having a diverse antenna arrangement having at least two antenna

- 25 branches, the method comprising: the radio unit taking plural signal samples on each of a plurality of different radio frequencies; wherein the signal samples for at least one of the 5 frequencies include samples taken from at least two different antenna branches of the diverse antenna arrangement. 11. A communications unit comprising: 10 a diverse antenna arrangement having at least two antenna branches; means for controlling the communications unit to take plural signal samples on each of a plurality of different radio frequencies, and for controlling the 15 communications unit to take the samples such that the signal samples for at least one of the frequencies include samples taken from at least two different antenna branches of the diverse antenna arrangement; and means for using the signal samples to estimate the 20 strength of the signal on each frequency.

12. The communications unit of claim 11, wherein the means for controlling the communications unit to take the signal samples controls the communications unit such 25 that the signal samples for each frequency include samples taken from at least two different antenna branches of the diverse antenna arrangement.

13. The communications unit of claim 11 or 12, further 30 comprising means for controlling the communications unit to take a signal sample from each of at least two different antenna branches of the diverse antenna arrangement when it tunes to a given frequency.

35 14. The communications unit of claim 11, 12 or 13, comprising means for tuning the communications unit to a frequency, sampling the signal received on one antenna

- 26 branch of the communications unit's diverse antenna arrangement, and then sampling the signal received on a second different antenna branch of the communications unit's diverse antenna system, before retuning to a 5 different frequency.

15. A mobile unit of a mobile communications system comprising a communications unit as claimed in any one of claims 11 to 14.

16. A mobile unit of a mobile communications system, in which system plural frequencies are used for communication, the mobile unit comprising: means for storing a list of frequencies used by the 15 communications system; a diverse antenna arrangement having at least two antenna branches; means for controlling the mobile unit to take plural signal samples on each frequency in the stored 20 list, and for controlling the mobile unit to take the samples such that the signal samples for at least one of the frequencies include samples taken from at least two different antenna branches of the diverse antenna arrangement; and 25 means for using the signal samples to estimate the strength of the signal on each frequency.

17. The mobile unit of claim 16, further comprising means for tuning to each frequency on the list in turn; 30 and means for, while tuned to each given frequency, sampling the signal on that frequency as received from a first antenna branch of the diverse antenna arrangement, and for sampling the signal on the frequency as received via a second, different antenna branch of the diverse 35 antenna arrangement, before retuning to the next frequency on the list.

- 27 18. A radio unit, comprising: a diverse antenna arrangement having at least two antenna branches; and means for controlling the radio unit to take plural 5 signal samples on each of a plurality of different radio frequencies, and for controlling the radio unit to take the samples such that the signal samples for at least one of the frequencies include samples taken from at least two different antenna branches of the diverse lo antenna arrangement.

19. A computer program element comprising computer software code portions for performing the method of any one of claims 1 to 10 when the program element is run on 15 data processing means.

20. A method of operating a radio unit substantially as hereinbefore described with reference to the accompanying drawing.

21. A radio unit substantially as hereinbefore described with reference to the accompanying drawing.