GB2343331A

GB2343331A - Base station signals to switch mobile terminals out of a power saving mode to ensure that its own power budget is not exceeded

Info

Publication number: GB2343331A
Application number: GB9826569A
Authority: GB
Inventors: Anthony Peter Hulbert
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1998-10-27
Filing date: 1998-12-04
Publication date: 2000-05-03
Anticipated expiration: 2018-12-04
Also published as: GB9826569D0; GB2343331B

Abstract

If a cell (10 in fig.1) is supporting the maximum number of calls for the capacity of the cell it is essential that the performance of mobile terminal receivers (30-40, fig.1) is high to minimise interference. However, if the system is not fully loaded, mobile terminals may operate with degraded receiver performance, interference still remaining low. In this system if the base station (20) is not operating at its overall power limit, it signals to individual mobile terminals that they may operate at a lower receiver performance and save their own battery power. This requires an increase in the power transmitted by the base station to those mobile terminals. Once, however, the base station reaches its power 'budget' (for example by additional mobile terminals entering the cell), the base station may require terminals in power saving mode to return to operate at full power.

Description

IMPROVEMENTS IN OR RELATING TO MOBILE TELECOMMUNICATIONS SYSTEMS The present invention relates to improvements in or relating to mobile telecommunications systems, and is more particularly concerned with such systems having power save operation modes.

The capacity of a code division multiple access (CDMA) system is limited by interference between the different signals using the same frequency. This is known as multiple access interference. This interference has properties, similar, but not identical to noise. The capacity of the system is therefore maximised by providing receiver demodulators that achieve the best practical performance in this type of interference. As such CDMA systems are generally based on direct sequence spread spectrum (DSSS) and the radio paths are multipaths, a near optimum receiver could be a so-called'rake'receiver which is well known to persons skilled in the art. A'rake'receiver effectively comprises a plurality of spread spectrum receivers, each receiver receiving the output of a different radio path. Typically, the outputs are maximal ratio combined to provide the overall decision variable. Within the receiver architecture, accurate tracking of code and carrier phase is required to optimise performance. Many other factors have also to be included in order to optimise performance, for example, signal path linearity and matched filter accuracy.

All of the above factors contribute to the power consumed by both the radio frequency (RF) path and by the digital baseband processing in the receiver. In the initial deployment of universal mobile telecommunication system (UMTS) which is based on CDMA, power consumption, and therefore battery life, is likely to be a problem area for the mobile terminals. In the longer term, advances in digital technology are expected to alleviate these problems considerably although power consumption is still likely to be an issue for the higher bit rate services, particularly those involving the transmission of data on multiple codes in parallel.

In a fully loaded system, that is, a system which is supporting the maximum possible number of calls for the services in use (defined by the capacity of the system) at a particular time, it is essential that all mobile terminal receivers operate with a performance for which the capacity was defined. If this does not happen, excessive interference will result and either the bit error rate (BER) requirements will not be satisfied or calls will need to be dropped, that is, the capacity of the system will not be met.

However, when the system is not fully loaded, it would be possible for the mobile terminals to operate with somewhat degraded receiver performance since the level of multiple access interference is lower.

It is therefore an object of the present invention to provide an improved mobile terminal which operates in a'power save'mode in accordance with the loading of the communications cell of which it forms a part.

It is another object of the present invention to provide a base station which controls the operation mode of mobile terminals in its cell in accordance with the loading of the cell.

It is a further object of the present invention to provide a telecommunications cell in which the base station and terminals thereof operate in accordance with the loading of the cell.

In accordance with one aspect of the present invention, there is provided a base station for a telecommunications cell characterised by having a downlink transmitter power'budget'and in that the base station has signalling means for signalling mobile terminals in the cell to substantially ensure that the power'budget'is not exceeded.

Advantageously, the signalling means provides'power save' mode signalling to switch the mobile terminals into and out of'power save'mode in accordance with the power'budget'.

Additionally, the base station may further include ranking means for producing a ranked list of mobile terminals in the cell in ascending order of the power each mobile terminal requires in'normal'mode. The ranked list is updated when a call is set up or torn down.

The base station may further include comparison means for comparing the total power required by the mobile terminals in the cell in 'normal'mode with the power'budget'and for effecting'power save' mode signalling of the mobile terminals so that each mobile terminal, in turn, is switched into'power save'mode until all the mobile terminals are in'power save'mode or the power'budget'is reached, whichever happens first.

The signalling means may utilise a broadcast channel for the telecommunications cell.

The power'budget'may be variable. This has the advantage of allowing for'cell breathing'.

In accordance with another aspect of the present invention, there is provided a mobile terminal for a telecommunications cell having a 'power save'mode, characterised in that the mobile terminal is switched into and out of'power save'mode in accordance with signalling information received from the base station of the cell.

It is preferred that the mobile terminal further includes indicating means for indicating to the base station that it has a'power save'mode.

In accordance with a further aspect of the present invention, there is provided a telecommunications cell comprising a base station and a plurality of mobile terminals as described above.

A mobile terminal which is operating in a'power save'mode has reduced performance characteristic and consumes less power than when it is not operating in the'power save'mode. The'power save'mode being selectable when the power'budget'of the base station has sufficient spare power capacity so that the base station can transmit at a sufficient power level in order to support the'power save'mode of the terminal without the base station exceeding its transmit'power'budget. The reduced performance characteristic requires a higher signal to noise plus interference (SNI) ratio to be received at the mobile terminal.

The terminal is part of a communications system and selection of the'power save'mode is additionally dependent upon a radio propagation path loss of the terminal in relation to radio propagation path losses of other terminals in the system. The terminal can enter the'power save'mode in preference to another terminal not having a'power save' mode and a smaller radio propagation path loss than the terminal.

The base station may signal either individually or broadcast to all terminals the instruction to enter or leave the'power save'mode.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which: Figure 1 illustrates a telecommunications cell; Figure 2 is a graph illustrating the power distribution for the telecommunications cell shown in Figure 1; Figure 3 is a block diagram illustrating a known power control subsystem for a mobile terminal and the base station in the telecommunications cell shown in Figure 1; Figure 4 is a block diagram illustrating the'power save'mode for a mobile terminal in accordance with the present invention; and Figure 5 is a block diagram illustrating'power save'control generation in the base station in accordance with the present invention.

The present invention provides a novel signalling capability for a base station in a telecommunications cell to mobile stations or terminals in the cell. Figure 1 illustrates a telecommunications cell 10 comprising a fixed location base station 20 and six mobile terminals 30, 32, 34, 36, 38, 40. It will readily be appreciated that a telecommunications cell may include different numbers of mobile terminals, and that the number of mobile terminals in the cell may vary with movement of the terminals into and out of the area defined by the cell. However, the present invention will be described with reference to six terminals in a cell.

As shown in Figure 1, each terminal 30,32,34,36,38,40 has a respective transmit/receive path 50,52,54,56,58,60 connecting it to the base station 20. In this particular cell arrangement, each transmit/receive path 50,52,54,56,58,60 has a different length due to the positioning of the respective terminals. However, it will readily be appreciated that the transmit/receive paths between the terminals and the base station may be constantly changing due to changes in the positioning of the terminals.

As discussed above, the terminals may also move into and out of an area covered by a particular cell. In the illustrated cell arrangement, terminal 32 is the closest to the base station 20, terminal 40 is the furthest away from the base station 20 and the other terminals 30,34,36,38 are at distances in between these two extremes.

In each cell, the base station 20 has a power'budget'. The power 'budget'is determined by two factors: a) the maximum power which the base station can output; and b) temporary restrictions in the maximum permissible output power below the maximum power which can be output (as in a) above), which are required to permit'cell breathing'.

If a particular cell is surrounded by heavily loaded cells, the base stations in the heavily loaded cells signal the relevant base station to reduce its power'budget'if it is necessary to avoid compromising traffic in any of the heavily loaded cells. This is known as'cell breathing'.

It will readily be appreciated that the concept of'cell breathing' can also be applied to sectoring where sector power'budgets'are considered. This becomes simple as, to a first order, a sector transmitter only affects communications in one or two neighbouring cells.

Generally, the power'budget'for a base station has to be allocated between all the terminals in the cell so that each terminal can receive and transmit with a satisfactory performance. In the illustrated example, each of the six mobile terminals 30,32,34,36,38,40 requires a different amount of power for a satisfactory performance due to the differences in distances from the base station 20. The distance of a terminal from the base station is one factor which affects the satisfactory performance of the terminal. Naturally, other factors may also affect the amount of power required to ensure satisfactory operation, for example, obstacles, such as hills or tall buildings, located along the transmit/receive path connecting any terminal with the base station, and other conditions which may interfere with signals passing between the terminal and the base station. However, for ease of explanation, it will be assumed that the only factor affecting satisfactory performance of a terminal within the cell is its distance from the base station.

The power'budget'for the base station 20 is illustrated graphically in Figure 2. Here, the maximum power which the base station can output is indicated by a dotted line, and a maximum power value of 1 COW is also given by way of example. For each of the terminals 30,32,34,36,38,40, the portion of the power'budget'which is being utilised is shown by the relevant block. In this case, the base station 20 is not operating at its maximum capacity. However, there may be instances where the base station 20 is required to output more power than its power'budget', for example, if more mobile terminals enter the cell or others already in the cell move further away from the base station.

For a fully loaded cell, all mobile terminals must operate with the performance for which the capacity of the cell was defined, otherwise the capacity will not be met. However, when the cell is not fully loaded, it is possible for the terminals to operate with somewhat degraded performance since the level of multiple access interference is lower. In accordance with the present invention, a signalling capability is provided from the base station to mobile terminals supported in the cell.

In one embodiment, the base station may set one or more thresholds of traffic loading, that is, the amount of power required to be output in accordance with the number of terminals in the cell at any one time and the power requirements of each terminal, for example, the distance each terminal is from the base station. When the traffic loading crosses one of these thresholds, the base station the mobile terminals supported in its cell.

If the traffic loading changes to a lower level, the base station signals the mobile terminals that they can now operate in a degraded reception performance mode.. If the traffic loading changes to a higher level, the base station signals the mobile terminals that they must now operate with in a normal reception performance mode. The broadcast channel for the cell can be used to effect signalling between the base station and the terminals so that the information can be communicated.

In response to signalling permitting a degradation in receiver performance, a mobile terminal may simplify the operation of its receiver in order to reduce its power consumption at the expense of degraded performance. Such simplifications may include, for example, reducing the number of active'rake'fingers.

Other simplifications, given by way of example and not exhaustive, may also include: a) shortening the receiver chip matched filter, b) making the receiver matched filter symmetrical thereby removing any compensation for a previous analogue filter's phase nonlinearity and allowing the number of multiplications to be halved using a pre-addition structure, c) reducing the update rate for a searcher for additional multipath components, d) reducing arithmetic word widths, e) reducing the oversampling rate for selecting for code tracking, and f) reducing the current in one or more of the amplifiers in the receive path.

As a mobile terminal estimates the signal-to-noise plus interference ratio (SNI) at its output and uses this to generate transmit power control (TPC) data, the effect of any of these changes will be to increase the transmit power requested from the base station. If the number of'rake'fingers is reduced then the order of diversity will also be reduced and an outer loop responsible for maintaining the required frame error rate will demand an increase in the output SNI. Either way, the degraded terminal receiver performance will result in an increase in the power transmitted from the base station to that terminal.

In accordance with the present invention, a base station should be allowed to increase its transmit power above the absolute minimum to one or more terminals on occasions whenever this will not compromise the ability of the cell to support the current traffic load.

In another embodiment, the base station may nominate particular mobile terminals whose performance is allowed to change at a given time and broadcast an appropriate signal to these individually. For example, when a mobile terminal moves closer to the base station, the power which the base station needs to transmit to that terminal will fall to a very small proportion of the total. If the performance of such a mobile terminal degrades and more power needs to be transmitted to it to achieve an acceptable performance, this may not significantly affect the total power output by the base station and may be acceptable.

Alternative signalling methods, such as regular confirmation of the current condition of each terminal within the cell, are not precluded by the above approach.

In further embodiment, the broadcast messages may be sent to the terminals in response to changes in the power output by the base station in relation to its power'budget'.

It may be the case that one or more of the terminals in a cell can switch to a'power save'mode where the terminal saves battery power.

The base station will then need to output more power to transmit to such terminals. In accordance with the present invention, the base station is provided with a facility for ranking mobile terminals in its cell in ascending order of the power which it is transmitting to each terminal in normal mode. The base station signals to each terminal, starting with the first, permitting it to enter'power save'mode. As each terminal enter 'power save'mode, the base station records the effect this has on its total transmitter power comparing it against its present power'budget'. If there is still a power margin, the base station allows the next lowest terminal in ranking to enter'power save'mode and so on until either all terminals are in'power save'mode or the power'budget'is reached.

By observing the effect of signalling terminals to go into'power save'mode, the base station can determine an average factor for the increase in transmitter power involved. Alternatively, a fixed global average figure can be used. In this way, the base station can compared the estimated'non power save'mode powers of terminals which are in 'power save'mode with those which are not. This can be done by dividing the power for terminals in'power save'mode by the average factor. As terminals move within a cell, by maintaining the ranked list for all terminals, it is possible to determine those terminals which are currently in'power save'mode but which should not be and vice versa.

When a call is set up or torn down, the ranked list can be updated and the appropriate'power save'mode signalling performed.

By way of example, for a cell having seven terminals, an'ideal' situation with regard to the power'budget'in respect of the ranking is shown in Table 1.

Table 1.'Ideal'situation.

Power (compensated) Terminal No. Power save? 0. 01 7 Y 0.03 3 Y 0.1 1 Y 0.3 2 Y ------------------------------------------------------------------------- 1.0 5 N 3.0 4 N 10.0 6 N Here, it can readily be appreciated that the terminals requiring the least power are in'power save', that is, terminals 1,2,3 and 7. However, this may not be the situation in the cell following the movement of terminals so that a'non-ideal'situation will arise as shown in Table 2.

Table 2.'Non-ideal'situation.

Power (compensated) Terminal No. Power 0. 01 7 Y 0.03 3 Y 0.1 1 N 0. 3 2 Y 1.0 5 N 3.04Y 10.0 6 N Here, terminals 2,3,4 and 7 are in'power save', but terminal 4 requires more power in'power save'than terminal 1 does. When the base station carries out its update of the rankings, it will signal to terminal 1 that it should go into'power save'and to terminal 4 that it should come out of'power save'.

It will readily be appreciated that for a given power'budget', the maximum number of mobile terminals which may operate in'power save'mode will depend on the ranking of each terminal by the base station. If the average increase in required base station transmitter power for a terminal operating in'power save'mode is, for example, 3dB, then the maximum number of ordered terminals which can be placed into 'power save'mode is the number which keeps the total transmitted power within the overall power'budget'for the ranked list. As the total power transmitted by the base station approaches its overall power'budget' (which may be variable according to the traffic activity levels in the surrounding cells as discussed above), the terminal receiving the highest transmitter power may be switched out of its'power save'mode. As the total power falls away from the overall'budget', the terminal receiving the lowest transmitter power out of those not currently in'power save' mode may be switched into its'power save'mode.

As mentioned above, the'power save'mode provides an option for mobile terminal manufacturers to increase their battery life. However, it will be appreciated that it is not intended that the provision of this mode should be a mandatory feature on a mobile terminal. It is, however, desirable that the base station should be able to distinguish between those terminals which do support a'power save'mode and those terminals which do not support a'power save'mode in order to be able to exclude the latter from the ranked list of terminals. In this way, only those terminals which support'power save'mode will be signalled to enter it.

The terminals may signal the base station indicating whether or not they support a'power save'mode either upon entering a cell or upon setting up a call in that cell.

A known power control subsystem for a base station and mobile terminal will now be described with reference to Figure 3. In Figure 3, a mobile terminal 100 is shown which comprises an antenna 110 for receiving signals from a base station 200 and for transmitting signals thereto as indicated by arrows'R'and'T'respectively. The mobile terminal 100 also comprises a duplexer 120 for receiving signals from the antenna 110 and for passing signals to the antenna for transmission.

When receiving, the duplexer 120 passes the incoming signals to a receiver 130. The receiver 130 then passes the received signal to a signal-to-noise estimator 140. The output from the signal-to-noise estimator 140 is an indication of the amount of noise present with the received signal and this forms one input to a comparator 150. A preset threshold signal is connected to provide the other input to the comparator 150. The comparator 150 compares the output from the estimator 140 with the threshold signal and provides an output signal when the output from the estimator is above that of the threshold signal. The output signal from the comparator 150 is then sampled at 160 to provide transmit power control (TPC) data for a multiplexer 170. In the multiplexer 170, the TPC data is mixed with other data to provide an output signal with is then passed to a modulator 180. The modulated signal is then amplified by amplifier 190 before being passed to the duplexer 120 prior to transmission.

Figure 3 also shows components of a base station 200. Base station 200 comprises a receiving antenna 210 which is connected to a receiver 220. The received signal is then passed to a demodulator 230 having TPC. The output from the demodulator 230 is passed to an accumulator 240, the output from which is used to control amplification of the signal source in adjustable amplifier 250. The amplified output is then transmitted by transmitting antenna 260.

In accordance with the present invention, receiver 130 in the mobile terminal 100 operates in a power save mode. Figure 4 illustrates a receiver 300 which has a power save mode. The receiver 300 comprises a down converter 310 connected to an analogue-to-digital converter 320.

The digitised signal is then filtered by filter 330 and passed to a'rake' finger arrangement 340,350,360. The'rake'finger arrangement comprises a plurality of'rake'fingers-only three of which are shown for clarity. The first'rake'finger 340 is connected to received the output from the filter 330 and passes a signal to one input of an adder 380. The second and subsequent'rake'fingers 350, 360 also provide inputs to adder 380. However, the second and subsequent'rake'fingers are only connected to adder 380 when enabled by power save control unit 370.

Adder 380 provides an output signal which is the sum of the inputs from the enabled'rake'fingers.

Figure 5 illustrates a flow diagram for power save control generation in a base station. In block 400, the base station ranks the terminals in order of the power transmitted to them. The ranking is adjusted to compensate for terminals which are in'power save'mode.

The adjustment is carried out by an average factor as described above.

The total power transmitted is estimated in block 410 and if it is less than a first threshold value, T ;, a message is broadcast to the mobile terminal requiring the least power and which is not in'power save'mode to switch to'power save'mode. If the total power transmitted is greater than a second threshold value, T2, a message is broadcast to the mobile terminal in'power save'mode which requires the most power to switch out of 'power save'mode. The second threshold value, T2, is greater than the first threshold value, Tu, but is less than the power'budget'for the base station.

If the total power required is less than the first threshold value, TI, then no action needs to be taken in respect of switching mobile terminals into and out of'power save'mode.

It will be appreciated that as the power utilised by the base station approaches its power'budget'value, there is a need to reduce the amount of power being output without disconnecting from any of the terminals where possible. This is effected by switching the terminal requiring the most power out of'power save'mode so that it requires less power from the base station. Similarly, as the power output by the base station falls from its power'budget'value, it is possible to utilise some of the spare capacity by switching the terminal requiring the least power into its 'power save'mode.

A further reduction in mobile terminal power consumption may be obtained by intelligently controlling the onset of so-called'soft handoff (SHO). In'soft handoff, a mobile terminal is receiving its signal from two or more base stations contemporaneously. This mode requires more operations in the baseband processing as it is necessary to run two or more rake receivers at the same time. If the onset of'soft handoff can be restricted to a smaller proportion of the cell when the cell is lightly loaded then again, the power consumption will be reduced.

Because it is likely that cells will be operating significantly below their maximum loading for the greater part of the time, the present invention should lead to considerable reductions in the average mobile terminal receiver power consumption and therefore improvements in the mobile terminal battery life.

It will readily be appreciated that a variable total downlink transmitter power'budget'as described above can also be used either with or without'power save'mode to implement a call admission regime.

If'power save'mode is being used, as the traffic loading increases, more and more terminals will be forced out of'power save'mode. Eventually, the power'budget'will be reached with all terminals not operating in 'power save'mode. At this point, no further call will be admitted. The ability to signal a variation in the power'budget'allows variable traffic loading priorities to be given to particular cells or groups of cells. For example, if there was a major accident in a particular cell, the power 'budgets'of the surrounding cells could be temporarily lowered to allow higher traffic to be supported in that cell.

Claims

CLAIMS : 1. A base station for a telecommunications cell characterised by having a downlink transmitter power'budget'and in that the base station has signalling means for signalling mobile terminals in the cell to substantially ensure that the power'budget'is not exceeded.
2. A base station according to claim 1, wherein the signalling means provides'power save'mode signalling to switch the mobile terminals into and out of'power save'mode in accordance with the power'budget'.
3. A base station according to claim 1 or 2, further including ranking means for producing a ranked list of mobile terminals in the cell in ascending order of the power each mobile terminal requires in'normal' mode.
4. A base station according to claim 3, wherein the ranked list is updated when a call is set up or torn down.
5. A base station according to claim 3 or 4, further including comparison means for comparing the total power required by the mobile terminals in the cell in'normal'mode with the power'budget'and for effecting'power save'mode signalling of the mobile terminals so that each mobile terminal, in turn, is switched into'power save'mode until all the mobile terminals are in'power save'mode or the power'budget'is reached, whichever happens first.
6. A base station according to any one of the preceding claims, wherein the signalling means utilises a broadcast channel for the telecommunications cell.
7. A base station according to any one of the preceding claims, wherein the power'budget'is variable.
8. A mobile terminal for a telecommunications cell having a 'power save'mode, characterised in that the mobile terminal is switched into and out of'power save'mode in accordance with signalling information received from the base station of the cell.
9. A mobile terminal according to claim 8, including indicating means for indicating to the base station that it has a'power save'mode.
10. A telecommunications cell comprising a base station according to any one of claims 1 to 7, and a plurality of mobile terminals according to claim 8 or 9.