GB2341049A

GB2341049A - Mobile terminal transmission power control

Info

Publication number: GB2341049A
Application number: GB9818385A
Authority: GB
Inventors: Anthony Peter Hulbert
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1998-08-25
Filing date: 1998-08-25
Publication date: 2000-03-01
Anticipated expiration: 2018-08-25
Also published as: GB9818385D0; GB2341049B

Abstract

A mobile radio system comprising a base station arranged for communication with a plurality of mobile terminals wherein the system includes, 'closed loop power control' to control the RF transmission power of the mobile terminals in dependence upon signal to noise ratio as determined at the base station and wherein immediately prior to the activation by the base station of a new mobile terminal user, a signal to noise ratio demand signalled to existing mobile terminal users, is temporarily increased thereafter to be reduced again consequent upon activation by the base station of the said new mobile terminal user.

Description

IMPROVEMENTS IN OR RELATING:r TO MOBILE RADIO SYSTEMS This invention

relates to mobile radio systems and more 1 especially but not exclusively it relates to Code Dix.1sion l.lultiple. 2Xccess (CDi'\L\), cellular mobile radio systems Code Di,,,isioii.?\,lultiple Access, cellular mobile radio systems, having several mobile terminals in communication with a base station, commonly use 'closed loop power control' to control the transmitted radio signal power of each mobile terminal so that the 'signal to interference plus noise ratio' of signals received at the base station, which will herein after be called 'signal to noise ratio' for simplicity, is z c z ' '.

the minimum ratio required to support the service. The required RF transmitter power for a given mobile terminal is a function of path loss bet,Afeen that mobile terminal and its base station, as well as total interference at that base station. The interference at a base station, is in turn a function of the number of currently ac live mobile terminals communicating through that base station and to a smaller degree, the number of currently active mobile terminals communicating through surrounding base stations.

a call in progress from a mobile terminal is 'torn down', the required RF power for each of the remaining mobile terminals to maintain a prescribed signal to noise ratio at the base station is reduced slightly. This is because the interference introduced by the 'tom down ' mobile terminal is no longer present. Similarly, when a i-ie,\, mobile 1 terminal call is set up, the power required from the other existing mobile terminals will increase due to the additional interference contribution of the nes\. mobile terminal. As traffic loading approaches the maximum capacity of a system, this RF power change can become quite significant.

In a syl, stem such as UNITS supporting a mixture of bit rates, Yhe effect of a new high bit rate user becoming active under conditions of high traffic loading, could be to require a -\,-,,ery significant increase in RF power from existing users in order to maintain for each existing user, a prescribed signal to noise ratio as measured at the base station.

- Because closed loop power control cannot operate instantaneously, the signal to noise ratio requirement for the other users will be temporarily unsatisfied. This could result in transmission frames following the acti-, i.,,ation of a new user ha-ving uncorrectable errors. In the case of services requiring low 'frame error rate' (FER), this could seriously adversely affect performance and in order to provide for this eventuality, it might be necessary to introduce a significant signal to noise ratio margin for normal operation, so that a required signal to noise ratio can be maintained under all circumstances. This, in turn, would lead to a loss in handling capacity of the system.

It is an important object of the present invention to provide a mobile radio system wherein these problems associated with system operation at or approaching the limits of handling capacity are largely obviated.

According to the present invention a mobile radio system comprises a base station arranged for communication with a pluraliq, of mobile terminals wherein the Sy stem includes, 'closed loop power control' to control the RF transmission power of the mobile terminals in dependence upon the signal to noise ratio of signals received therefrom as determined at the base station, and wherein immediately prior to the activation by the base station of a new mobile terminal user, a signal to noise ratio demand signalled to existing mobile terminal users in accordance with which mobile terminal RF transmission power is set, is temporarily increased thereafter to be reduced again consequent upon activation by the base station of the said new mobile terminal user.

As will hereinafter be explained, in this system, it is arranged that the increase in interference resulting from activation of a new mobile terminal user will compensate for the reduction in signal to noise ratio requirement in such a way that actual RF transmission power required from the existing mobile terminals will stay substantialI3, constant during a transition period whilst the new mobile terminal is acth.-ated.

One embodiment of the invention M11 now be described by wa, of example only with reference to the accompanying drawings in which:

Figure I is a generally schematic block diagram of a known CDNLA mobile radio base station arrangement; Figure 2 is a generally schematic block diagram of a known CDNLA mobile terminal arrangement; Figure 3 is a graph showing the power increase requirement of mobile terminal users in the known system of Figure 2 as the number of mobile terminal user numbers increases, Figure 4 is a generally schematic block diagram of a CD,NL-,-\ base station arrangement in accordance with one embodiment of the invention; and Figure 5 is a timing diagram.

Referring now to Figure 1, a base station of a CDNLA mobile radio system in communication with several mobile terminals, which in combination are normally described as a cell, comprises separate transmit and receive antennas 1, and 2. Communications traffic for the mobile terminals of the cell is fed on a line 3 via a multiplex/ modulate unit 4 and an amplifier 5 to the transmit antenna 1. In order to control the RF power transmitted by individual mobile terininals of the cell, RF signals received therefrom at the antenna 2, are fed to a signal processor 6 which provides on lines 7,8 and 9, complex base-band signals, one for each mobile terminal in the cell, which are fed to a plurality of code units, only three of which are shown for simplicity of description and convenience, designated 10, 11, and 12. Since all of the code units are similar, for convenience, the code unit 10 only is shown in detail and it comprises a despreading demodulator 13, a signal to noise measurement unit 14, a comparator 15 and a coding switch 16. The signal to noise measurement unit 14 sen,7es to compare received energy after despreading on a line 13a with received energy before despreading on a line 13b, wl- dch provides a measure of the signal to noise ratio. The measured signal to noise ratio is fed on a line 14a to the comparator 15 and compared with a predetermined threshold signal N'thl., on a line 14b. Although the threshold used in all code units may be the same, in practice the threshold might be different for each mobile terminal. Thus in code unit 10 the threshold Vth I is used, 7th2 is used and so on, so that the whereas in code unit 2 a threshold N1 code unit 12, which is the Nth code unit, uses a threshold VthN. The comparator unit 15 is arranged to set the sxNritc h 16 to a binary '1' or '0' in dependence upon whether or not the signal to noise ratio as indicated on the line 14a is greater than, or equal to a required signal to noise ratio, as indicated b3, the threshold signal on the line 14b. The binary signal produced b, the switch 16 is fed on a line 17 to the multiplex/modulate unit 4 wherein it is included with signals for transmission to the mobile terminal to which it relates, wherein it is used to control signal transmission power so as to effect 'closed loop C power control', whereby signal to noise ratio is maintained at the minimum value necessary in order to maintain good error free communications.

It will be readily appreciated that the binary power control signal produced by the switch 16 is transmitted to the appropriate mobile terminal for each of the received signals. If direct sequence spread spectrum CDNIA is also used for the down link, then the power control bit for each for each mobile terminal is multiplexed into the down link transmission using the spreading code applicable to that mobile terminal, this operation being carried out in the multiplex./modulate unit 4.

6- Referring now to Figure 2, each mobile terminal comprises a transmit, recei-x7e antenna 18, received signals from which are fed to an RF signal processor 19 wherein it is converted to complex base-band. The base-band signal is fed to demultiplexer 20 wherein relevant power control bits are demultiple\ed from the appropriate spread spectrum signal and applied to a binar, to bipolar converter 2 1, accumulated in an accumulator 22 and scaled in a multiplier 23 which is fed on a line 24 with a signal providing a required scaling factor. An output control signal from the multiplier 23 is fed to a gain controlled amplifier 2 5, to which signals for transmission are fed on a line 2 6, the gain controlled amplifier 25 being arranged to feed the antenna 18 via an appropriate non- returnable junction device 27. Thus as hereinbefore described, closed loop RF power control is automatically effected.

In order to facilitate a better understanding of the system, overall operation of automatic RF power control will now be considered.

Consider a simple case in which all terminals require the same despread signal to noise, 'y'. If the processing gain is 'PU, the received power from the 'i th' mobile terminal is 'Pi ' and the received noise is 'M and if there are 'n' users then the total interference will be given by:

n + pi Ordinarily a signal does not interfere with its own reception but with spread spectrum signals in multipath, a reasonable assumption is that a signal does interfere with its own reception to the same extent as any other signal. The signal to noise ratio at the output of the despreader will be:

I PG. Pi Power control operates to set this to the required signal to noise ratio, Thus:

Y= (PG. Pi This will work to set all the recehed powers to the same value, P and we will hax7e:- 3-PG. P/ (N+nP) which we can easily solve for P, since:

P=(j\.,/PG),,' [1-(ny"PG)] The maximum number of users is given when P goes to infinity, i.e. when n = PG,"Y. Let this number given by n max = PG/y. We can then easily show that:

P = N,'(n max- n) As an example, consider a system capable of supporting 100 users (i.e. n max= 100). We can plot P as a function of n. If we let N = I then we are plotting the received signal relative to the receiver noise, as shown in Figure 3, wherein it can be seen that the signal power rises sharply as the number of users approaches the maximum capacit, of,the system.

In order to avoid this, it is proposed that, during a transmission frame immediately preceding the activation of a new mobile terminal, the signal to noise ratio for all mobile terminals be temporarily increased by a suitable factor, as hereinbefore described with reference to Figure 4. At the instant of activation of the new mobile terminal., this increase is removed. However, the increase in interference resulting from the activation of the new mobile terminal will compensate for the reduction in signal to noise ratio target in such a way that the actual transmitter powers required from each of the mobile terminals will stay substantially constant over the transition.

The required temporary global increase in signal to noise ratios across the cell can be computed as follows:

where n is the number of active users, yi is the Eb/No normally, required for the ith user, Bi is the bit rate of the i th user, and the n + 1 th user is the user about to be activated. In this way the total power before the activation of the n + lth will correspond to the power required after that user has been activated. It will be appreciated that no modifications are required in the mobile terminal and the required additions to the base station are shown in Figure 4.

The call management processor 2 8, shown in Figure 4, provides an additional output, the 'Threshold Adjustment', which serves to alter all of the thresholds by the appropriate factor as computed above.

For a better understand of the timing of operations, reference will now be made to Figure 5. A call from a new mobile terminal is due to start operating in a Frame N+1. Therefore the threshold is increased throughout a Frame N. At the end of the Frame N the threshold is reduced so that the signal to noise ratio required of all the mobile terminals in the cell is returned to normal but the mobile terminals' powers will have been increased. As the new call starts, it will generate increased interference which will be approximately, matched by- the effect of the increased mobile terminals' powers.

Claims

1. A mobile radio system comprising a base station arranged for communication with a pluralit, of mobile terminals wherein the system includes, 'closed loop power controlto control the RF transmission power of the mobile terminals in dependence upon the signal to noise ratio of signals received therefrom as determined at the base station and wherein immediately prior to the activation by the base station of a new mobile terminal user, a signal to noise ratio demand signalled to existing mobile terminal users in accordance with which mobile terminal RF transmission power is set, is temporarily increased thereafter to be reduced again consequent upon activation by the base station of the said new mobile terminal user.

2. A mobile radio system as claimed in Claim 1, wherein the system operates in accordance with a Code Division Multiple Access principles.

3. A mobile radio system as claimed in Claim 1 or Claim 2, wherein the base station includes a call management processor which is operative to monitor the number of active mobile terminals in the cell of which it forms a part, and in dependence thereon modifies the signal to noise ratio demand signalled to existing mobile terminal usersP when a call is to be set up for a new mobile terminal, by a factor calculated in accordance with the traffic contribution expected from the new user and the total number of active mobile terminals in the cell.

4. A mobile radio system as claimed in Claim 1 and substantially as herein described with reference to the accompanying drawings.