GB2402579A

GB2402579A - Method of communication in cellular communication systems

Info

Publication number: GB2402579A
Application number: GB0312971A
Authority: GB
Inventors: Anthony Peter Hulbert
Original assignee: Siemens AG; Roke Manor Research Ltd
Current assignee: Siemens AG; Roke Manor Research Ltd
Priority date: 2003-06-06
Filing date: 2003-06-06
Publication date: 2004-12-08
Anticipated expiration: 2023-06-06
Also published as: GB2402579B; GB0312971D0; WO2004112276A1

Abstract

A method of communication in a cellular communication system, the method comprising setting up uplink and downlink dedicated channels (DCH) for data transmission between one or more mobile stations and a base station; wherein after setup or the end of data transmission, the downlink DCH is removed; wherein the or each mobile station reduces its transmission power on the uplink DCH; wherein the base station provides a common broadcast power control channel (BPCCH) to the or each mobile station; wherein the power level from each mobile station is monitored and modifications of the power level from the or each mobile station are requested via the BPCCH as necessary to maintain a minimum power level, such that synchronisation between the or each mobile station and the base station is maintained.

Description

2402579:

A METHOD OF COMMUNICATION IN A CELLULAR COMMUNICATION

SYSTEM

This invention relates to a method of communication in a cellular radio communication system, in particular for 3r Generation mobile systems, such as Universal Mobile Telecommunications System (UMTS).

UMTS uses wideband code division multiple access (CDMA) technology.

CDMA systems have two requirements if there is a call in progress. The first is that the receiver needs to be synchronised to the transmitted signal and the second is that the transmit power needs to be controlled so that it is at the correct level. This means that the power should be sufficient to avoid producing an unacceptably high error rate, but not so high that the error rate is significantly lower than tolerable. Both of these set up requirements take a finite amount of time, but if the system is working as a burst mode transmission, the time taken for the set up can result in unacceptable latency.

There are two conventional solutions to these problems. The first is to put up with the delay in the set up, but for some applications, such as gaming this is totally unacceptable. The second is to leave the transmitter, from a mobile station of the system, constantly on. This removes the problem of delay caused by the set up procedure, but also ties up capacity unnecessarily. In an example of a system with a low duty cycle of the order of 10%, then the effect is to use 10 times the minimum required capacity, reducing the maximum number of users and so the possible revenue.

In accordance with the present invention, a method of communication in a cellular communication system comprises initially setting up uplink and downlink dedicated channels (DCH) for data transmission between one or more mobile stations and a base station; wherein after initial set-up or the end of each data transmission, the downlink DCH is removed; wherein the or each mobile station reduces its transmission power on the uplink DCH; wherein the base station provides a common broadcast power control channel (BPCCH) to the or each mobile station; wherein the power level from each mobile station is monitored and modifications of the power level from the or each mobile station are requested via the BPCCH as necessary to maintain a minimum power level, such that synchronization between the or each mobile station and the base station is maintained.

Preferably, the base station monitors the uplink DCH for an indication that the À '.. e.. a: e. À ce. amp: e. a: À . . . . mobile station wishes to transmit data.

Preferably, the indication that the mobile station wishes to transmit data comprises an increase in power level followed by transmission of an access message.

Preferably, the base station responds to the access message by sending a forward access channel signal to the mobile station, receives immediate confirmation of synchronization on the uplink dedicated channel and thereafter permits transmission of data from the mobile station.

Preferably, the communication system is a code division multiple access system.

Preferably, the communication system is a UMTS system, the base station is a Node B and the mobile station comprises user equipment.

Preferably, the power on the uplink DCH is reduced by increasing a spreading factor.

The present invention provides a solution to the need to maintain communication between the mobile station and the base station without tying up an excessive amount of resource unnecessarily. The broadcast power control channel operates continuously when the system is activated, but runs at significantly lower power than is required for normal data traffic. This enables synchronization and power control signals to be provided between the mobile station and the base station whilst no data is being transmitted, so that when the next data transmission request is received it can be acted upon immediately, without the delays caused by setting up the connection afresh.

An example of a method of communication will now be described with reference to the accompanying drawings in which: Figure 1 illustrates operation of one example of a conventional transition from Cell forward access channel (Cell_FACH) state to Cell dedicated channel (DCH) state; and, Figure 2 is a state diagram illustrating the method of the present invention.

The present invention is particularly applicable to, although not limited to, UMTS systems, so the following examples are described with respect to such systems,.

In UMTS, the base station is commonly known as Node B and the mobile station is known as user equipment (UP). Wideband CDMA or UMTS can operate in two basic ces.. .:. Àe:e . . . À modes. These are Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode. Within the FDD mode of UMTS, there is the need to send bursts of data. An emerging concept is known as the Enhanced Uplink Dedicated Transport Channel (EUDTCH). This provides transport of data from the user equipment to the Node B in this burst manner. In order to minimise interference to other users and thus to maximise capacity it is desirable that the transmissions between the UE and the Node B associated with the EUDTCH are substantially eliminated when there is no data to send. On the other hand, the applications that generate data which must be sent using this channel may require low latency. Thus, when there is data to transport again, it is necessary to re-establish the data channel rapidly.

Operation of the current, unenhanced uplink dedicated transport channel has two states, known as cell forward access channel (Cell_FACH) and cell dedicated channel (Cell_DCH). Conventionally, the transition from Cell_FACH state to Cell_DCH state is slow, taking up to one second. There are six phases to the set up of the channel for data transmission, as shown by Fig. 1. Available uplink access slots are derived in the random access channel (RACH), then a preamble is transmitted using the selected uplink access slot, signature, and preamble transmission power (steps 1 & 2).

If no positive or negative acquisition indicator is received at the UE in the acquisition indicator channel (AICH) corresponding to the selected signature (step 3), then steps 1 and 2 are repeated with increased power. If an acquisition indicator is received in step 3, then a random access message (of 10 or 20ms) is transmitted after the uplink access slot of the last transmitted preamble (step 4). The system then waits for a radio link control (RLC) reconfiguration in the FACH message (step 5) and thereafter a radio link (RL) synchronization phase is started. This RL synchronization phase may take up to Is and the data transmission can start only after the "in-sync" is reported to the higher layers, so there may be a significant delay in beginning transmission after the initial request.

The delay in the above procedure is caused by the time taken to ramp up the UE transmitter power and to control it so that its level is substantially as required. This time delay is needed because in a CDMA system it is essential that the transmitted power be minimised at all times in order to maximise capacity. On the other hand there is a minimum received signal power at the Node B for acceptable operation, so until that power is reached no data can be transmitted. In order to satisfy the conflicting r.e car re.' r r r < r requirements of maximising capacity and achieving the minimum power level for transmission, the UE estimates the approximate power that it will need to transmit to provide acceptable operation at a chosen Node B. This is based on, so called, open loop measurements where the UE uses its measurement of the downlink power, in conjunction with knowledge of the Node B transmitted power, to estimate the path loss between itself and the Node B. This estimate is then used in conjunction with knowledge of the interference environment at the Node B to estimate the median power, which should be transmitted. The UK, however, reduces its start transmitter power by several decibels below the estimated median power level in order to ensure that there is no danger of transmitting too much power because of either a change in the path loss, non-reciprocity of the uplink and downlink paths due to frequency difference, or measurement errors.

The power from the UE is ramped up in stages. When the received power at the Node B reaches the minimum acceptable, the Node B signals back to the UE on the AICH and the UE then holds its power constant. The UE then sends a message requesting set up of a DCH. The Node B responds by providing the necessary information on the FACH. The UE then transmits on an uplink DCH, but because the timing of this transmission is asynchronous, synchronization can take some time.

Moreover, after synchronization the power control must be closed around the uplink to allow the UE to adjust its power more accurately to match the minimum requirement.

This involves an outer control loop that adjusts the set point for the power control loop.

The outer control loop counts cyclic redundancy check (CRC) success and failures in order to set a target frame error rate. Inevitably this is a slow process.

The whole process may take from 200 ms to 1 second and the actual time at which the UE is allowed to transmit data is set to 1 second later to ensure that data is not lost under the worst case condition. However, this delay is too long for time critical services, such as gaming applications. This means that the only way to support time critical bursty services is to stay continuously in the Cell_DCH state, but this conflicts with the requirement to minimise interference, which would be done by switching to the Cell_FACH state when there is no data to send, so freeing up additional capacity.

Figure 2 is a state diagram illustrating how the present invention is able to overcome these problems by operating in a third state which might be termed Cell_sub- DCH state. In this state a broadcast power control channel (BPCCH) is provided which :ec:. te.e.. e. :e runs at a significantly lower power than the normal traffic channel, with sufficient power for synchronization and power control, but too low a power for data transmission, so optimising the conflicting requirements of reducing delay and reducing interference. Keeping the power low prevents the power control from causing interference, whereas the power can easily be scaled up to allow transmission of data traffic when required, without losing the required transmit power for the mobile station and having to go through the number of iterations conventionally used to regain power level and synchronization.

Following a cessation in data available to transmit (step A), the UE continues to transmit dummy data with reduced power on the uplink DCH channel and the downlink DCH channel is removed. Uplink power control is maintained, but not by the downlink DCH. Instead, the BPCCH is introduced and capacity assigned in the BPCCH for that UK. The BPCCH acts as a holding channel as part of a closed loop system which ensures that the necessary power level between all users that are in Cell_sub-DCH state IS and the base station is maintained, instead of having to use one channel between each user and the base station.

This approach has the benefit that downlink DCH channels are given up which reduces downlink interference and also returns spreading codes to the available pool for other channels. Furthermore, the uplink DCH transmit power is reduced by increasing the spreading factor as will be described below, an approximate required UE transmitter power is maintained during breaks in data transmission and the Node B receivers stay synchronized to the UK.

The base station receives a signal from the mobile station which consists of a number of spread spectrum bits. These bits are correlated in the base station receiver to obtain a score. This score is checked to see if it is above or below O to give a binary data decision. The base station will have prior knowledge of the message it would expect from a mobile station, so the correlation score allows the base station to measure the power and synchronization of the signal from the mobile station.

The spreading factor is increased for the uplink DCH whilst operating in sub DCH state. It is preferred that there is an integer relationship between spreading factor period and slot period. In FDD there are IS slots in 10 ms and the chip rate is 3.84 Mcps (mega chips per second). Thus the number of chips per slot is 2560. It is possible to transmit dummy data on the uplink DCH, so the Node B can correlate coherently 6 t 6 8 6 6 4 j 4 6 6 j 8 6 tl4 686 e 6 over any period it chooses since the dummy data is known at the Node B. In one embodiment the data is either all '1's or all '0's. The purpose of the Node B receiving the dummy data is to measure its power for the purposes of power control and to remain in synchronization.

The receiver is not limited to correlation over individual bits, but can add correlation scores together to get a higher spreading factor or to calculate over a longer time period, so it is a very flexible system. The mobile station transmits dummy data, which is already known to the base station node B. so that the base station correlates for any length of signal at its choice. The ability to correlate over a longer than usual time period allows transmissions to be made at a lower power.

The accuracy of the power control depends upon the accuracy of the power measurement at the receiver, the less power available, the less accurate the result. The Node B can measure the power in a number of different ways. It could simply average the square of the decision variable. Alternatively it could measure the power indirectly.

1 S The purpose of power control is to maintain a bit error ratio (BER). One approach of power control would be to measure the BER directly. As described in our UK patent No. 2292289 there is a method using analysis of bit error rate (BER), whereby a signal is sent back in each time period to indicate "error" or "no error". If there is an error, the mobile station transmitter will increase its power by a large amount, but if there is no error, then it decreases its power by a small amount. In this approach the Node B would measure errors. Now there is no coherent reference on the uplink DCH in this mode because the data is known. The output of the receiver correlator will be a sequence of phasors, one for each correlation period. These phasors will have a common, slowly moving (due, e.g., to Doppler spread) component on top of the noise component. The slowly moving component can be estimated by averaging. 'Errors' can then be detected as phasors whose phase departs from within + /2 of the estimated slowly moving component. The power control sub system then interprets 'correct' phasors as power reduction commands and 'errored' phasors as power increase commands.

With multiple mobile stations making use of the same channel to receive power control and synchronization signals from the base station, it is necessary to synchronise the signals in some way to ensure that the correct instruction goes to the correct mobile station. One possibility is to put the control data for a particular mobile station into a ee À ce he c. e À e e ee À À À Àe e e À À À À e e e specific point in the frame i. e. using time division multiple access (TDMA), but the bursty nature of the data transmissions to the mobile stations may generate errors in other channels due to downlink multiple access interference both intra and inter cell. It is necessary to prevent interference in users who are receiving data at that time.

The BPCCH is able to multiplex the power control signals in a number of different ways. The simplest approach is to time division multiplex the power control bits to each of the UEs in a sequential fashion. A framing code or pulse would define the start of each frame and each UE would be allocated the Nth bit where N is UE specific. There is a disadvantage in this approach in that, for lightly loaded cells, many of the bit positions would be empty whereas for heavily loaded cells there might be too few bit positions available. The lightly loaded channel would still generate interference during the transmissions which would adversely affect capacity. The overload problem could only be overcome by providing a high capacity channel, which would be wasteful of resources. The problem in this case could be overcome by pseudo randomly assigning bit positions to UEs in a time varying sequence so that the interference, either intra or inter cell, is averaged and therefore proportional to the number of UEs using the BPCCH. In this way the high potential capacity channel would only generate maximum interference when fully loaded. In addition it would be possible to lower the power control update rate when the loading of the BPCCH reached a high level (e.g. above 50% of capacity) .

In an alternative embodiment, the power control update rate is made adaptive and UE-specific. En this approach the BPCCH only signals a power control update to a UE when the power adjustment is needed at that UK. The BPCCH then needs to identify the UE concerned with a UE code prefacing (or associated with) the power control instruction (usually one bit). This approach is only attractive when the majority of Node B to UE paths are relatively static. It also has the disadvantage that it would be error prone since any errors could lead to the correct UE failing to adjust its power or, even worse, another UE adjusting its power arbitrarily.

When a UE in Sub-DCH state needs to transmit data (step B) it can indicate its desire to do so by increasing its uplink DCH transmitter power as necessary and transmitting an access message - effectively obtaining the equivalent of a RACH channel without using the actual RACH channel. For example, a predetermined increase of power of N dB would be known to the mobile station and this increase À À À . À À : : would be applied as soon as the mobile station had data to transmit. Note that for the purpose of this approach the Node B receiver, which is looking after the Sub-DCH signal, must also, simultaneously, be looking for any possible access messages. The Node B responds to the access messages by sending a FACH signal (step C) as in Fig. 1 and the procedure continues as before except that there is substantially no power control settling time or synchronization time. The Node B sets up the downlink DCH again and signals resumption of full DCH to the UE on the FACH. The UE detects the FACH signalling (step D) and data is transmitted on the DCH.

Claims

À: ': ace I. as: À À À À À À À À À À À À :e.:e À À. À. .e CLAIMS

1. A method of communication in a cellular communication system, the method comprising setting up uplink and downlink dedicated channels (DCH) for data transmission between one or more mobile stations and a base station; wherein after set up or the end of data transmission, the downlink DCHis removed; wherein the or each mobile station reduces its transmission power on the uplink DCH; wherein the base station provides a common broadcast power control channel (BPCCH) to the or each mobile station; wherein the power level from each mobile station is monitored and modifications of the power level from the or each mobile station are requested via the BPCCH as necessary to maintain a minimum power level, such that synchronization between the or each mobile station and the base station is maintained.

2. A method according to claim 1, wherein the base station monitors the uplink DCH for an indication that the mobile station wishes to transmit data.

3. A method according to claim 2, wherein the indication that the mobile station wishes to transmit data comprises an increase in power level followed by transmission of an access message.

4. A method according to claim 2 or claim 3, wherein the base station responds to the access message by sending a forward access channel signal to the mobile station, receives immediate confirmation of synchronization on the uplink dedicated channel and thereafter permits transmission of data from the mobile station.

5. A system according to any preceding claim, wherein the communication system is a code division multiple access system.

6. A system according to any preceding claim, wherein the communication system is a UMTS system, the base station is a Node B and the mobile station comprises user equipment.

7. A system according to any preceding claim, wherein the power on the uplink DCHis reduced by increasing a spreading factor.