WO2002065666A1

WO2002065666A1 - Method and system of throughput control of shared channels

Info

Publication number: WO2002065666A1
Application number: PCT/SE2002/000229
Authority: WO
Inventors: Andreas Andersson; Patrik Karlsson
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2001-02-12
Filing date: 2002-02-08
Publication date: 2002-08-22
Also published as: SE0100476D0

Abstract

A method and system of dynamic throughput control allocating transmission resources on the basis of shared channel throughput in cellular mobile radio systems.

Description

Method and system for througput control of shared channels.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to cellular mobile radio systems, and more especially to common or shared channel throughput in such a system, particularly in a Universal Mobile Telecommunications System, UMTS.

BACKGROUND AND DESCRIPTION OF RELATED ART

Use of shared or common channels, particularly for signaling purposes is known from prior art . A shared channel or common channel is shared between, or common to, several users. A dedicated channel is dedicated to a particular user or a particular purpose. A shared channel or common channel can be used for transmission of user data. When a user has traffic or data to send it can be transmitted on the shared channel if there is idle capacity in the shared resource.

International patent application WO0038351 utilizes the interrelationship between transmission power, bit energy and transmission bit rate for dedicated user data channels on a shared transmission medium.

U. S. Patent US6067458 relates to power control when transmission data rate changes. Power control operates in one of two modes and the receiver can be notified in advance of the data rate to be used.

European patent application EP0886389 discusses the interrelationship between transmission power, spreading factor and data rate.

European patent application EP0876008 discusses the interrelationship between spreading factor and data rate. None of the cited documents reveal a method and system of throughput control, estimating shared channel throughput and dynamically adjusting transmission power or spreading factor and related throughput of the shared channel.

SUMMARY OF THE INVENTION

A shared channel or common channel is shared between, or common to several users. A dedicated channel is dedicated to a particular user or a particular purpose. Transmission of traffic or data on the shared channel is advantageous to simplify and reduce the system resources allotted for channel allocation. Data is queued until it can be transmitted. Consequently, queuing time depends on traffic load and throughput of the shared channel.

According to prior art the shared channel is set to a fixed size in terms of data rate. This setting may be re-configured. If the shared channel capacity is set to a high bit rate, the capacity of other channels or resources reduces correspondingly. If, on the other hand, channel size is insufficient for actual data rates, transmission delay will be excessive.

When propagation and interference conditions vary the shared channel throughput and delay will vary as well.

Consequently, an object of this invention is to dynamically allow for throughput preserving measures of a shared chan- nel.

It is also an object to dynamically adjust shared channel capacity on the basis of estimated throughput demands.

A further object is to introduce transmission power control on basis of shared channel throughput. Finally, it is an object to introduce means for adaptively varying the capacity or corresponding throughput settings, of the shared channel in relation to throughput demands, and actual throughput and transmission delays.

These objects are met by estimating shared channel throughput and dynamically adjusting transmission power or spreading factor of the shared channel in relation to available resources .

Preferred embodiments of the invention, by way of examples, are described with reference to the accompanying drawings below.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 displays an exemplary code tree and corresponding spreading factors, according to prior art.

Figure 2 illustrates a flowchart of a first preferred embodiment of the invention.

Figure 3 shows a flowchart of a second preferred embodiment of the invention.

Figure 4 shows a flowchart of a third preferred embodiment of the invention.

Figure 5 shows a network element, according to the invention, for practicing the method of figures 2 and 3.

DESCRIPTION OF PREFERRED EMBODIMENTS

Data traffic tends to occur in bursts. In a cellular radio communications system, the traffic time-variation in a cell can be substantial. Consequently, it is difficult to foresee the required capacity for data traffic of a particular cell when the system is set up. Also, the demand of an in- dividual user as regards throughput and delay will vary over time.

This invention simplifies and reduces the system resources allotted for channel allocation and yields a stable air-in- terface load control. A single shared traffic channel is used for data traffic in downlink. According to prior art solutions, the shared channel is of a predetermined size, in terms of capacity, and channel structure. Data is queued until it is transmitted. Queuing time will depend on traffic load and throughput at a given shared channel capacity. In prior art solutions, capacity of the shared channel is not determined adaptively over time on basis of amount of data traffic in a cell.

Problems of prior art solutions, as indicated above, are solved by allowing for different shared channel capacity settings that can be adaptively varied, the adaptation being based upon actual throughput and delay, thereby not occupying more system resources than necessary and dynamically making freed channel resources available to other (dedicated) channels.

In Code Division Multiple Access, respective digital user signals are separated by individual spreading codes. Each code represents a spreading factor. Sub-channels or lower rate channels are separated by sub-codes with a greater spreading factor. Figure 1 illustrates the interrelations between data rates/sub-channels and sub-codes/spreading factors. It displays a binary code tree where a spreading factor, SF, equal to one occupies the entire code tree, i.e. all levels indicated by SF=2 , SF=4, SF=8 and SF=16. A spreading factor equal to one translates into the highest user data rate. A sub-code with a spreading factor equal to two occupies only half the tree, the leftmost sub-tree or the rightmost sub-tree, the respective sub-tree occupying the levels indicated by SF=4, SF=8 and SF=16 preventing other users from using the corresponding codes. Similarly, if a user is assigned a sub-code corresponding to a spread- ing factor equal to four, he occupies a sub-tree (and corresponding sub-channels) originating from the level indicated by SF=4 down to SF=16. Spreading factor four represents a medium user data rate. A user allocated a channel of spreading factor 16 occupies the lowest data rate in this exemplary figure. Spreading codes for SF=1, SF=2 and SF=4 are denoted at the root of the corresponding sub-tree. This code tree is only an example not limiting the scope of the invention.

Total transmitted downlink power reflects a margin to con- gestion. A low spreading factor corresponds to a high bit rate. The lower the smallest spreading factor available the larger the sub-tree of a code tree available. Correspondingly, for a power-limited channel, a bit rate increase implies a requirement for transmission power to in- crease, in order not to increase the error rate.

This patent application distinguishes three categories of "throughput" :

- desired throughput, i.e. throughput corresponding to dynamically reserved resources for the shared channel;

- demand for throughput on the shared channel, i.e. transmission resources corresponding to a maximum delay; and

- actual throughput achieved on the shared chan- nel. According to a first embodiment of the invention, as schematically illustrated in figure 2, throughput demand and channel quality is measured and shared channel capacity is adjusted accordingly by setting the channel spreading fac- tor and transmission power level of the channel, as available. The objectives, and characteristics valid also for the first embodiment of the invention, are explained in further detail in relation to a second and a third embodiment of the invention.

According to the second embodiment of the invention, for which a flow chart is depicted in figure 3 , shared channel actual throughput, as adjusted in relation to estimated demand, is maintained at a desired level during channel variations by using power control where transmission power is controlled on basis of actual shared channel throughput. Actual throughput, TP, is measured. The measurement data could be averaged over a time span. Desired throughput of the shared channel is determined in relation to spreading factor and transmission power available. At a given spreading factor, if actual throughput is less than desired, DSCH transmission power is increased provided that total downlink transmission power is less than an upper power limit of the downlink, and the transmission on the downlink shared channel is less than the upper power limit of the channel. The upper power limit depends on how power transmission is implemented. If there is a dedicated transmitter for the shared channel there will be an upper power limit for the particular channel. If transmitters are shared for several downlink channels there will be a total upper power limit for these downlink channels. The power control must not increase transmission power beyond any upper power limit. In a reversed situation where actual throughput, TP, exceeds desired throughput, it is advantageous to decrease transmission power of the shared channel in order to reduce interference to other users or channels and maintain capacity of real time services. However, transmission power must not be decreased below a minimum transmission power level for the users to be able to legibly receive shared channel data, including a margin for variations in transmission path loss and interference level, in accordance with system specifications.

Figure 4 shows a flow chart describing a third embodiment of the invention. Shared channel actual throughput, as ad- justed in relation to estimated demand, is maintained at a desired level during channel variations by adjusting spreading factor of a spreading code. Actual throughput, TP, is measured. The measurement data could be averaged over a time span. Desired throughput of the shared channel is determined in relation to spreading factor and transmission power available. At a given transmission power, if actual throughput is less than desired, DSCH spreading factor is decreased provided a smaller spreading factor is available. In a reversed situation where actual through- put, TP, exceeds desired throughput, it is advantageous to increase the spreading factor of the channel in order to free resources to other users/channels and maintain capacity of real time services.

As a substitute for actual throughput an estimated demand for throughput can be used. Such an estimate can be achieved from measurements on the total amount of buffered data related to transmissions on the shared channel.

Figure 5 shows a radio communications network element according to the invention. The network element instruments the invention as described above. The network element includes means 1 for measuring or estimating shared channel throughput. It also has means 2 for measuring, or receiv- ing information on, channel quality. For evaluation purposes means 3 retrieves information on available spreading factor or transmission power resources. Processing means 4 evaluates the various measured, estimated or retrieved pa- rameters and adjusts the spreading factor and transmission power in accordance with a desired throughput demand. Transmission power is increased or spreading factor is reduced, if actual throughput is less than desired, and there are free resources available. If not immediately avail- able, resources can in some cases be freed by handing over traffic or users to other channels. If actual throughput is greater than a target level, transmission power will be reduced or spreading factor will be increased so as to reduce excessive use of resources. However, resources will not be released to such a large extent that minimum requirements for operation are jeopardized. Means related to adjustment of transmission power and spreading factor are preferably located in the same network element. However, the invention also covers network elements related to adjustment of only one of transmission power and spreading factor.

The invention is not intended to be limited only to the embodiments described in detail above. Changes and modifications may be made without departing from the invention. It covers all modifications within the scope of the following claims .

Claims

1. A method of dynamic throughput control of a shared channel in a radio communications system, the method c h a r a c t e r i z e d i n that it comprises the following steps:

- shared channel throughput demand is measured or estimated;

- channel quality is measured or channel quality information is received;

- availability of at least one of spreading fac- tor and transmission power is determined; and

- at least one of spreading factor and transmission power of the shared channel is adjusted in accordance with shared channel throughput demand, channel quality and availability of spreading factor and transmission power.

2. The method according to claim 1 c h a r a c t e r i z e d i n that shared channel throughput demand is measured or estimated by time-averaging actual throughput.

3. A method of dynamic throughput control of a shared channel in a radio communications system, the method c h a r a c t e r i z e d i n that it comprises the following steps:

- a desired throughput level is determined or received;

- actual shared channel throughput is measured or estimated;

- if the measured or estimated actual throughput is less than the desired throughput level, a first action of control is undertaken; and - if the measured or estimated actual throughput is greater than the desired throughput level, a second action of control is undertaken;

4. The method according to claim 3 c h a r a c t e r - i z e d i n that the first action of control comprises augmentation of shared channel transmission power if total downlink power is less than an upper downlink transmission power limit and shared channel transmission power is less than an upper shared channel transmission power limit.

5. The method according to claim 3 or claim 4 c h a r a c t e r i z e d i n that the second action of control comprises reduction of shared channel transmission power if shared channel transmission power is greater than a lower shared channel transmission power limit.

6. The method according to claim 5 c h a r a c t e r i z e d i n that the lower shared channel transmission power level depends on channel quality.

7. The method according to any of claims 3-6 c h a r a c t e r i z e d i n that the first action of control comprises maintenance of shared channel transmission power if not the total downlink power is less than an upper downlink transmission power limit and the shared channel transmission power is less than a shared channel transmission power limit.

8. The method according to any of claims 3-7 c h a r a c t e r i z e d i n that the second action of control comprises maintenance of shared channel transmission power if shared channel transmission power is not greater than a lower shared channel transmission power limit.

9. The method according to any of claims 3-8 c h a r a c t e r i z e d i n that the first action of control comprises selection of a spreading code, for which the spreading factor is smaller than current spreading factor, if such code is available or can be made available.

10. The method according to any of claims 3-9 c h a r a c t e r i z e d i n that the second action of control comprises selection of a spreading code, for which the spreading factor is greater than current spreading factor, if such code is available or can be made available.

11. The method according to any of claims 3-10 c h a r a c t e r i z e d i n that the first action of control comprises maintenance of the spreading code if no spreading code of smaller spreading factor is available.

12. The method according to any of claims 3-10 c h a r a c t e r i z e d i n that the second action of control comprises maintenance of spreading code if no spreading code of greater spreading factor is available.

13. The method according to any of the preceding claims c h a r a c t e r i z e d i n that the shared channel throughput demand is measured or the shared channel actual throughput is estimated by measuring queuing time or amount of buffered data for the shared channel.

14. A network element of dynamic throughput control of a shared channel in a radio communications system, the network element c h a r a c t e r i z e d b y means for:

- measuring or estimating shared channel throughput demand;

- measuring channel quality or receiving channel quality information; - determining information on availability of at least one of spreading factor and transmission power; and

- adjusting at least one of spreading factor and transmission power of the shared channel in accordance with shared channel throughput demand, channel quality and availability of spreading factor and transmission power.

15. A network element of dynamic throughput control of a shared channel in a radio communications system, the network element c h a r a c t e r i z e d b y means for:

- determining or receiving a desired throughput level;

- measuring or estimating actual shared channel throughput;

- undertaking a first action of control if the measured or estimated actual throughput is less than the desired throughput; and

- undertaking a second action of control if the measured or estimated actual throughput is greater than the desired throughput;

16. A radio communications system c h a r a c t e r i z e d b y means for carrying out the method in any of claims 1-13.