WO2009099367A1

WO2009099367A1 - Power control method for cs over hspa

Info

Publication number: WO2009099367A1
Application number: PCT/SE2008/050825
Authority: WO
Inventors: Hans Hannu; Per Synnergren; Daniel ENSTRÖM
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2008-02-05
Filing date: 2008-07-02
Publication date: 2009-08-13

Abstract

In a method and mobile station for transmitting speech data using a speech codec enabled to code coding speech data at varying speech rates, the transmission power is set in a power controlstate in response to a current signal quality measure and in a rate control state wherein speech transmission is controlled by switching the speech rate in response to the signalqualitymeasure and the transmission power is kept essentially constant Using the method will prevent power control based on the signal quality such as the SIR target when instead it would be better to reduce the used transport block size, i.e. reduce the speech codec rate. As a result of the improved use of radio resources, the voice capacity of a cellular system can be increased, while maintaining speech quality.

Description

Power Control method for CS over HSPA

TECHNICAL FIELD

The present invention relates to a method and a device for controlling power when transmitting Circuit Switched (CS) data.

BACKGROUND

Cellular Circuit Switched (CS) telephony was the first service introduced in the first generation of mobile networks. Since then CS telephony has become the largest service in the world.

Today, it is the second generation (2G) Global System for Mobile Communication (GSM) network that dominates the world in terms of installed base. The third generation (3G) networks are slowly increasing in volume, but the early predictions that the 3 G networks should start to replace the 2G networks already a few years after introduction and become dominating in sales has proven to be wrong.

There are many reasons for this, mostly related to the costs of the different systems and terminals. But another reason may be that the early 3G networks was unable to provide the end user the performance they needed for IP services like e.g. web surfing and peer-to-peer IP traffic. Another reason may also be the significantly worse battery lifetime of a 3G phone compared to a 2G phone. Some 3G users actually turn of the 3G access, in favor for the 2G access, to save battery. Later 3 G network releases includes High Speed Packet Access (HSPA), HSPA enable the end users to have bit rates that can be compared to bit the rates provided by fixed broadband transport networks like Digital Subscriber Line (DSL). Since the introduction of HSPA, a rapid increase of data traffic has occurred in the 3G networks. This traffic increase is mostly driven by lap-top usage when the 3 G telephone acts as a modem. In this case battery consumption is of less interest since the lap-top powers the phone.

After HSPA was introduced, battery consumption became a focus area in the standardization. This lead to the opening of a working item in the 3rd Generation Partnership Project (3GPP) called Continuous Packet Connectivity (CPC). This working item aimed to introduce a mode of operation where the phone could be in an active state but still have reasonably low battery consumption. Such state could for instance give the end- user a low response time when clicking a link in a web page but still give a long stand by time.

The features developed in the CPC working item were successfully included in the 3GPP Release 7 specifications. But, the gain of CPC could only be utilized when running HSPA. This means that battery lifetime increase cannot be achieved for users using the CS telephony service.

In order to be able to increase the talk time of CS telephony another working item has been open that aims to make CS telephony over HSPA possible.

From a high-level perspective a CS over HSPA solution can be depicted as in Fig. 1. An originating mobile station connects via HSPA to the base station NodeB. The base station is connected to a Radio Network Controller (RNC) comprising a jitter buffer. The RNC is via a Mobile Switching Center (MSC)/Media Gateway (MGW) connected to an RNC of the terminating mobile station. The terminating mobile station is connected to its RNC via a local base station (NodeB). The mobile station on the terminating side also comprises a jitter buffer.

In the scenario depicted in Fig. 1, the air interface is using Wideband Code Division Multiple Access (WCDMA) HSPA, which result in that:

- The uplink is High Speed Uplink Packet Access (HSUPA) running 2 ms Transmission Time Interval TTI and with Dedicated Physical Control Channel (DPCCH) gating.

- The downlink is High Speed Downlink Packet Access (HSDPA) and can utilize Fractional Dedicated Physical Channel (F-DPCH) gating and Shared Control Channel for HS-DSCH

(HS-SCCH) less operation, where the abbreviation HS-DSCH stands for High Speed Downlink Shared Channel.

- Both uplink and downlink uses Hybrid Automatic Repeat Request (H-ARQ) to enable fast retransmissions of damaged voice packets.

The use of fast retransmissions for robustness, and HSDPA scheduling, requires a jitter buffer to cancel the delay variations that can occur due to the H-ARQ retransmissions, and scheduling delay variations. Two jitter buffers are needed, one at the originating RNC and one in the terminating terminal. The jitter buffers use a time stamp that is created by the originating terminal or the terminating RNC to de-jitter the packets.

The timestamp will be included in the Packet Data Convergence Protocol (PDCP) header of a special PDCP packet type. A PDCP header is depicted in Fig. 2.

In communication systems based on some cellular radio communication standards e.g. Code Divisional Multiple Access (CDMA), power control is used to meet the desired quality of service targets. The power control may be implemented both in the user equipment to meet the downlink quality target and also in the base station to meet the uplink quality target. It is important that the power control is able to maintain the desired quality of service target despite varying radio conditions, which is often the case in wireless communication systems.

In for example CDMA systems, the inner loop power control, also called fast power control, runs every time slot, which is typically less than lms (e.g. 0.67 ms in WCDMA). In

WCDMA the inner loop power control runs in both uplink and downlink. The fast inner- loop power control adjusts the transmit power of the sender towards a specific Signal to Interference and noise Ratio (SIR) target at the receiver. The aim of the uplink and downlink inner loop power controls is to counter the effect of fast fading, while maintaining the desired SIR target. During every slot the user equipment estimates the SIR on some known reference or pilot symbols and compares it with some SIR target corresponding to the given service (e.g. Block Error Rate (BLER), certain Bit Error Rate (BER) requirements and spreading factor used etc.).

The aim of the outer loop power control is to adjust the SIR target value used by the inner loop power control as previously explained, while maintaining a certain link quality. The quality target (e.g. BLER of the data) is set by the network and is expected from the user equipment to consistently maintain this target to ensure the desired quality of service is met throughout the session. Due to the varying radio link conditions e.g. user mobility, fast fading etc, the mapping between the SIR target and BLER changes over time. This is a key point as it requires frequent adjustment of the SIR target to maintain the desired quality value, e.g. BLER. This mechanism of adjusting the SIR target is also referred to as outer loop power control, quality control or outer loop scheme.

In systems such as enhanced uplink (EUL) version of WCDMA, the outer loop power control is configured to fulfill a quality target based on number of transmission attempts i.e.: "after x targeted transmissions, the residual block error rate should be y %". If the transmission is not successfully decoded after the stipulated maximum number of transmissions, the SIR target is increased by some value, e.g. 0.5 dB. For every successfully decoded transmission, the corresponding SIR target is decreased, for example by a factor inversely proportional to the error probability, e.g. about 0.01 dB if the error rate is 2%. The uplink outer loop power control for enhanced uplink channels adjusts the uplink Dedicated Physical Control CHannel (DPCCH) SIR target so the residual error rate after the stipulated maximum number of transmissions is fulfilled.

The transmission of data over the air in a wireless communication system is performed by using a plurality of different physical channels, for example Dedicated Physical Control CHannel (DPCCH), Dedicated Physical Data CHannel (DPDCH), Enhanced Dedicated Physical Control CHannel (E-DPCCH) and Enhanced Dedicated Physical Data CHannel (E- DPDCH). The power consumptions of these are related to each other by power offsets, e.g. β-values or gain factor relative the power level of the DPCCH.

An exemplary state-of-art implementation of the outer loop power control for EUL WCDMA increases the DPCCH SIR target when the number of transmission attempts is larger than TA target.

The increase in SIR target will cause the user equipment to use more power in order to fulfill the target. In certain high load situations this may be undesired as the power increment will increase the interference level in the system. The increased interference may eventually lead to that the system will not allow more user into the system, and the user already in the system will have worsen quality. Also, for a speech user the increased interference may lead to lost or damaged speech frames.

Hence there exists a need for an improved method of transmitting speech data. SUMMARY

It is an object of the present invention to provide an improved method for transmitting speech data.

This object and others are obtained by the method, User Equipment and radio system node as set out in the appended claims. Thus, by transmitting circuit speech data using a speech codec enabled to encode speech at varying bit-rates, wherein the transmission power is set in a power control state in response to a current signal quality measure transmission can be improved. This is obtained by switching to control transmission in a rate control state wherein speech transmission is controlled by switching the speech rate in response to the signal quality measure and the transmission power is kept essentially constant in response to some predetermined event. The event can for example be that a measured signal quality is within some predetermined range or some other indication that it is better to control the speech transmission using speech rate control rather than power control.

The invention also extends to a User Equipment and a Radio system node such as a Radio Network Controller adapted to transmit speech data in accordance with the above method.

Using the invention will prevent power control based on the signal quality such as the SIR target when instead it would be better to reduce the used transport block size, i.e. reduce the speech codec rate. As a result of the improved use of radio resources, the voice capacity of a cellular system can be increased, while maintaining speech quality.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which:

- Fig. 1 is a general view of a system used for packeized voice communication,

- Fig. 2 is a view of a Packet Data Convergence Protocol (PDCP) header, - Fig. 3 is a view of an exemplary User Equipment in accordance with the present invention,

- Fig. 4 is a view of an exemplary Radio Network Controller in accordance with the present invention,

- Fig. 5 is a flowchart illustrating different procedural steps performed when transmitting speech data, and

- Fig 6 is a view of an exemplary scenario.

DETAILED DESCRIPTION

The AMR codec and other codecs support different rates of coded speech, i.e. the speech is coded with different amount of bits. This is useful as it allows for more bits to channel coding, typically used in GSM, or smaller transport block which is useful in HSPA as smaller transport blocks requires less power to reach a given SIR. This can be used to maintain a given speech quality but still maintaining a certain system voice user capacity as a lower codec mode may perform equally or better at a higher Block Error Rate (BLER) than a higher codec mode at a lower BLER.

As has been identified by the inventors a SIR target increase in the uplink for example the HSPA EUL may cause a service to decrease in quality. A decreased service quality may in turn cause a drop in capacity. In accordance with the present invention the signal quality measure such as the SIR target is prevented to be changed during some circumstances and in particular when it is determined that a changed transport block size will provide more efficient transportation. Hence the present invention provides for a changed speech codec rate instead of a SIR target adjustment when some predetermined conditions are fulfilled.

In Fig. 3 an exemplary User Equipment (UE) 300 in accordance with the present invention is depicted. The UE 300 comprises a speech codec 301. The UE 300 further comprises a state controller 303 adapted to switch between at least two defined Outer Loop Power Control states. Also the UE 300 comprises a power control module 305. The state controller 303 is connected to the speech codec 301 and the power control module 305. The output from the module 301 determines how a changed SIR target is used to control transmission from the UE. The output from the state module 303 can be:

- A state where the OLPC commands are generated in a conventional way as described above and the transmission power is set in response to the signal quality such as the SIR target, or

- A state where speech codec rate switching is performed while keeping the transmission power essentially constant.

When in the state where speech codec rate switching is performed the SIR target for an UE can be frozen at its existing value resulting in an essentially constant transmission power. In accordance with another embodiment the SIR target is set to a pre-defined value or it may be updated using normal procedures when the UE is in a state where speech codec rate switching is performed. If this approach is chosen the SIR target can change, which will lead to a decrease or increase in transmission power on average during a time period until the new SIR target is reached (steady state).

The invention can also be used in the downlink by a system node of a cellular radio system. For example the codec rate switching decision can be placed within a Radio Network Controller RNC, as that node being responsible for the possible transcoding and the jitter buffer handling, in for example CS voice over HSPA. Also, the RNC is the common placement of the OLPC algorithm. In Fig 4 a Radio Network Controller 400 is depicted. The RNC 400 comprises a speech codec 401. The RNC 400 further comprises a state controller 403 adapted to switch between at least two defined Outer Loop Power Control states. Also the RNC 400 comprises a power control module 405. The state controller 403 is connected to the speech codec 401 and the power control module 405. The output from the module 401 determines how a changed SIR target is used to control transmission in the downlink. .The output from the state module 403 can be: - A state where the OLPC commands are generated in a conventional way as described above and the transmission power is set in response to the signal quality such as the SIR target, or

Speech codec rate switching can be performed in various ways. In Fig. 5 a flow chart illustrating steps performed when executing power control including Speech codec rate switching in a User Equipment in accordance with exemplary embodiments of the present invention. First in a step 501 the UE is in a conventional state where the transmission power is set in response to the SIR target. For example the UE may be using AMR4.75. Next in a step 503 it is checked if the SIR target has changed to a value below some predefined value X or above some predefined value Y. If the SIR target has not exceeded the value Y or is not below the value X, the procedure stays in the state with conventional power control where the transmission power is set in response to the SIR target and the procedure returns to step 501. If the SIR target has exceeded the value Y or is below the value X, the procedure continues to a step 505.

In step 505 the procedure switches to a state involving speech codec rate switching. Optionally in step 505 the SIR target is frozen or set to a predefined value. The procedure then continues to a step 507. In step 507 the speech codec rate is set in response to the underlying SIR target. For example, when in the speech rate switching mode, the currently used speech codec is AMR7.95 and the underlying SIR target increases above a predetermined value the speech codec is switched to another speech codec for example AMR4.75. In the speech rate switching mode the transmitter switches between at least two speech codec rates. In accordance with one embodiment the transmission power is kept essentially constant while in the speech rate switching mode and a changing underlying SIR target is accounted for by switching to another codec rate. The speech rate switching mode can advantageously be used in SIR target ranges close to a border between different codec rates.

Next in a step 509 it is checked if the underlying SIR target is below a predetermined value V. If the underlying SIR target is below such a predetermined value V the procedure leaves the speech rate switching mode and returns to the conventional dedicated state and the procedure returns to step 501. If the underlying SIR target is not below such a predetermined value V the procedure continues to a step 511. In step 511 it is checked if checked if the underlying SIR target is above a predetermined value Z. If the underlying SIR target is not above such a predetermined value V the procedure remains in the speech rate switching mode and the procedure returns to a to step 505. If the underlying SIR target is above such a predetermined value Z the procedure leaves the speech rate switching mode and returns to the conventional dedicated state and the procedure returns to step 501.

In Fig 6 the procedure in Fig. 5 if further exemplified. For purposes of the present example it is assumed that a The UE is in a conventional dedicated state and the SIR target is decreasing and it uses, at reference numeral 1. The SIR target is continuously decreasing and at a threshold X a codec rate switching up to AMR7.95 is performed and the OLPC goes into a speech rate switching state, at reference numeral 2. The used SIR target can then be frozen to value X or set to a predefined value such as V. If the underlying SIR target is continuously decreasing and reaches a threshold V, at reference numeral 3, the UE OLPC is changed to a conventional dedicated state. However, if the underlying SIR is instead increasing and reaches threshold Y at reference numeral 4 a codec rate switching down to 4.75 is performed in accordance with the present example.

If the underlying SIR target is then continuously decreasing and reaches threshold value Z, at reference numeral 5, the UE OLPC is changed to a conventional dedicated state. However, if the underlying SIR target is instead decreasing the process repeats, and the procedure will again be at reference numeral 2. In accordance with one embodiment the used SIR target can be set to the next threshold value in the direction that the SIR target is going instead of freezing the used SIR target to the threshold value until the OLPC state is changed. Further, the value of the SIR thresholds may be different and for example depend on if a codec rate up- or down switch is eminent or has been performed recently or not.

Using the method and system as described herein will prevent power control based on the signal quality such as the SIR target when instead it would be better to reduce the used transport block size, i.e. reduce the speech codec rate. As a result of the improved use of radio resources, the voice capacity of a cellular system can be increased, while maintaining speech quality.

Claims

1. A method of transmitting circuit switched speech data using a speech codec enabled to encode speech at varying bit-rates, wherein the transmission power is set in a power control state in response to a current signal quality measure characterized by the step of: - in response to a predetermined event switching (503, 505) to control transmission in a rate control state wherein speech transmission is controlled by switching the speech rate in response to the signal quality measure and the transmission power is kept essentially constant.

2. The method according to claim 1, wherein the signal quality measure is a Signal to Interference Ratio, SIR target.

3. The method according to claim 1 or 2, wherein the rate control state is entered when the signal quality measure has changed to a value below some predefined value X or above some predefined value Y.

4. The method according to any of claims 1 - 3, wherein the signal quality measure is frozen or set to a predefined value.

5. The method according to any of claims 1 - 4, wherein the speech is coded using an Adaptive Multi Rate, AMR, codec.

6. A User Equipment (400) adapted to transmit circuit switched speech data using a speech codec enabled to encode speech at varying bit-rates, the user equipment further being adapted to set the transmission power in a power control state in response to a current signal quality measure characterized by:

- means (403) for switching to a rate control state in response to a predetermined event, wherein speech transmission is controlled by switching the speech rate in response to the signal quality measure and wherein the transmission power is kept essentially constant.

7. The User Equipment according to claim 6, wherein the signal quality measure is a Signal to Interference Ratio, SIR target.

8. The User Equipment according to claim 6 or 7, further comprising means for entering the rate control state when the signal quality measure has changed to a value below some predefined value X or above some predefined value Y.

9. The User Equipment according to any of claims 6 - 8, further comprising means for freezing the signal quality measure is frozen or setting the signal quality measure to a predefined value.

10. The User Equipment according to any of claims 6 - 9, comprising an Adaptive Multi Rate, AMR, codec for coding the speech.

11. A control node(500) in a cellular communication system adapted to control transmission of circuit switched speech data using a speech codec enabled to encode speech at varying bit-rates, the control node further being adapted to set the transmission power in a power control state in response to a current signal quality measure characterized by: - means (503) for switching to a rate control state in response to a predetermined event, wherein speech transmission is controlled by switching the speech rate in response to the signal quality measure and wherein the transmission power is kept essentially constant.

12. The node according to claim 11, wherein the signal quality measure is a Signal to Interference Ratio, SIR target.

13. The User Equipment according to claim 11 or 12, further comprising means for entering the rate control state when the signal quality measure has changed to a value below some predefined value X or above some predefined value Y.

14. The node according to any of claims 11 - 13, further comprising means for freezing the signal quality measure is frozen or setting the signal quality measure to a predefined value.

15. The node according to any of claims 11 - 14, comprising an Adaptive Multi Rate, AMR, codec for coding the speech.