WO2009099364A1

WO2009099364A1 - Method and device for jitter buffer control

Info

Publication number: WO2009099364A1
Application number: PCT/SE2008/050594
Authority: WO
Inventors: Daniel ENSTRÖM; Per Synnergren; Hans Hannu
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2008-02-05
Filing date: 2008-05-21
Publication date: 2009-08-13

Abstract

In control of a jitter buffer of a mobile station during transmission of circuit switched data in a cellular radio system the jitter buffer is controlled in response to radio layer measurements. Hereby an improved transmission can be obtained. In particular the carrier over interference, C/I measurement can be used as a measurement of the radio layer.

Description

METHOD AND DEVICE FOR JITTER BUFFER CONTROL

TECHNICAL FIELD

The present invention relates to a method and a device for controlling a jitter buffer of a mobile station during transmission of circuit switched data in a cellular radio system.

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 3 G 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 3 G phone compared to a 2G phone. Some 3 G users actually turn of the 3 G access, in favor for the 2G access, to save battery.

Later 3G 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 DSL. Since the introduction of HSPA, a rapid increase of data traffic has occurred in the 3 G 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 the 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.

As is described above, there is a need to use dual jitter buffers to be able to run standard CS telephony over HSPA. One will be located in the RNC while the other is located in the receiving terminal. By the very nature of jitter buffers there is always a need to have some safety margin in the buffering depth in order to avoid excessive speech frame losses when an increase of the jitter magnitude is experienced. With dual buffers, dual such margins are in principle needed which will give an additional latency in the system compared to a single jitter buffer design. Traditional CS telephony has typically been transported using low-jitter dedicated channels where any jitter management scheme could be quite simple and non-adaptive since the channel was actually designed for low-jitter dedicated telephony transport. In the CS over HSPA case, the situation is different. The shared channels used in HSPA are by their very nature high jitter channels which makes CS transport challenging since an application such as CS telephony is designed to be used on low jitter dedicated channels.

In order to keep the end-to-end delay as low as possible in the CS over HSPA case, the depth of the respective buffer needs to be as low as possible, hence the requirements on the buffers are strict in terms of the adaptation of the buffering level. This problem has been addressed for Packet Switched (PS) transport using IP but not for CS transport.

For PS transport, the information in the packetization such as RTP time stamp and the measured arrival time can be used to estimate the current jitter level whereby the client end point can get a fairly reliable end-to-end estimation of the jitter. However, this information is not available for CS transport although a "simulated" Real-time Protocol time stamp (RTP TS) is present in the Adaptive Multi Rate (AMR) counter of the PDCP header as is seen in Fig. 2. However, even with that information, there is a problem to correctly adjust the jitter buffer level in a CS terminal, i.e. the jitter buffer in the receiving mobile station, in an optimized delay-minimized way.

Hence, there exists a need to minimize the mobile station (UE) jitter buffer level in a CS over HSPA terminal in order to keep the end-to-end delay as low as possible.

SUMMARY

It is an object of the present invention to provide an improved cellular radio system.

This object and others are obtained by the method and mobile station as set out in the appended claims. Thus by controlling a jitter buffer of a mobile station during transmission of circuit switched data in a cellular radio system in response to radio layer measurement an improved transmission can be obtained. In particular the carrier over interference, C/I measurement can be used as a measurement of the radio layer.

In accordance with one embodiment of the present invention the jitter buffer depth is increased in response to a radio layer measurement indicating a worse radio channel.

In accordance with another embodiment of the present invention the jitter buffer depth is decreased in response to a radio layer measurement indicating a better radio channel.

Using the jitter buffer control method and mobile station as described herein enables utilization of the vertical integration of a CS service running over a packet data connection such as HSPA to control the jitter buffer level in the mobile station. By utilizing radio layer measurements such as C/I, the buffer adaptation algorithm can be proactive and hence also allow a smaller safety margin for late losses compared to other algorithms for determining the jitter buffer size/depth. The result will be a telephony service with lower end-to-end latency and reduced risk for media degradation due to too late arriving speech frames to the speech decoder.

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. l is a general view of a Circuit switched mobile connection,

- Fig. 2 is a view of a Packet Data Convergence Protocol (PDCP) header,

- Fig. 3 is a view of a controller for a jitter buffer, and - Fig. 4 is a flowchart illustrating procedural steps performed when controlling a jitter buffer of a mobile station.

DETAILED DESCRIPTION In accordance with the present invention radio layer measurements are used to control the jitter buffer in transmission of CS data over HSPA. The radio layer measurement can in accordance with one embodiment be the carrier over interference (C/I) level. In accordance with other embodiments other radio measurements such as Outer Loop Power Control (OLPC) settings including values of TTA and TTE may also be used.

For HSPA channels, radio layer measurements such as the C/I value in the receiver is an indirect estimator of the experienced jitter on the downlink since a low C/I indicates a need for more re-transmissions. In the AMR rate control case, a decrease in C/I translated into a lower media bit-rate used for the session with an increased level of redundancy (or channel coding) for the transmission. In the HSPA case, there are other alternatives to secure end- user quality.

A decrease in C/I can be met by increasing the tolerance for jitter, i.e. increasing the jitter buffer level in the receiver. Hence, instead of reducing the media bit-rate to provide more error robust coding, the buffering level can be increased to provide more jitter tolerant transmission.

An increase in jitter is normally managed by an adaptive jitter buffer by measuring the arrival rate of the data packets and reacting when that indicates an increase of re- transmissions, i.e. jitter. However, by using the C/I measurement or other radio measurements, this buffer increase can be made proactive. This means that the safety margin in the jitter buffer can be reduced in a way not possible where no radio parameters derived from radio layer measurements are taken into account when determining the target jitter buffer level. In accordance with one embodiment, a lower C/I do not automatically mean that the jitter will increase immediately with a certain amount, but indicates that the probability of re-transmissions have increased, hence also the probability for a jitter increase.

In accordance with the present invention, the control algorithm for a jitter buffer used in a mobile station also termed User Equipment (UE) in the CS over HSPA case is adapted to take into account radio layer measurements.

In Fig. 3 view of a receiver 300, such as a mobile station for CS over HSPA including a controller 301 for controlling a jitter buffer of a mobile station is depicted. The controller 301 in accordance with one embodiment receives input from a statistics module 303 and an operation parameters module 305. The operation parameters module 305 can provide information relating to loss rate, adaptation restrictions and similar. In addition the controller is connected to a module 307 providing information relating to radio measurements received via L1/L2 layer as indicated by block 309. The controller 301 is further connected to control a jitter buffer 311. In accordance with one embodiment the controller 301 also controls a speech decoder 313 and or a Time scaling unit 315. The time scaling algorithm unit 315 is a unit which can be replaced by any other algorithm with the purpose to manage the media aspects of jitter buffer adaptation. The receiver 300 also typically also comprises an audio output 317 for output of the received signal.

In accordance with one embodiment the jitter buffer control algorithm is adapted to take the information related to radio measurements into account when determining a proper buffering depth. The determination of the proper buffering level is one important aspect of a jitter buffer control algorithm and the actual mapping between the information available and the proper buffering depth is not deterministic.

By taking radio measurements into account fast adaptation control can be performed to handle jitter build-up resulting from a decrease in for example CIl. If radio layer measurement information is not taken into account it will either lead to an excessive buffering depth i.e. longer end-to-end delay or to speech frame losses (hence poor media quality) due to a too low buffering level.

In Fig. 4 a flow chart illustrating an exemplary implementation of a jitter buffer control algorithm for CS over HSPA is depicted. First in a step 401, the control algorithm is initiated. Next in a step 403 radio measurements data is obtained. The radio measurement data can in accordance with one embodiment comprise the previous C/I value, a C/I threshold value for triggering a jitter buffer investigation, a previous buffer target value, and a difference threshold value T for buffer target depth change.

Next, in a step 405, an updated C/I value is obtained via radio measurements. Next, in a step 407, the difference between the old C/I value and the new C/I value are compared to the C/I threshold value for triggering a jitter buffer investigation. If the difference calculated in step 407 is above the threshold value the procedure continues to a step 409. In step 409 a new buffer target level is determined. Next, in a step 411 the estimated jitter magnitude for the new C/I is stored. Next, in a step 413, the difference between the estimated jitter magnitude for the new C/I and the previous buffer target value is calculated as a difference value V. Next, in a step 415 it is checked if the value V is above the difference threshold value T for buffer target depth change. If in step 415 it is determined that the difference is not above the difference threshold value T for buffer target depth change the procedure continues to a step 417.

In step 417 the new buffer target level is set to the same as the old buffer target level. The procedure then continues to a step 421. If in step 415 it is determined that the difference is above the difference threshold value T for buffer target depth change the procedure continues to a step 419. In step 419 the new buffer target level is set to the estimated jitter magnitude for the new C/I. The procedure then continues to a step 421. In step 421 the buffer target for the jitter buffer is set to the new buffer target. Next, in a step 423 the buffer target is stored and the procedure ends in a step 425.

If the difference calculated in step 407 is not above the threshold value the procedure continues to a step 408. In step 408 in step the buffer target for the jitter buffer is set to the old buffer target and the procedure continues directly to step 423.

In accordance with one embodiment the actual mapping between the change in the C/I and the anticipated jitter increase can be set to depend on the C/I value. In accordance with one embodiment the mapping is not linear between C/I change and anticipated jitter magnitude.

Using the jitter buffer control method as described herein enables utilization of the vertical integration of a CS service running over HSPA to control the jitter buffer level in the mobile station. By utilizing radio layer measurements such as C/I, the buffer adaptation algorithm can be proactive and hence also allow a smaller safety margin for late losses compared to other algorithms for determining the jitter buffer size/depth. The result will be a telephony service with lower end-to-end latency and reduced risk for media degradation due to too late arriving speech frames to the speech decoder.

Claims

1. A method of controlling a jitter buffer (311 ) of a mobile station (300) during transmission of circuit switched data in a cellular radio system, characterized by the steps of:

- obtaining (405) measurement of the radio layer, and - controlling (423) the jitter buffer in response to the radio layer measurement.

2. The method according to claim 1, characterized in that the measurement of the radio layer is a measurement of the carrier over interference, C/I measurement.

3. The method according to any of claims 1 or 2, characterized by the step of:

- increasing (419) the jitter buffer depth in response to a radio layer measurement indicating a worse radio channel.

4. The method according to any of claims 1 or 2, characterized by the step of: - decreasing (419) the jitter buffer depth in response to a radio layer measurement indicating a better radio channel.

5. The method according to any of claims 1 - 4, characterized in that the transmission of circuit switched data is transmitted using a packet data connection in particular a High Speed Packet Access, HSPA, connection.

6. A mobile station (300) adapted to receive circuit switched data in a cellular radio system, characterized by:

- means (307) for obtaining measurement of the radio layer, and - means (301) for controlling the jitter buffer in response to the radio layer measurement.

7. The mobile station according to claim 6, characterized in that the measurement of the radio layer is a measurement of the carrier over interference, C/I measurement.

8. The mobile station according to any of claims 6 or 7, characterized by:

- means (301) for increasing the jitter buffer depth in response to a radio layer measurement indicating a worse radio channel.

9. The mobile station according to any of claims 6 or 7, characterized by:

- means (301) for decreasing the jitter buffer depth in response to a radio layer measurement indicating a better radio channel.

10. The mobile station according to any of claims 6 - 9, characterized in that the mobile station is adapted to receive circuit switched data via a packet data connection in particular a High Speed Packet Access, HSPA, connection.