GB2330486A

GB2330486A - Delay Control for Seamless Handover

Info

Publication number: GB2330486A
Application number: GB9721952A
Authority: GB
Inventors: Jonathan Alastair Gibbs
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1997-10-17
Filing date: 1997-10-17
Publication date: 1999-04-21
Also published as: GB9721952D0

Abstract

Smooth transitions of delay in the speech signal transmission of different speech encoders (codec) are facilitated as a mobile communication unit 110 roams from a first communication system 120 having a first speech encoder to a second communication system 130 having a second speech encoder. As the mobile communication unit 110 roams into the second communication system 130, the first communication system 120 will hand over the communication signal to the second communication system 130. The amount of delay in the differing speech paths of the two codecs in the handover is determined and a change of delay between the speech paths is measured. Once the change of delay is measured, the second codec determines if the change of delay is greater than a threshold. If so, the second codec gradually changes from the delay of the speech path of the first codec to the delay of the speech path of the second codec.

Description

DELAY CONTROL FOR SEAMLESS SPEECH HANDOVER Field of the Invention The present invention relates generally to quality of speech in a communication system, and in particular to reduction of delay contrast in a handover involving two speech connections with different end-to-end delays.

Background of the Invention In mobile communication systems, a mobile unit will be roaming about an area and communicating with base stations located throughout that area. The base stations transmit information to the mobile unit which transmits information to the base stations. Active two-way communications are being performed between the mobile unit(s) and the base stations using radio frequency (RF) signals.

Communication between mobile units and the base station rely upon speech encoders to transform speech in its analog form to data streams which can be modulated onto some RF carrier signal. The system requires that both the base stations and the mobile units include speech encoders (codecs) for transmission and reception of transmitted speech.

In order to overcome the effect of RF channel impairments between the base stations and mobile units, it is common to employ forward error correction codes (FEC) and limited interleaving. The FEC introduces additional redundency bits into the data path, adding structure to that data, prior to modulating the bits onto the RF carrier for transmission. In the ideal case, through FEC, the number and location of the errors in the speech path can generally be determined at the decoder and corrected.

However, in general, there will be periods when too many errors are introduced due to the signal fades for the FEC both detect and correct the data. In such cases, the FEC will not be effective. Since bursts of speech data modulated onto the RF carrier are so short (usually less than 20 ms in length), significant amounts of data is lost in the maximum fade periods. Accordingly, the FEC encoded speech burst is interleaved prior to modulation onto the RF carrier. However, inherent with the use of speech codecs, FEC and interleaving, is time delay between the time that the audio signal was encoded at one end of the speech circuit and the time that the signal is resynthesised at the decoder. Although the length of delay may seem insignificant, in communication systems a small amount of delay can result in significant loss in quality of speech, particularly in the presence of an echo path back to the source and hence it desirable to minimize the end-to-end delay if possible.

Some communication systems, such as UMTS (Universal Mobile Telephone System), allow movement between different types of systems, such as between cordless/short range type systems and cellular systems. When a user moves from one system to another, the systems must communicate and there must be a transfer from the first to the second. The transferring of a mobile unit from one system to another is called a hand over.

As an example of an entire system allowing roaming from one established system to another, the UMTS system is designed to allow a user to move about world-wide with a single mobile telephone unit and be able to communicate in any system in that area. The UMTS system also allows links to different systems in a single area, such as a cordless system and a cellular system. In this type of "universal" system, a user may begin a call on their cordless system and then begin moving out of range of that system and into a cellular system. In order to maintain the call, the call is handed over from one system to the other, such as from the cordless system to the cellular system. Similarly, the call can move from the cellular system back to the cordless system as the user moves into the range of the cordless area.

Most systems having varying design parameters for the codecs, FEC and interleavers which operate within them. It follows, then, that each system will have a different end-to-end delay for the audio signals between the calling parties. This creates a unique problem as the communication industry moves from mobile units capable of operating in only a single system to mobile units which range from system to system, such as the UMTS system. With each system having a different, even slightly different, end-to-end delay in their components, a user moving from one system to another system may perceive the delay change at a handover. However, the goal of UMTS and other such systems is to establish a smooth, non-detectable transition from one system to the other during handover so that the user is unaware of the handover. In addition, changes in delay from one codec to another can affect the operation of echo cancellation.

Brief Description of the Drawings FIG. 1 is a diagram of a mobile unit during a handover.

FIG. 2 is a diagram of delays from two different codecs.

FIG. 3 is a block diagram of a transceiver according to a preferred embodiment of the present invention.

FIG. 4 is a flow chart of the operation of the preferred embodiment of the present invention.

Detailed Description of a Preferred Embodiment The present invention provides a method and apparatus to allow smooth transitions of delay in the speech signal transmission of different speech paths (codec) as a mobile communication unit 110 (of FIG. 1) roams from a first communication system 120 having a first speech encoder to a second communication system 130 having a different end-to-end delay. As the mobile communication unit 110 roams into the second communication system 130, the first communication system 120 will establish a simultaneous call to the second communication system 130. The amount of delay in the two speech paths of the two systems is determined and a change of delay between the speech paths is measured. Once the change of delay is measured, the second codec determines if the magnitude of the change of delay is greater than a threshold. If so, the delay of the speech path is gradually changed from that of the first system to the delay of the second speech path.

FIG. 2 shows a simplified example of delays from two separate codecs. For explanation purposes, it will be assumed that delay 1 (210) is from the first communication system (120) having a first codec, and delay 2 (220) is from the second communication system (130) having a second codec. In this example, delay 1 (210) is assumed to be greater than delay 2 (220) such that a sudden change from delay 1 to delay 2 would cause a difference recognizable by a user. As mentioned earlier, the goal with handovers is to complete a handover without the user noticing a difference or change. For the delays of the example, then, there needs to be a gradual change from the delay of speech path 1 (210) to the delay (220) of speech path 2.

FIG. 3 shows a block diagram of a transceiver 300 receiving a handover from base station 120 to base station 130 according to a preferred embodiment of the present invention. Transceiver 300 comprises delay measure 310, comparator 320, filter control 330, polyphase all pass filter (APF) 340, and a by-pass 350. The explanation of transceiver 300 will be in conjunction with the explanation of the operation of the present invention as shown in FIG. 4.

The delay in the data stream of both speech path 1 and speech path 2 is determined (410). With these delays, delay measure 310 measures the change (delta) of delay between delay 1 and delay 2 (420). Comparator 320 then compares the change of delay to a predetermined threshold (430). If the magnitude of the change of delay does not exceed the predetermined threshold (440), the signal is transmitted along by-pass 350 and delay 2 is transmitted (450).

If the magnitude of the change of delay between delay 1 and delay 2 does exceed the predetermined threshold (440), filter control 330 determines if delay 1 is greater than delay 2 (460). If delay 1 is greater than delay 2, filter control 330 directs all pass filter 340 to match the delay of system 1 (470). A seamless handover from base 120 to base 130 is now performed since the two delays are equalised (480). Once the handover has been performed, APF 340 adjusts the additional delay to delay 2 gradually in decreasing amounts until the total added delay equals zero (490).

If delay 2 is greater than delay 1 (460), filter control 330 directs APF 340 to gradually increase an additional delay to delay 1 (462) until the total end-to-end delay for the two systems are equal. A seamless handover from base 120 to base 130 is now performed (464).

When echo cancellation is operating, APF 340 makes the transition from one code to the other much slower than is done when echo cancellation is not operating.

The method described above for carrying out the present invention can be stored on a storage medium such as a storage diskette, hard drive, or other medium for storing electronic operations.

By gradually changing the delay from one codec to another in a hanover situation, a user will not be able to notice the handover and will not be able to detect the change from one to the other.

What is claimed is:

Claims

CLAIMS:

1. A method comprising the steps of: determining an amount of delay of differing speech paths in a handover; measuring a change of delay between the speech paths; determining if the change of delay is greater than a threshold; gradually increasing a delay of a first of the speech paths until equal to a delay of the second speech path before performing the handover when the delay of tne first of the speech paths is less than the delay of the second speech path; and matching the delay of the second speech path to the delay of the first speech path, performing the handover and gradually reducing the additional delay to zero.

2. A method according to claim 1 wherein the step of determining if the change of delay is greater than a threshold comprises comparing the change of delay to a maximum delay for echo cancellation.

3. A method according to claim 1 wherein the step of determining if the change of delay is greater than a threshold comprises comparing the change of delay to a threshold of detection.

4. A method according to claim 3 wherein the step of comparing the change of delay to a maximum delay comprises comparing the change of delay to a threshold for human perception.

5. A transceiver comprising: a comparator comparing a change of delay between different speech encoders to a threshold; and a filter coupled to the comparator, the filter adjusting the delay of a speech path to equalise it with that of a second if the change of delay exceeds the threshold.

6. A transceiver according to claim 5 wherein the transceiver further comprises a filter control, the filter control controlling adjustments by the filter.

7. A transceiver according to claim 5 wherein the transceiver further comprises a by-pass wherein the filter is bypassed from the comparator if the change of delay does not exceed the threshold.

8. A system comprising: first and second communication systems each having different speech encoders; and a communication device communicating with the different speech encoders during a handover, the speech encoder of the second communication system gradually adjusting a delay of the speech encoder from a delay of the speech encoder of the first communication system to the delay of the speech encoder of the second communication when a change of delay between the speech encoders of the first and second communication systems is greater than a threshold.

9. A storage medium having stored thereon the steps of: determining an amount of delay of differing speech paths in a handover; measuring a change of delay between the speech paths; determining if the change of delay is greater than a threshold; and gradually changing from the delay of a first of the speech paths to the delay of the second speech path.