WO2006133631A1

WO2006133631A1 - Method for performing a handoff in a telecommunication system

Info

Publication number: WO2006133631A1
Application number: PCT/CN2006/001235
Authority: WO
Inventors: Yingmin Wang; Guiliang Yang; Yuanxin Qiao
Original assignee: Shanghai Ultimate Power Communications Technology Co., Ltd.
Priority date: 2005-06-16
Filing date: 2006-06-07
Publication date: 2006-12-21
Also published as: KR100939996B1; EP1891814A4; EP1891814B1; US8089935B2; EP1891814A1; CN100464603C; US20080102836A1; CN1882168A; JP2008547253A; KR20080031869A

Abstract

A method is disclosed for performing a handoff in a telecommunication system. The method comprises: Step I: performing a first multi-cell joint detection process for decoding for the communications within the home cell when a mobile terminal enters a handoff region between a home cell and one or more neighboring cells; Step II: performing a handoff process for a mobile terminal when in the home cell to join a target neighboring cell; and Step III: performing a second multi-cell joint detection process for decoding for the communications between the mobile terminal and the target neighboring cell. Therefore, the present invention can solve the problems in handoff of cells in telecommunication system, improve the decoding for the uplink or downlink communications, and enhance the throughout and performance of the telecommunication system.

Description

Method for Performing a Handoff in a Telecommunication System

Field of the Invention

This invention relates generally to wireless communication systems, and more particularly to a method and system for performing a handoff procedure by using the multi-cell joint detection process. Background

In cellular telephone or other wireless voice or data communication systems, a service area is generally divided into cells, each of which may be further divided into sectors.

Each cell may be served by one or more base stations (BSs), which is further connected to a message switching center ("¹MSC"), a subscriber management system ("SMS"), or a data router via a base station controller ("BSC"). A plurality of mobile communication devices/ terminals ("MT") are connected to the MSC, the SMS, or the router by establishing radio links with one or more nearby base stations.

One commonly used type of cellular radiotelephone communication system is referred to as a Code Division Multiple Access (CDMA) system, in which the radio signals of different users share the same frequency spectrum at the same time, in contrast to previous Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) systems.

In a typical CDMA cellular radiotelephone communication system, an MT communicates with a base station having the strongest available signal. In order to track the available signals, the MT maintains a list of available base stations. Specifically, each base station in the CDMA system transmits an unmodulated "pilot" signal at a set of predetermined frequencies. MT receives the pilot signals and determines which pilot signals are the strongest. A "searcher" unit located in the MT commonly performs the signal detection and intensity measurement functions. The results of the searcher are reported to the current base station. The current base station then instructs the MT to update a list of available base stations maintained by the MT. Typically, the list is further divided into three operative sets, an active set, a candidate set, and a neighbor set. The active set contains a list of the base stations with which the MT is currently communicating. The candidate set is a list of base stations which may change into the active set, and the neighbor set is a list of base stations which are being monitored, but less frequently.

Since the coverage area of a telecom system is traditionally divided into cells, an MT may move from one cell to another. As an MT moves and the signal from its currently active base station weakens, the MT must access a new base station. Based upon the results of the searcher and the instructions received back from the new base station, the MT will communicate with a different base station. The process of switching the communication link from one base station to another is known as a handoff process.

In order to transfer an MT user's communication without interruption, the communication link must be switched to the next base station. There are generally two types of handoff, the soft handoff and the hard handoff. If a new link is established before current link is terminated, it is known as the soft handoff. In contrary, in a hard handoff, the current link will be terminated first and then a new link with the new base station is established. Since the hard handoff will cause a service interruption for the MT user, although temporarily, it lowers the quality of service (QOS) for the cellular telephone user. For example, if the MT is engaged in a voice service, the user will most likely experience degraded voice quality or even call drop. If an MT is transmitting data, significant transmission delays (due to retransmission errors) will be likely to occur. The handoff usually occurs when an MT travels to a border area where base stations in multiple neighboring cells are now ready to be switched to. When the MT is in the border area, signal interferences from multiple cells are the severest, and the handoff procedure directly impacts on the communication quality.

An improved method and system for effectively performing a handoff procedure to achieve better communication quality is needed accordingly. Brief Description of the Invention

A method is disclosed for performing a handoff in a telecommunication system.

According to the first aspect the invention, the method for performing a handoff in a telecommunication system comprising: Step I: performing a first multi-cell joint detection process for decoding for the communications within the home cell when a mobile terminal enters a handoff region between a home cell and one or more neighboring cells; Step II: performing a handoff process for a mobile terminal when in the home cell to join a target neighboring cell; and Step III: performing a second multi-cell joint detection process for decoding for the communications between the mobile terminal and the target neighboring cell.

According to the second aspect the invention, the method for performing a handoff in a telecommunication system comprising: Step I: performing a first multi-cell joint detection process when a mobile terminal enters a handoff region between a home cell and one or more neighboring cells by performing multi-cell channel estimation and code channel selection for decoding for communications within the home cell; Step II: performing a handoff process for the mobile terminal when in the home cell to join a target neighboring cell; and Step III: performing a second multi-cell joint detection process for decoding for the communications between the mobile terminal and the target neighboring cell.

According to the third aspect the invention, the method for performing a handoff in a telecommunication system comprising: Step I: performing a first joint detection process including a first multi-cell channel estimation when a mobile terminal enters a handoff region between a home cell and one or more neighboring cells, selecting one or more code channels from the neighboring cells, and decoding for the communications within the home cell; Step II: performing a handoff process for the mobile terminal when in the home cell to join a target neighboring cell; and Step III: performing a second multi-cell joint detection process including a second multi-cell channel estimation, selecting one or more code channels from the neighboring cells, and decoding for the communications within the target cell between the mobile terminal and the target neighboring cell.

Therefore, the present invention can solve the problems in handoff of cells in telecommunication system, improve the decoding for the uplink or downlink communications, and enhance the throughout and performance of the telecommunication system. Brief Description of the Drawings

FIG. 1 is a sehematic diagram showing a wireless communalization network.

FIG. 2 is a flow diagram for performing an improved handoff process according to one embodiment of the present invention. Detailed Description of the Invention

Although the present invention is illustrated below with regard to a few embodiments, it should be understood that the present invention is applicable to any multiple access technologies, which implement frequency division or time division for multiple access channelization. The description below with regard to embodments of the present invention may also be applied in the enviroments of CDMA systems.

FIG. 1 illustrates a telecommunication system 100 arranged for providing voice and data communications to one or more mobile terminals. For the illustration purpose, only three cells Cl, C2, and C3, are shown. Each cell has a base station (BS) 102A-102C that is responsible for communicating with all mobile terminals 104 within the boundaries of the cell. When MT 104, which is initially operating in cell Cl, moves close to a border area 106, a handoff procedure will be performed to connect MT 104 to a neighboring cell depending on in which direction it is moving. Assuming MT 104 is moving to cell C3, a necessary handoff would be performed to connect MT 104 to BS 102C. However, conventionally, BS 102A treats the signals from BSs 102B and BS 102C as noise while evaluating handoff possibilities. It should be understood that the signal interferences are significant at the border area 106.

In a TD-CDMA system, as it inherently has the characteristics of both Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) systems, several users share the same frequency band, but transmit and receive in different slots (bursts) or codes by their specific signature sequences. The performance of a TD-CDMA system in a multipath delay spread environment is limited by the multiple access interference (MAI) due to the lack of orthogonality of spreading codes at the receiver and by the inter-symbol interference (ISI) between consecutive symbols of a single user.

As understood in the art, the inter-symbol interference refers to the fact that symbols within each of the individual spreading signals interfere with their respective adjacent symbols. On the other hand, multiple-access interference refers to the fact that the code sequences are no longer orthogonal after they propagate through the composite propagation channel and arrive at the receiver, although a predetermined set of code sequences are designed to be mutually orthogonal at the transmitter. However, as a result of non-orthogonality, symbols of the spreading signals interfere with one another, and each acts as a noise source for others.

In the case that the length of the spreading code sequence is comparable with the length of the composite propagation channel, the received signals are not mutually orthogonal, and thus the output of a matched filter contains interferences from the remaining user signals. In order to eliminate this continuous MAI as well as the inevitable ISI, it is necessary to perform additional operations at the output of the matched filters. The combined operation of matched filtering and MAI-ISI elimination is referred to as joint-detection. Joint-detection schemes attempt to eliminate MAI and ISI by suppressing the mutual interference existing among all the transmitted symbols carried by the user signals.

Usually, conventional single user CDMA detectors, such as the matched filter and the rake receiver, are optimized for detecting the signal of a single user in the absence of ISI and taking the MAI as additional noise. Instead, the joint detection process, by exploiting the knowledge about the signature sequences and channel impulse responses, deals with both ISI and MAI. As the MT travels within the border area and a handoff is to be performed, there will be various signal interferences from various sources including the aforementioned inter-cell interference.

In the CDMA system shown in Fig. 1, since the same frequency is used by all the three cells, no frequency change is needed although the code channel needs to be altered for handoff. That is, when performing the handoff, the new base station from the new cell will provide a new code channel for the communications. If a hard handoff is performed, due to the inter-cell interference, the communication quality cannot be assured. If a soft handoff is performed, because more code channels have to be involved, the inter-cell interference is more severe.

The present invention provides an improved handoff process by using a multi-cell joint detection process while an MT is in the border area so as to improve the communication quality during the handoff. Traditionally, a joint detection process is limited to analyze multiple MTs in a particular cell, but the improved handoff process proposed by the present invention includes a multi-cell joint detection process which analyzes multiple communication signals from multiple cells.

Fig. 2 is a flow diagram 200 illustrating an improved handoff process according to one embodiment of the present invention. When a mobile terminal enters a handoff region such as a border area from its home cell in step 202, it is first determined whether a handoff is to be performed in step 204. This is generally determined by the communications between the BS and the MT. While the MT is still in its home cell, a first multi-cell joint detection process is performed in step 206. When the MT is in the border area, there are communication signals from multiple sources (e.g., from multiple cells) interfering with the communication between the MT and its current home cell BS. When the first multi-cell joint detection process is carried out, it considers code channels from the neighboring cells. The multi-cell joint detection process provides necessary information for devising a first decoding mechanism to reduce interferences, which enhances decoding for the communications within the home cell. For example, after a multi-cell channel estimation, one or more code channels from the neighboring cells are identified, especially the one the mobile terminal is moving to.

The detailed multi-cell joint detection process and the multi-cell channel estimation can refer to Chinese Patent Application No. 200410080196.6 filed by the applicant of the present application. By further selectively grouping these code channels for performing the first multi-cell joint detection process, the interference originated from the target neighboring cell will be treated differently and isolated from the rest of the noise. The decoding mechanism such as a decoding matrix can be obtained and used to process the communication between the MT and the BS. Based on this decoding mechanism, a handoff process 208 can be completed. As a result of the enhanced decoding mechanism, the MT communicates with the BS with improved signal quality when being in the home cell, and it further switches over to the target neighboring cell to complete the handoff process. The handoff process can be either a hard handoff or a soft handoff. After the handoff is performed, a second multi-cell joint detection process 210 is then performed for devising a second decoding mechanism for communications between the MT and the target neighboring cell. The result of the second multi-cell joint detection process, like the first multi-cell joint detection process, enhances the second decoding mechanism, which is used for future communications between the MT and the target BS in the target neighboring cell. When the MT moves further away from the handoff region in step 212, the home cell will no longer be taken into account in any future joint detection process, and code channels from the neighboring cells of the said target neighboring cell will also no longer be taken into account of any future joint detection processes when the mobile terminal leaves the handoff region and moves in the target neighboring cell, in other words, when the intensity of the interference signal from the corresponding neighboring cell drops below a second predetermined threshold.

It should be understood that this improved handoff process is better than the traditional handoff processes in that it considers both home and target neighboring cells as interference sources for devising a much more accurate decoding mechanism. For example, when the first decoding mechanism is devised by performing the first multi-cell joint detection process, the interference from the target neighboring cell is excluded and clearly identified without being combined with other noise. When the joint detection process is performed based on pilot channel estimation codes of the home cell and various neighboring cells, a multi-cell channel estimation process can be implemented to obtain pilot channel estimation of the home cell as well as the neighboring cells. Then, according to the correlation between the code channels and the correlation between the pilot channel estimation codes, one or more arrangements for grouping the code channels are selected to designate different code channels for the cells neighboring to each other. In other words, code channels are selected carefully for performing the multi-cell joint detection. Finally, based on the pilot channel estimation result and the code channel grouping result and their correlation, multi-channel signals are processed to obtain detection results based on the multi-cell joint detection process. Obviously, the selection of different code channels can vary significantly depending on the design of the telecommunication system. The selection can be based on cell boundary, code channel power intensity, relative signal intensity, or any of the combinations. It should be also understood that the multi-cell joint detection process can vary in its operation during handoff. In short, this multi-cell joint detection process, which considers a selected number of neighboring cells assures that the decoding mechanism is enhanced for better communications. Similarly, when an MT is in the new cell, the home cell interference is restrained in the second multi-cell joint detection process so the second decoding mechanism is also enhanced for future communications.

It is to be noted that the aforementioned identification and selection can use command exchange between the home cell and the neighboring cells for identifying and selecting. And code channels can be identified and selected by means of, for instance, identifying spreading code information of the neighboring cells through predetermined one-to-one correlation based on channel estimation.

While the first multi-cell joint detection process is being performed, the code channels are carefully selected for the purpose of analysis. For example, when the first multi-cell joint detection is being prepared, if to be selected, code channels used by neighboring cells are required to be above certain threshold signal intensity (a first threshold). In other words, only those signals that impose significant impact on the communication of the MT will be considered. As the threshold can be varied, different numbers of code channels can be involved in the process. It should be understood that the more channels are involved, the more resources have to be used for the process. It is up to the telecom system designer to determine the threshold based on a cost-benefit analysis. The system can also predetermine a fixed number of channels to be considered for the first or second multi-cell joint detection process. Or, the system can rank all the neighboring cells in terms of their signal intensity for analysis purposes, but only select a group of cells having the highest rankings. As stated above, because an MT and BS are aware of a list of other BSs that are available to the MT, the system can prioritize the BS or cells based on the signal intensity of their pilot or code channels.

Similar to that in the first multi-cell joint detection process, the number of the code channels included in the second multi-cell joint process can be selected intelligently, although can be easily different from those used in the first multi-cell joint detection process. It can be seen that when an MT moves further away from its home cell and moves further into the new cell, the signals from the home cell will fade, and at a certain point, for example the signal intensity is below a second threshold normally smaller than the first threshold, their code channels will not be taken into account in the multi-cell joint detection any longer. It should be understood that the boundaries of the border areas are virtual and no physical line can be defined. Whether an MT has entered the virtual border area can be determined by its signal intensity with regard to its communication with its original (home) BS. For example, a signal-noise-ratio (SNR) can be one norm to determine whether the MT has entered the border area. Other similar criteria including other forms of interference signal intensity measurement can be used to determine the border area. For example, it is also true that when an MT drops its original home cell from the joint detection process, it has left the border area.

The first or second multi-cell joint detection process can be implemented for uplink or downlink communications, or both. As such, the BS, the MT or both can carry out the multi-cell joint detection process. As to the interferences sources detection, they can be provided or detected through various methods. For example, the communications among different BSs can be used to identify some characteristics of certain signal interferences. The telecom system can also preset some parameters for easily identifying sources of the signals from different BSs. Also, the interference parameters can also be obtained through testing.

In order to perform the multi-cell joint detection process, a joint detection receiver can be contained in the BS or the MT, or both. A typical joint detection receiver is provided with all the codes and is therefore able to decode all the channels within a given time slot. One known method of estimating the unknown transmitted symbol sequence in a joint-detection receiver is zero-forcing block-linear equalization. Normally, the joint-detection equations or arithmetic can be solved by using propagation channel estimation and the knowledge of the used spreading codes. According to the present invention, during the multi-cell joint detection process, the channel estimation codes of the home cell and the neighboring cells are used to obtain the channel estimation for both the home cell and the neighboring cells through methods such as the multi-set channel estimation method. Then, based on the predetermined arrangement between the code channels and channel estimation codes of the home cell and the neighboring cells, code channels are divided into different groups. The grouping can be realized based on various criteria such as cell separation, code channel power intensity, or any other reasonable methods. Based on the channel estimation of the home and neighboring cells, the multi-cell joint detection process is performed to examine the signals in different code channels.

It is to be noted that different choices of multi-cell joint detection can be performed based on different situations. The first or the second multi-cell joint detection process mentioned above can be based on any multi-cell joint detection process (for example, that in the Chinese Patent Application No. 200410080196.6). However, in other modification embodiments, the first multi-cell joint detection process can further includes multi-cell channel estimation or code channel selection or both for the communications within the home cell; or for the communications within the home cell, the first multi-cell joint detection process including a first multi-cell channel estimation can be first performed, and then one or more code channel are selected, and for the communications within the target cell, the second multi-cell joint detection process including a second multi-cell channel estimation can be first performed, and then on or more code channels are selected. Therefore, such different arrangements in multi-cell joint detection can further reduce the calculation time and expense in multi-cell detection.

The above disclosure provides many different embodiments, or examples, for implementing different features of the invention. Also, specific examples of components, and processes are described to help clarify the invention. These are, of course, merely examples and are not intended to limit the invention from that described in the claims.

While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims

1. A method for performing a handoff in a telecommunication system comprising:

Step I: performing a first multi-cell joint detection process for decoding for the communications within the home cell when a mobile terminal enters a handoff region between a home cell and one or more neighboring cells;

Step II: performing a handoff process for a mobile terminal when in the home cell to join a target neighboring cell; and

Step III: performing a second multi-cell joint detection process for decoding for the communications between the mobile terminal and the target neighboring cell.

2. The method as defined in claim 1, wherein,

Step I further includes performing multi-cell channel estimation.

3. The method as defined in claim 2, wherein, Step I further includes Step Ia: identifying and selecting one or more code channels from the neighboring cells for performing the multi-cell joint detection process.

4. The method as defined in claim 3, wherein,

Step Ia further includes using command exchange between the home cell and the neighboring cells for identifying and selecting.

5. The method as defined in claim 3, wherein,

Step Ia further includes identifying spreading code information of the neighboring cells through predetermined one-to-one correlation based on channel estimation.

6. The method as defined in claim 3, wherein,

Step Ia further includes selecting the code channels from the neighboring cells if the intensity of the interference signal or SNR from the corresponding neighboring cell is above a first predetermined threshold.

7. The method as defined in claim I₅ wherein,

Step I includes an uplink multi-cell joint detection process.

8. The method as defined in claim 1, wherein,

Step I includes a downlink multi-cell joint detection process.

9. The method as defined in claim 1, wherein,

Step I is performed at either a base station or the mobile terminal.

10. The method as defined in claim 1, wherein,

Step I further includes excluding the interferences of the target neighboring cell from noise for decoding the communications within the home cell.

11. The method as defined in claim 1, wherein,

Step II further includes terminating communications between the mobile terminal and the home cell prior to connecting the mobile terminal to the target neighboring cell.

12. The method as defined in claim 1, wherein,

Step II further includes terminating communications between the mobile terminal and the home cell after connecting the mobile terminal to the target neighboring cell.

13. The method as defined in claim 1, wherein,

Step III further includes identifying interferences from the home cell.

14. The method as defined in claim 1, wherein,

Step I further includes excluding the interferences of the home cell from noise to decode for the communications within the target neighboring cell.

15. The method as defined in claim 1 further comprising excluding code channels from the neighboring cells from any future joint detection processes when the mobile terminal leaves the handoff region and moves in the target neighboring cell.

16. The method as defined in claim 1 further comprising excluding code channels from one or more selected neighboring cells from any future joint detection processes when the intensity of the interference signal or SNR from the corresponding neighboring cell drops below a second predetermined threshold.

17. A method for performing a handoff in a telecommunication system comprising:

Step I: performing a first multi-cell joint detection process when a mobile terminal enters a handoff region between a home cell and one or more neighboring cells by performing multi-cell channel estimation and code channel selection to decode for communications within the home cell;

Step II: performing a handoff process for the mobile terminal when in the home cell to join a target neighboring cell; and

18. The method as defined in claim 17, wherein, the code channel selection further includes using command exchange between the home cell and the neighboring cells for identifying and selecting one or more code channels.

19. The method as defined in claim 17, wherein, the code channel selection further includes identifying spreading code information of the neighboring cells through predetermined one-to-one correlation based on channel estimation.

20. The method as defined in claim 17, wherein, the code channel selection further includes selecting the code channels from the neighboring cells if the intensity of the interference signal or SNR from the corresponding neighboring cell is above a first predetermined threshold.

21. The method as defined in claim 17, wherein, Step III further includes: identifying interferences from the home cell; and excluding the interferences from the home cell from noise to decode for the communications within the target neighboring cell.

22. A method for performing a handoff in a telecommunication system comprising:

Step I: performing a first joint detection process including a first multi-cell channel estimation when a mobile terminal enters a handoff region between a home cell and one or more neighboring cells, selecting one or more code channels from the neighboring cells, and decoding for the communications within the home cell;

Step III: performing a second multi-cell joint detection process including a second multi-cell channel estimation, selecting one or more code channels from the neighboring cells, and decoding for the communications within the target cell between the mobile terminal and the target neighboring cell.

23. The method as defined in claim 22, wherein,

Command exchanges between the home cell and the neighboring cells for selecting are used to select one or more code channels.

24. The method as defined in claim 22, wherein, selecting one or more code channels further includes identifying spreading code information of the neighboring cells through predetermined one-to-one correlation based on channel estimation.

25. The method as defined in claim 22, wherein, selecting the code channels from the neighboring cells if the intensity of the interference signal or SNR from the corresponding neighboring cell is above a first predetermined threshold.

26. The method as defined in claim 22, wherein,

Step I includes an uplink multi-cell joint detection process.

27. The method as defined in claim 22, wherein,

Step I includes a downlink multi-cell joint detection process.

28. The method as defined in claim 22, wherein, the first multi-cell joint detection process excludes the interferences of the target neighboring cell from noise to decode for the communications within the home cell.

29. The method as defined in claim 22, wherein, the second multi-cell joint detection process excludes the interferences of the home cell from noise to decode for the communications within the target neighboring cell.

30. The method as defined in claim 22 further comprising excluding code channels from the neighboring cells from any future joint detection processes when the mobile terminal leaves the handoff region and moves in the target neighboring cell.

31. The method as defined in claim 22 further comprising excluding code channels from one or more selected neighboring cells from any future joint detection processes when the intensity of the interference signal or SNR of the corresponding neighboring cells drops below a second predetermined threshold.