WO2001095656A1

WO2001095656A1 - Methods for assigning radio resources

Info

Publication number: WO2001095656A1
Application number: PCT/CN2000/000140
Authority: WO
Inventors: Daoben Li; Shiu Ming Joseph Ho; Haiyang Gao; Yongsheng Zhang; Weidong Chen; Yuanhua Deng; Frederic Leroudier
Original assignee: Linkair Communications, Inc.
Priority date: 2000-06-05
Filing date: 2000-06-05
Publication date: 2001-12-13
Also published as: AU2000250580A1; EP1305965A1; CN1241450C; CN1433654A

Abstract

This invention describes a method for adapting the radio channel to the radio propagation conditions of different mobile terminal under an IFW wireless system. This is achieved by assigning different Interference Free Windows to different mobile terminals. This invention provides a method for supporting one of more IFW within the cell location and thus allowing the air interface to support mobile terminals with different IFW requirements. This invention also describes the IFW selection procedure to be performed during call setup and the IFW handoff procedure that is performed when the network determines that the IFW of the mobile terminal should be changed during a call or a data session. These two procedures allow the dynamic adaptation of IFW so that the channel can be dynamically adapted to the changing propagation behavior of the mobile terminals.

Description

METHODS FOR ASSIGNING RADIO RESOURCES

Field of the Invention:

The present invention relates to the methods for assigning radio resources in a wireless system, and more particularly to the methods and means for assigning radio resources in a wireless system utilizing the orthogonal spread codes with an Interference Free Window (IFW) property.

Background of the invention:

In PCT application PCT/CN00/00028, invented by Li Daoben and entitled "A Method for Spread Spectrum Multiple Access Coding with Zero Correlation Window," a coding scheme called LS code was disclosed. LS code has the special property that the auto correlation of an LS code is zero, except at the origin, when the two copies of the LS codes are synchronized to with a window of n chips. In addition, LS code has another special property that the cross correlation between two different LS codes are zero when the two codes are synchronized to the same n-chip window. Thus, as described in PCT/CN00/00028, there is a Zero Correlation Window of size n chips. In the context of this invention, the term Interference Free Window or IFW is used in place of the term Zero Correlation Window.

Thus when remote units transmit to a base station signals that are modulated using a set of LS codes that have a Interference Free Window of [-n, +n], these signals will not interfere with each other as long as they arrive at the receiving base station within n chips with respect to each other. This eliminates inter-symbol interference and multiple access interference when multipath signals from a same remote unit and signals from different remote units arrive within a Interference Free Window.

Hereinafter the wireless systems, the signals, the air interface which utilize LS codes as the orthogonal spread codes will be respectively referred to as LS coded wireless systems, LS coded signals, and LS coded air interface.

Hereinafter the wireless systems, the signals, the air interface which utilize orthogonal spread codes with an IFW property will be respectively referred to as IFW wireless systems, IFW signals, and IFW air interface.

Because of the special IFW property of LS code as described in PCT/CNOO/00028, any LS coded signal arriving at a receiver will result in no interference after de-correlation for a particular mobile or fixed transmitter. This is true for reflected signals from the transmitter or for signals from another transmitter. As such, in order to minimize Multiple Access Interference (MAI) and Inter-Symbol Interference (ISI), and to achieve best transmission quality with an LS coded wireless system, two conditions must be met:

1) The majority of the energy from reflected signals must fall within a predetermined time window. This means that the maximum delay spread (or multi-path spread) must be lesser than the Interference Free Window.

2) The transmission from two mobile terminals utilizing LS codes must be synchronized within the IFW to each other.

Any uplink signal, either from the same or a different mobile terminal which is not received within this window by the Base Station will result in MAI and ISI, thus reducing transmission quality and system capacity.

To the best of our knowledge, the present invention contains the first proposal that allows the adaptation to the radio propagation conditions of different mobile terminal using the new concept of Interference Free Window.

Summary of the invention:

It is an object of the present invention to provide a method that allows the LS coded air interface to adapt to the radio propagation conditions of different mobile terminals, and in particular to the delay distribution characteristics of this environment.

It is another object of this invention to provide a method on how different IFW can be allocated to different mobile terminal within an LS coded wireless system cell area.

It is a further object of this invention to provide the steps and procedures for the adaptive adjustment of IFW based on the latest interference characteristics of each mobile terminal with the cell.

The present invention involves a method for adapting the IFW air interface to the radio propagation conditions of different mobile terminals within the same cell. This is achieved by first utilizing the special feature of LS coded wireless system where an IFW of a given size can be associated with the LS codes. A larger IFW can tolerate larger delay spread of the transmitted signal. However, there is a trade off between the IFW size and the number of available LS codes and consequently system capacity. The larger the IFW, the smaller the number of available LS codes. It is thus advantageous to limit the IFW size to the minimal required value so that larger number of LS codes will be available for carrying user traffic.

In a wireless communication system, the radio propagation condition of each mobile terminal varies depending on its location and other factors. This means that the minimum IFW that can achieve acceptable performance, such as bit error rate, varies from user to user (or user group to user group). This invention provides a method and means for achieving the followings in a LS coded wireless system:

• Support different IFW size for different mobile terminals in the same cell;

• Dynamically select the IFW of the mobile terminals based on the radio propagation environment that the user is experiencing; • Provide the procedure for selecting and modifying the optimal IFW for each mobile terminal. This invention enables the dynamic adaptation of IFW for different users based on the measured radio propagation characteristic of each individual user. This results in higher system capacity and better quality of service for each user.

The invention relies on the notion that the transmission from a fixed or mobile transmitter of an LS coded wireless system can be separated by the receiver from the transmission from other transmitters by a number of different means. This separation can for example be realized: in the time domain by limiting some transmitters to transmit within certain time slots; - in the space domain by using adaptive antenna arrays to distinguish areas of different delay spreads; in the frequency domain by allocating different RF carriers to transmitters with different delay properties; by allocating different code divided channels to mobiles belonging to different delay spread categories.

The method according to the present invention is composed of the following steps:

1. Divide the air interface into a number of partitions in either the time, the space, and the frequency domains. Each partition supports a different IFW. For example, in the time domain, this can be achieved by allocating different IFW to different SF. In the space domain, this can be achieved by allocating different IFW to different directional antenna. In the frequency domain, this can be achieved by allocating different IFW to different carrier.

2. When the mobile terminal request for radio channel resource, the IFW assignment procedure is performed such that the mobile terminal is assigned radio resource that supports an IFW that is most suitable to the radio propagation condition of the mobile terminal. 3. During the communication session of mobile terminal, the network constantly evaluates the radio propagation condition of the mobile terminal. If there is a change in the radio propagation environment such that the interference level of the mobile terminal improves or degrades, the mobile terminal is assigned to another radio resource with a different IFW. This procedure is called the IFW handoff procedure.

Brief description of the attached drawings:

The accompanying drawings which are incorporated in and constitute a part of this specification, illustrate particular embodiments of the invention, and together with the description, serve to explain, but not restrict, the principles of the invention.

Figure 1 describes a LAS-2000 frame structure and shows how the frame is further divided into sub-frames and time-slots according to a preferred embodiment of the present invention.

Figure 2 shows the generating tree of the LS code and demonstrates the relationship between the Interference Free Window and the number of available LS codes.

Figure 3 shows an example of the allocation of different IFW windows to different SFs within the 20 ms LAS-2000 frame structure such that each 20 ms sub- frame supports 2 rFWs.

Figure 4 shows an example of the allocation of different IFW windows to different SFs within the 20 ms LAS-2000 frame structure such that each 20 ms sub- frame supports 3 IFWs.

Figure 5 shows the steps of the IFW assignment procedure that is performance when a channel resource request is received from the mobile terminal.

Figure 6 shows the steps of the IFW handoff that is performance when a voice call or a data session has been established between the mobile terminal and the network.

Detailed Description of the Invention:

According to the design of IFW wireless system, the system can be configured to tolerate different interference condition of the mobile terminals. In a CDMA based cellular system, interference may originate from several sources. The primarily sources of interference include Inter-Symbol interference (ISI), Multiple Access Interference (MAI), Adjacent Cell Interference (ACI), and noise. ISI is caused by the existence of multiple reflected paths of the same transmission associated with a mobile terminal. Due to the unequal time dispersion characteristics of the different reflected paths, neighboring symbols of the same transmission overlaps each other and results in interference. MAI is the results of interference between the transmission paths of different mobile terminals within the same cell. It has been demonstrated that it is difficult to design codes that has high tolerance for both ISI and MAI. ACI is generated by the transmission of the mobile terminals in a neighboring channel or cell, while noise is generated by sources that do not belong to the wireless communications system under discussion.

The IFW wireless system has a special characteristic that if the multiple transmission paths, either belonging to the same or different mobiles, are synchronized to within a given Interference Free Window of n chips, then both ISI and MAI can be eliminated. In the invention, a method for adapting the IFW to the ISI and MAI conditions of the mobile terminal is provided such that the best ISI and MAI characteristics can be achieved without unnecessary reduction in the total capacity of the IFW wireless system. Figure 1 describes the LAS-2000 (Large Area Synchronous) frame structure and shows how the frame is further divided into sub-frames and time-slots according to the present invention. LAS-2000 is an operation model of IFW wireless system that is compatible to IS-2000. Other operation modes, such as Wideband-LAS (W-LAS) that is compatible with Wideband CDMA as defined in the UMTS standards, exist. The following descriptions are presented based on LAS-2000. However, the application of this invention is not limited to LAS-2000. It can be applied to W-LAS and other IFW wireless system operation modes. The LAS-2000 frame is 20 ms long and is divided into 10 Sub-Frames (SFs). Each 20 ms frame consists of 24576 chips. The first SF consists of 1545 chips and is used for carrying the Broadcast and Synchronization Channel in the forward link and the Access and Synchronization Channel in the reverse link. The remaining 9 SFs, each consists of 2559 chips, are used for carrying user traffic and other signaling channels, such as common control channels and pilot channel.

Each SF is divided into 17 time-slots (TSs). TSO is primarily used for carrying the pilot channel, even though it can be used for other purposes as well. TSs 1 to 16 are used for carrying user traffic as well as signaling information. The specific starting position of the TSs within the SF is governed by the LA code to be used.

The LA code was disclosed in PCT application PCT/CN98/00151 entitled "A Spread Spectrum Multiple Access Coding Method," that was invented by Li Daoben. The present invention utilizes the LA code as described in PCT/CN98/00151 such that each SF carries a (136, 17, 2259) LA code. This LA code consists of 17 pulses with a period of 2559 chips. Each LA pulse corresponds to a time-slot within the SF. As described earlier, the first time-slot may be used as the Pilot and the remaining 16 time-slots can be used for carrying user data or signaling information. Each TS carries an LS code, the LS code is divided into the C and S components, each of 64 chips long. There are two 4-chip gaps within the LS code, one located before the C component and another one located between the C and the S component.

Figure 2 shows the generating tree of the LS code, it can be seen that, at the root of the tree, there is a pair of LS codes called the originators. The number of LS codes doubles when the tree goes down by one layer and the LS code length doubles. There are altogether 128 LS codes of 128 chips long (C and S components together) at the bottom of the tree when a root of length 2 is used. The minimum IFW of a tree or subtree depends on how the LS codes are selected within the overall tree. If all LS codes are made available for use with a carrier, the IFW size is equal to 1 chip or [0, 0]. The minimum size of the IFW can be increased to 3 chips or [-1,+1] when only half of the 128 LS codes are used, for example LS codes 1-64, or LS codes 65-128. Similarly, if only one quarter of the 128 LS codes are used, either LS codes 1-32, 33-64, 65-96, or 97-128, then the IFW size will increase to 7 chips or [-3, +3], and so on. There is thus a trade of between IFW size and the number of available LS codes.

This invention proposes a method for adapting the set of available LS codes according to the measured delay spread for each mobile or fixed transmitter. The invention relies on the separation of the mobiles depending on their multi-path transmission characteristics. One way to achieve this is to use the time domain by first partitioning the SFs into two or more groups where each group is associated with an LFW, then allocate SFs from different groups to different mobile terminals according to the radio propagation condition experienced by the mobile terminal, as illustrated by the following preferred embodiments.

In one of the preferred embodiment of the present invention, each 20 ms frame carries two voice channels per LS code. The first four SFs are used to carry one voice channel, the next four SFs are used to carry another voice channel, and SF9 is used for signaling or other purposes. It is then possible to allocate different IFWs to the two channels within the 20 ms frame. In this case, SFs 1 to 4 can be assigned a 1 chip IFW with a maximum of 128 LS codes for carrying voice traffic, and SFs 5 to 8 can be assigned a 3 chips IFW with a maximum of 64 LS codes for carrying voice traffic. This is illustrated in Figure 3.

In another preferred embodiment of the present invention as shown in Figure 4, the 20 ms LAS-2000 frame is divided into 3 Data Frames (DFs), DF1, DF2 and DF3. Each DF consists of 3 SFs. In this case, different IFW size can be assigned to different DFs. For example, DF1 can be assigned an IFW size of 1 chip, DF2 is assigned an LFW size of 3 chips, and DF3 is assigned an LFW size of 7 chips. When allocating channel resource, mobile terminals are allocated DFs based on their radio propagation condition. Mobile terminals that are in a good radio propagation environment with very small delay spread between it multiple reflected paths can be assigned to DF1. On the other hand, mobile terminals that are experiencing high delay spread on its reflected paths can be allocated to DF2 or DF3.

Note that the IFW size is also affected by other design parameters, such as the gap between the C and S components in the LS code. The IFW size mentioned here is the maximum available size given the number of available LS codes. Figures 3 and 4 shows only two particular embodiments of the present invention. Other arrangements, such as assigning a different IFW size to each SF or supporting more than 3 IFW sizes within the 20 ms frame, are possible and are not covered in the examples given in Figures 3 and 4.

It should be noted that other methods can be used to separate different groups of mobile terminals displaying different propagation delay characteristics in order to realize the invention covered by this patent exist. Such methods include the use of adaptive antenna arrays using measured multi-path characteristics to separate mobiles exhibiting different multi-path characteristics. Since the signals from transmitters can be separated by these antennas, they do not interfere with transmitters belonging to a different group of multi-path characteristics. Within a group of mobiles with uniform delay characteristics, a minimum IFW can be used according to the delay spread of the group of transmitters.

Other methods involve the use of different RF carriers assigned dynamically to mobiles belonging to groups of homogeneous multi-path characteristics, or the use of orthogonal codes to create distinct groups of transmitters where an optimal IFW can be used without risking interference from mobiles with wider delay spread.

The adaptation of LFW to the radio propagation environment of each mobile terminal can be achieved by two additional procedures: the IFW assignment and the LFW handoff procedures. The IFW assignment procedure is performed during connection setup or resource assignment. The initial radio propagation condition of the mobile terminal is estimated and the mobile is assigned resource in particular SFs, transmitter, or carrier (depending on the particular implementation) that has an IFW that is most suitable for the propagation condition of the mobile terminal. The IFW handoff procedure is executed when the radio propagation environment of the mobile terminal has changed during a communication session. If the condition has worsened such that the multi-path delay spread has increased, the mobile terminal can be handed off to SFs, transmitters, or carriers with a larger IFW. On the other hand, if the condition has improved such that a smaller LFW is sufficient for provide low interference for the mobile terminal, the mobile terminal can be handed off to SFs, transmitters, or carriers with smaller LFW. In this way, larger IFW is assigned to a mobile terminal only when necessary. This results in better utilization of the wireless channel resources. The followings describe in more details the preferred embodiment of the IFW assignment and IFW handoff procedures.

Figure 5 shows the flow diagram for the IFW assignment procedure. The IFW assignment procedure includes three steps: resource request, IFW decision, and resource assignment. The resource request step begins when the network receives a request from the mobile terminal for radio resource. This can be a voice channel request message that is generated by mobile when the user initiates a call, or it can be a packet data channel resource request message generated by the mobile terminal when the user initiates a data session. In addition, the resource requests can be initiated due to many other reasons, such as in response to a polling from the network, and is not described in details here.

During the IFW decision step, the network makes a decision on the IFW that is most suitable for the current radio propagation environment of the mobile terminal. This step may involve a channel measurement sub-step that is used to evaluate the channel propagation condition of the mobile terminal. The IFW decision can be made in a number of ways including, but not limited to, (i) a table lookup procedure such that the channel condition is mapped to a IFW value, or (ii) a worst case assignment procedure such that the mobile is always assigned the largest available LFW. If it is later on determined that a smaller IFW is sufficient for the mobile terminal, an LFW handoff procedure can be performed to reassign the mobile terminal to SFs, transmitter, or carrier with smaller LFW.

The resource assignment step is initiated once the resource request and LFW decision steps are complete. During this step, the network sends signaling messages to the mobile terminal indicating the resource assigned to the mobile terminal. The mobile terminal may send an acknowledgement to the network and begins transmission on the assigned resource.

The exact resource request and resource assignment procedure depends on the specific wireless communication standards to be used. The above descriptions provide a general view on how the IFW assignment procedure can be performed in a generic wireless network.

The IFW handoff procedure is performed when channel resource has been allocated to the mobile terminal and the mobile terminal has begun transmission using the allocated radio resource. Figure 6 shows the flow diagram of the IFW handoff procedure. This procedure consists of several steps: channel measurement, IFW handoff decision, IFW handoff indication, and IFW handoff.

The channel measurement step consists of the continuous measurement and evaluation of the channel condition. For the forward link, the mobile terminal performs the channel measurement; the result of the measurement is then sent to the network. For the reverse link, the network performs the channel measurement.

After each measurement period, the network enters into the IFW handoff decision step. During this step, the networks determines the appropriate forward and reverse link LFW sizes for the mobile terminal, and make a decision on whether a IFW handoff should be performed by the mobile terminal. Note that depending on the network, LFW handoff may be enabled for the forward link only, reverse link only, or both forward and reverse. In addition, whether IFW handoff should be performed depends on a number of factors including, but not limited to, the severity of interference condition of the mobile terminal and the traffic load of the channels. Thus an IFW may not be performed even if it is determined that a different IFW is more suitable for the current interference condition of the mobile terminal.

If IFW handoff is required, the network enters into the IFW handoff indication step. During this step, the network indicates to the mobile terminal that an IFW handoff should be performed and provides the mobile terminal all the relevant parameters, such as the identity of the new channel, new transmitter, or new carrier, so that the mobile can perform the IFW handoff.

The final step is the IFW handoff step when the mobile terminal performs the IFW handoff as indicated by the network.

It will be apparent to those skilled in the art that various modifications can be made to the present cell selection method without departing from the scope and spirit of the present invention. It is intended that the present invention covers modifications and variations of the systems and methods provided they fall within the scope of the claims and their equivalents. Further, it is intended that the present invention cover present and new applications of the system and methods of the present invention.

Claims

1. A method for the air interface to adapt to the radio propagation conditions of different mobile terminals, wherein the air interface is divided into a plurality of partitions in either the time domain, the space domain, the frequency domain, or any combination of them, each partition is allocated a set of orthogonal spread codes with an Interference Free Window, and at least two partitions are allocated different sets of orthogonal spread codes with different Interference Free Windows.

2. A method of claim 1, wherein the said air interface is divided into a plurality of sub-frames in the time domain.

3. A method of claim 1 or 2, wherein the said orthogonal spread codes are LS codes.

4. A method of claim 1 or 2, wherein the said plurality of partitions is partitioned into a plurality of groups, and each group is associated with an Interference Free Window.

5. A method of claim 4, wherein the said orthogonal spread codes are LS codes.

6. A method of assigning radio resources to a mobile terminal, wherein the air interface is divided into a plurality of partitions in either the time domain, the space domain, the frequency domain, or any combination of them, each partition is allocated a set of orthogonal spread codes with an Interference Free Window, wherein at least two partitions are allocated different sets of orthogonal spread codes with different Interference Free Windows, and the said mobile terminal is assigned a radio resource associated with an Interference Free Window that is suitable to the radio propagation condition of the said mobile terminal.

7. A method of claim 6, wherein the said mobile terminal is assigned a radio resource associated with an Interference Free Window that is most suitable to the radio propagation condition of the said mobile terminal.

8. A method of claim 6, wherein the initial radio propagation condition of the said mobile terminal is estimated before the assignment of the radio resource to the said mobile terminal.

9. A method of claim 6, wherein during the communication session of the said mobile terminal, the network constantly evaluate the radio propagation condition of the mobile terminal, and assign the said mobile terminal another radio resource associated to a different Interference Free Window in consideration of the current radio propagation condition.

10. A method of claim 6, wherein the said air interface is divided into a plurality of sub-frames in the time domain.

11. A method of claim 6 or 10, wherein the said orthogonal spread codes are LS codes.

12. A method of claim 6 or 10, wherein the said plurality of partitions is partitioned into a plurality of groups, and each group is associated with an Interference Free Window.

13. A method of claim 12, wherein the said orthogonal spread codes are LS codes.

14. A method of adjusting assigned radio resources, wherein the air interface is divided into a plurality of partitions in either the time domain, the space domain, the frequency domain, or any combination of them, each partition is allocated a set of orthogonal spread codes with an Interference Free Window, wherein at least two partitions are allocated different sets of orthogonal spread codes with different Interference Free Windows, and during the communication session of the said mobile terminal, the network constantly evaluate the radio propagation condition of the mobile terminal, and assign the said mobile terminal another radio resource associated to a different Interference Free Window in consideration of the current radio propagation condition.

15. A method of claim 14, wherein besides the consideration of the current radio propagation condition of the said mobile terminal, other factors should be also considered in assigning the said mobile terminal another radio resource associated to a different Interference Free Window.

16. A method of claim 15, wherein the said other factors including the traffic load of the channels.

17. A method of claim 14, wherein the said air interface is divided into a plurality of sub-frames in the time domain.

18. A method of claim 14 or 17, wherein the said orthogonal spread codes are LS codes.

19. A method of claim 14 or 17, wherein the said plurality of partitions is partitioned into a plurality of groups, and each group is associated with an Interference Free Window.

20. A method of claim 19, wherein the said orthogonal spread codes are LS codes.