WO2002011317A1

WO2002011317A1 - A tdd framing method for physical layer of a wireless system

Info

Publication number: WO2002011317A1
Application number: PCT/CN2000/000165
Authority: WO
Inventors: Daoben Li; Yongsheng Zhang
Original assignee: Linkair Communications, Inc.
Priority date: 2000-06-20
Filing date: 2000-06-20
Publication date: 2002-02-07
Also published as: CN1150697C; WO2002011317A8; HK1050771A1; CN1385013A; AU5387000A

Abstract

A TDD framing method for physical layer of a wireless system, wherein each frame comprises a plurality of burst structures of DPCH and at least a burst structure of CCPCH. Each of the said plurality of burst structures of DPCH can be allocated to either uplink or downlink and comprises a plurality of time slots, the number of the said time slots can be determined by the number of pulses of a LA code, the said time slot length varies with the variation of the pulse interval of the said LA code, while each time slot can be modulated with selected orthogonal spread spectrum codes. The said burst structure of CCPCH is allocated to downlink in the even/odd frames and to uplink in the odd/even frames. The present method can greatly reduce the ACI, the requirement for the number of spreading codes, as well as the requirement for the lengths of the spreading codes in a wireless system. At the same time, it can ideally support mobile IP services.

Description

A TDD FRAMING METHOD FOR PHYSICAL LAYER

OF A WIRELESS SYSTEM

FIELD OF THE INVENTION:

This invention relates generally to wireless communication systems, and in particular, to a method for providing a frame structure using in Time Division Duplex (TDD) mode, which can provide higher capacity and performance for mobile communication services, especially the IP-like services.

BACKGROUND OF THE INVENTION:

Today, mobile communications are becoming one of the important factors that influence our living styles. It is foreseen that the future telecommunication network will be the " mobile IP " network.

For " mobile ", the key is to support higher spectral efficiency and higher moving speed. For " IP ", the key is to support asymmetric traffic, higher throughput and smaller delay.

It is well known that the multiple access technology and the duplex technology are the key technologies for system design. FDD FDMA is used in the 1st generation system. FDD TDMA and FDD CDMA are used in the 2nd generation system. In the proposed 3rd generation system, FDD CDMA is adopted in both UTRA FDD (or W-CDMA) and IS-2000, and TDD CDMA/TDMA is adopted in both UTRA TDD and TD-SCDMA. From the technical viewpoint, the composite multiple access scheme FDMA/CDMA/TDMA with TDD can be capable of supporting the " mobile IP " services. It is well known that the capacity of a CDMA system is limited by the interferences. The interferences include Inter-Symbol Interference (ISI) among multipath signals from a same remote unit, Multiple Access Interferences (MAI) among signals from different remotes units in the service area of a same base station, and Adjacent Cell Interference (ACI) among signals from neighboring base stations and the remote units that they serve.

In current CDMA wireless systems, to reduce the ACI, different base stations in different, nearby cells have to use different spreading codes at a certain time. Thus the code length of spreading codes have to be very long to provide enough spreading codes. This greatly increases the complexity of the system.

Existing CDMA wireless systems use pseudo-random spreading codes that result in non-zero interferences. Even when orthogonal spreading codes such as Walsh codes are used that give zero interference, the orthogonal property can be destroyed when there are multipath signals from a same remote unit or the signals from different remote units are not synchronized, resulting in interferences among different signals.

In PCT application PCT/CN98/00151, invented by Li Daoben and entitled "A Spread Spectrum Multiple Access Coding Method," a coding scheme called Large Area code (LA code) was disclosed, wherein spread spectrum access code consists of basic pulses that have normalized amplitude and duration of 1 and polarity, the number of basic pulses is ascertained by such practical factors: the requested number of users, the number of usable pulse compression codes, the number of usable orthogonal carrier frequencies, system bandwidth and system maximal information rate, the intervals between these basic pulses on time axis are various, and coding just utilizes the dissimilarity of pulse positions and orders of pulses' polarities. Hereinafter such codes will be called LA codes or LA-CDMA codes, which have the same meaning. Table 1 shows a primary LA-CDMA code with 17 pulses with its corresponding sequence of 17 time slots with different lengths.

Table, 1 Primary LA-CDMA code

When relaxing the restriction of orthogonality, i.e. to adopt quasi-orthogonality which uses imperfect orthogonal codes, to increase the number of users. For example, considering an LA code with N pulses, as the order of N basic intervals has no affect on its auto-correlation and cross-correlation functions, it can be arbitrary. When a code group with various orders of basic intervals is exploited at the same time, the number of users will increase enormously.

The orthogonal characteristic or quasi-orthogonality of the LA codes can serve as a solution for reducing interference of adjacent service areas or channels.

SUMMARY OF THE INVENTION:

An object of the present invention is to provide a TDD framing method for physical layer of a wireless system.

Another object of the present invention is to provide a TDD framing method that could reduce the Adjacent Cell Interference (ACI) among signals from neighboring base stations and the remote units that they serve. The objects and advantages are realized by the following methods in accordance with the present invention.

A TDD framing method for physical layer of a wireless system, wherein each frame comprises a plurality of burst structures of DPCH and at least a burst structure of CCPCH. Each of the said plurality of burst structures of DPCH can be allocated to either uplink or downlink and comprises a plurality of time slots, the number of the said time slots can be determined by the number of pulses of a LA code, the said time slot length varies with the variation of the pulse interval of the said LA code, while each time slot can be modulated with selected orthogonal spread spectrum codes. And the said burst structure of CCPCH is allocated to downlink in the even/odd frames and to uplink in the odd/even frames.

The permutation position of the said LA codes can be recombined, and the permutation of the said time slot can be also recombined corresponding to it.

The pulse polarity of the said LA codes can be transformed, and the polarity of the said time slot can be also transformed corresponding to it.

Different base stations from different, nearby cells shall be assigned different LA codes so that adjacent cell interference can be reduced, while in nearby cells, the same spreading code can be assigned. Therefore greatly reduce the requirement for the number of spreading codes, as well as the requirement for the lengths of the spreading codes. At the same time, since each of the said plurality of burst structures of DPCH in each frame can be allocated to either uplink or downlink, it can ideally support asymmetric traffic, higher throughput and smaller delay, in other words, the " mobile IP " services.

BRIEF DESCRIPTIONS OF THE 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, and not restrict, the principles and advantages of the present invention.

Fig.l is a prior art frame structure of UTRA TDD defined in 3 GPP specifications.

Fig.2 is another prior art frame structure of TD-SCDMA defined in 3 GPP specifications.

Fig.3 is a TDD frame structure according to a preferred embodiment of this invention.

Fig.4 illustrates the detailed TDD radio frame according to the embodiment of Fig. 3.

Fig.5 is the burst structure of DPCH according to a preferred embodiment of the present invention.

Fig.6 is one type of burst structure of CCPCH according to a preferred embodiment of the present invention.

Fig.7 is another type of burst structure of CCPCH according to a preferred embodiment of the present invention.

DETAILED DESCRIPTIONS OF THE INVENTION:

Fig.l is the frame structure of UTRA TDD defined in 3GPP specifications. Fig.2 is the frame structure of TD-SCDMA defined in 3GPP specifications. While the TDD frame according to the present invention can be designed to be compatible with UTRA TDD with chip rate 1.28 Mcps, which will have the similar frame structures with multiple switching points.

For CDMA TDD system design, the frame structure is one of the key factors. Some of the main concerns are capacity, coverage, flexibility, and compatibility, which will be separately interpreted herewith.

The capacity is constrained by the interferences. The consequence of these interference sources is a negative impact on system performance and capacity. Many methods have been tried and used to reduce these interferences. For example, in UTRA TDD and TD-SCDMA, Joint Detection is used to reduce ISI and MAI, and smart antenna is also used in TD-SCDMA to reduce interferences. Although these technologies can improve the system performance with the expense of system complexity, the biggest problem is that all the above interferences cannot be eliminated to the ideal level due to the drawbacks of system design.

In CDMA TDD system, the coverage is mainly determined by the gap length between transmission and reception. The larger the switching gap is, the larger the coverage will be supported. However, the gap length is contradicted with the capacity or spectral efficiency. In UTRA TDD, the gap is so small that it is suitable for Pico-cell or micro-cell environment. In TD-SCDMA, the gap is large enough to support macro-cell environment, but it cannot efficiently support the smaller cell because the fixed gap length.

Concerning flexibility, it is highly required that flexible services can be supported in the TDD system in order to be capable for "mobile IP" applications. One common way used in TDD system is dynamic channel allocation. However, in UTRA TDD like CDMA TDD system, it is not so efficient because there will be additional interferences introduced into the system, and Joint Detection cannot work very well.

Concerning mobility, in the traditional CDMA TDD system, the fast close-loop power control cannot be achieved because the power control rate is determined by the frame length. The imperfect power control will result severe degradation for system performance. And higher speed mobility means fast channel fading, which is expected to be compensated with fast power control. Thus, it cannot support high-speed mobility. This is the case in UTRA TDD. In TD-SCDMA, another limitation for high speed mobility is because of smart antenna.

Concerning compatibility, FDD and TDD are usually regarded as two very different system, there are fundamental differences even between UTRA FDD and UTRA TDD. However, from the technical point of view , it is better that there are as many commonalities as possible , thus the compatibility between FDD and TDD can be achieved.

In a PCT Application with inventor, number and title of it respectively as Li Daoben, PCT-CN00/00028 and "A Scheme for Spread Spectrum Multiple Address Coding with Interference Free Window" , disclosed a kind of complementary orthogonal codes referred to here as LS codes. The LS codes have a "Interference Free Window" property, which is also referred to as "zero correlation window" property. As an illustration, consider the following four LS codes of length 8:

(Cl, SI) = (++-+, +-)

(C2, S2) = (+++-, +-++)

(C3, S3) = (-+++, --+-)

(C4, S4) = (-+--, -+)

The cross-correlation of any two of these codes is zero when the time shift between the two codes is within the (inclusive) window [-1, +1], and the auto-correlation of any of these codes is zero except when there is no time shift. Thus these four codes have a Interference Free Window of [-1, +1].

Similarly the following LS codes of length 16 have an Interference Free Window of [-3, +3]:

(Cl, SI) = (++-++++-, +—+-++)

(C2, S2) = (++-+-+, +--+-)

(C3, S3) - (+++-++-+, +-+++—)

(C4, S4) = (+++— +-, +-++-+++)

If we consider only (C1,S1) and (C2,S2), they have a Interference Free Window of [-7,+η.

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 interferences and multiple access interferences when multipath signals from a same remote unit and signals from different remote units arrive within an Interference Free Window.

As a first preferred embodiment of the present invention, the said selected orthogonal spread spectrum codes can be LS codes. And such a framing method, frame, or system that combines LA code and LS code in TDD mode will be referred to as LAS-CDMA TDD mode hereinafter.

In the above mentioned preferred embodiment of the present invention, ISI and MAI can be reduced to zero for all signals within a zero-correlation window, i.e., a time window within which there is zero-correlation, while ACI can be reduced to a marginal level. As long as multipath signals from a same remote unit and signals from multiple remote units are synchronized within a zero-correlation window, the ISI and MAI can be reduced to zero. Thus, high system performance and capacity can be ideally achieved. In the preferred embodiment of LAS-CDMA TDD mode according to the present invention, all the signals will be kept within an " interference free " time window via bi-synchronization. And fast power control is not needed in this preferred embodiment, only slow power control will be adopted to save power of mobile station. Therefore, high mobility speed can be easily achieved.

Concerning flexibility, in the preferred embodiment of LAS-CDMA TDD mode according to the present invention, the FDMA/TDMA/CDMA composite multiple access scheme can be adopted. The transmission/reception is based on the unit of " Sub-frame (Time Slot) - Code - Frequency ". With the modularity of data unit, it can be adapted to be capable of supporting variable data rate, especially the packet data. Since the switching point of uplink and downlink can be flexibly allocated within one frame, and all the sub-frames (time slots) can be flexibly allocated to either uplink or downlink, it can support the IP type asymmetric traffic. Therefore high flexibility can be easily achieved.

Preferably, the said LS codes fill the said time slot in form of an LS frame, which has a certain length and further includes C component for C code and S component for S code, while the C code and the S code of the LS code are filled in the said C component and S component separately.

Preferably, when the length of the said allocated LS codes is shorter than length of the said C component plus the said S component, multiple LS codes can be used to fill the said C component and the said S component of the said LS frame.

Preferably, a gap for TDD switching between transmitting and receiving is inserted between consecutive burst structures. As a second preferred embodiment of the present invention, the last several time slots or symbols within each burst structure can be kept as null or punctured, such that the said gap can be enlarged to support larger coverage. Therefore, the frame structure according to this preferred embodiment can provide the feasibility for dynamic coverage area, i.e., the system can be flexibly modified to be capable of different coverage area. It can support Pico-cell, micro-cell and macro-cell scenarios.

A third preferred embodiment of the frame structure according to the present invention is illustrated in Fig.3. Within 10 ms radio frame, 5 sub-frames (SF) will be available, and each sub-frame can be allocated to either uplink or downlink. The length of each sub-frame will be dependant on which type of transport channel is mapped on this sub-frame.

In Fig.4, the detail of the above radio frame is illustrated. Each sub-frame can be allocated to either uplink or downlink. Two 10 ms radio frames are considered. In the (2k)-th frame, BSCH is mapped to the 1st sub-frame, which will be used as downlink synchronization and broadcasting. In the (2k+l)-th frame, ACH is mapped to the 1st sub-frame, which will be used as uplink random access and synchronization. The remaining 4 sub-frames within each frame can be used as DCH to transmit either uplink traffic or downlink traffic.

The gap of TDD switching between transmitting and receiving is dependant on which type of transport channel is mapped in the sub-frame. The length of gap is used for TDD switching between transmitting and receiving, and timing adjustment.

Considering the normal case in example: For DCH, the length of gap is 36 Tc = 28.1 μs. For BSCH , the length of gap is 41 Tc = 32.0 μs . For ACH, the length- of gap is 44 Tc = 34.3 μs. Thus when the switching points are allocated flexibly, the maximum coverage (or cell radius) will be 4.2 km.

In order to support larger coverage, one simple way is that the last time slot within each sub-frame of BSCH, ACH or DCH can be kept as null or punctured, such that the gap can be enlarged and the tolerated round-trip propagation delay can be increased. However, the maximum coverage will also be determined by the window width of ACH. The burst structure of DPCH (Dedicated Physical Channel) that will be mapped from DCH is based on the composite LA/LS code with LA codes (17,136,2559). It is illustrated in Fig.5. Each sub-frame is filled with one LA code of length 2559 Tc (see Table 1). There are total 17 pulses for one LA code, and each pulse is substituted with LS code. So there are 17 time slots (TS) within one sub-frame. The last 36 Tc gap is used as the TDD switching gap. In default, the 1st time slot (TS) of each sub-frame is always used for pilot. In order to support high mobility, multiple pilots can be inserted into the sub-frame. The remaining 16 time slots are used for data transmission. Within each time slot, the available LS spreading codes will depend on the data rate, the propagation environment and so on. In order to support variable data rate, multiple LS code pairs can be adopted in one time slot. With this TDMA CDMA scheme, variable data rate can be easily supported in LAS-CDMA TDD mode.

The burst structure of CCPCH (Common Control Physical Channel) contains two types. Type 1 is used for BSCH mapping, and type 2 is used for ACH mapping. In Fig.6, burst structure type 1 is illustrated. In Fig. 7, burst structure type 2 is illustrated.

BSCH is mapped to CCPCH of burst type 1, in which 41 Tc gap is used for TDD switching between transmitting and receiving. There are total 87 symbols to be transmitted, and the 1st symbol is the reference symbol for DQPSK. So the data rate of BSCH is 8.6 kbps.

BSCH will be mapped to the 1st sub-frame of (2k)-th frame as illustrated in Fig.4. And base station (BS) will transmit it omni-directionally with full power level.

In order to support larger coverage, the last 2 symbols can be omitted or punctured, such that the gap can be enlarged.

ACH is mapped to CCPCH of burst type 2, in which 52 Tc gap is used for TDD switching between transmitting and receiving, and 16 Tc gap is used for guard period between ACH and DCH. There are total 13 access slots available, and there are total 650 access slots available in 1 second. ACH will be mapped to the 1st sub-frame of (2k+l)-th frame as illustrated in Fig.4. And mobile station (MS) will transmit it with highest power level.

When f = 2 GHz , λ = 0.15 m , each access slot is designed to support uplink γ synchronization within a cell of radius °

3 x lO^s ,s x 48

>^*o ^{= '} = 11.25 km 1.28MHz

According to a fourth preferred embodiment of the present invention, a method for transmitting downlink information and a method for transmitting uplink information using the framing method of the present invention are disclosed.

The method for transmitting downlink control and user information is composed of Broadcast and Synchronization Channel ( BSCΗ ) information transmitting and Control and user information transmitting.

Wherein Broadcast and Synchronization Channel ( BSCΗ ) information transmitting including:

Multiplexing and encoding downlink broadcast and synchronization channel information into a data stream;

Partitioning the data stream into super-frame and frame;

Selecting the 1st sub-frame in the even/odd frames for the broadcasting and synchronization information transmitting; Mapping the BSCH information onto this sub-frame according to the BSCH burst structure;

Wherein control and user information transmitting including:

Determining the required Interference-Free Window width according to the real-time multi-path delay measurements for the corresponding user;

Determining the required number of pilots in each sub-frame according to the real-time Doppler or mobility speed measurements for the corresponding user;

Determining the switching point between transmission and reception according to the traffic characteristics with the frame structure, which may be either single switching point or multiple switching points;

Determining the gap length between transmission and reception according to the coverage requirement, and the gap length can be adjusted via symbol puncturing in the corresponding sub-frames with either DPCH or CCPCH burst structures;

Determining the required number of sub-frames according to the service data rate with the capability of DPCH burst structure;

Partitioning the user information into different sub-frames;

Mapping the user information onto the available sub-frames. The method for transmitting uplink control and user information using the framing method of the present invention is composed of Random Access Information ( ACH ) information transmitting and control and user information transmitting.

Wherein Random Access Information ( ACH ) information transmitting, including:

Multiplexing and encoding uplink random access channel information into a data stream;

Partitioning the data stream into super-frame and frame;

Selecting the 1st sub-frame in the odd/even frames for the uplink random access information transmitting;

Mapping the ACH information onto this sub-frame according to the ACH burst structure;

Wherein control and user information transmitting, including:

Partitioning the user information into different sub-frames;

Mapping the user information onto the available sub-frames.

It will be apparent to those skilled in the art that various modifications can be made to the present methods 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 TDD framing method for physical layer of a wireless system, wherein each frame comprises:

A plurality of burst structures of DPCH, each can be allocated to either uplink or downlink and comprises a plurality of time slots, the number of the said time slots can be determined by the number of pulses of a LA code, the said time slot length varies with the variation of the pulse interval of the said LA code, while each time slot can be modulated with selected orthogonal spread spectrum codes, and

At least a burst structure of CCPCH, which is allocated to downlink in the even/odd frames and to uplink in the odd/even frames.

2. A method of claim 1, wherein the permutation position of the said LA codes can be recombined, and the permutation of the said time slot can be also recombined corresponding to it.

3. A method of claim 1, wherein the pulse polarity of the said LA codes can be transformed, and the polarity of the said time slot can be also transformed corresponding to it.

4. A method of claim 1, wherein the selected orthogonal spread spectrum codes are LS codes.

5. A method of claim 4, wherein the said LS codes fill the said time slot in form of an LS frame, which has a certain length and further includes C component for C code and S component for S code, while the C code and the S code of the LS code are filled in the said C component and S component separately.

6. A method of claim 4, wherein when the length of the said allocated LS codes is shorter than length of the said C component plus the said S component, multiple LS codes can be used to fill the said C component and the said S component of the said LS frame.

7. A method of claim 1 , wherein a gap for TDD switching between transmitting and receiving is inserted between consecutive burst structures.

8. A method of claim 7, wherein the last several time slots or symbols within each burst structure can be kept as null, such that the said gap can be enlarged to support larger coverage.

9. A method of claim 1, wherein the said frame is 10 milliseconds in length, and the said LA codes includes 17 pulse intervals with the minimum interval 136 chips.

10. A method of claim 1 or 4, wherein the said burst structure of CCPCH is allocated to downlink as BSCH which includes a plurality of symbols and to uplink as ACH which includes a plurality of access time slots.

11. A method for transmitting downlink information using the framing method of claim 1 or 4, in which the said burst structure of CCPCH is allocated to downlink as BSCH including a plurality of symbols, comprising steps of BSCH information transmitting, and control and user information transmitting, wherein:

The said BSCH information transmitting includes the steps of:

Multiplexing and encoding downlink broadcast and synchronization channel information into a data stream; Partitioning the data stream into super-frame and frame; Selecting the 1st sub-frame in the even/odd frames for the broadcasting and synchronization information transmitting; and Mapping the broadcasting and synchronization information onto this sub-frame according to the BSCH burst structure; The said control and user information transmitting includes the steps of:

Partitioning the user information into different sub-frames; and

Mapping the user information onto the available sub-frames.

12. A method for transmitting uplink information using the framing method of claim 1 or 4, in which the said burst structure of CCPCH is allocated to uplink as ACH including a plurality of access time slots, comprising steps of ACH information transmitting, and control and user information transmitting, wherein:

The said ACH information transmitting includes the steps of:

Partitioning the data stream into super-frame and frame; Selecting the 1st sub-frame in the odd even frames for the uplink random access information transmitting; and Mapping the random access information onto this sub-frame according to the ACH burst structure;

The said control and user information transmitting includes the steps of:

Determimng the required Interference-Free Window width according to the real-time multi-path delay measurements for the corresponding user;

Partitioning the user information into different sub-frames; and

Mapping the user information onto the available sub-frames.