WO2009045135A1

WO2009045135A1 - Pilot design for tdd ofdm systems

Info

Publication number: WO2009045135A1
Application number: PCT/SE2007/050710
Authority: WO
Inventors: Afif Osseiran; Jiann-Ching Guey
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-10-04
Filing date: 2007-10-04
Publication date: 2009-04-09

Abstract

According to teachings presented herein, pilot symbols are staggered in time, to better distribute them across the Orthogonal Frequency Division Multiplexing (OFDM) symbols within an ongoing series of transmitted OFDM frames. For example, the stagge insertion pattern can be defined to make the number of pilots transmitted in each OFDM symbol time equal, thereby equalizing the peak transmit power across time. Correspondingly, the data symbols for given users are also staggered accordingly, so th channel estimation by those users can be achieved by interpolation, which is often more accurate than extrapolation.

Description

PILOT DESIGN FOR TDD OFDM SYSTEMS BACKGROUND

Technical Field

[0001] The present invention generally relates to Orthogonal Frequency Division Multiplexing (OFDM) systems, such as cellular and other wireless communication networks that make use of OFDM-based transmissions, and particularly relates to pilot transmission in Time Division Duplex (TDD) OFDM systems. Background

[0002] Orthogonal Frequency Division Multiplexing (OFDM) is a technology that transmits multiple signals simultaneously over a wired or wireless communication medium. In wireless communications, the OFDM receiver is relative simple, because the multiple data streams are transmitted over a number of parallel flat fading channels. In fact equalization is not done in the time domain; instead, one-tap filters in the frequency domain are sufficient. [0003] In an OFDM system, known symbols referred to as "pilot symbols^' are transmitted across the time-frequency plane, for use by the receiving device in estimating estimate the overall OFDM channel's time-frequency response. In turn, that channel estimation allows the receiving device to perform coherent demodulation of data symbols. [0004] Because the channel's time-frequency response is a slow-varying two- dimensional process, the pilot symbols essentially sample this process and therefore need to have a density that is high enough for the receiving device to reconstruct (or interpolate) the full response. That is, the number and distribution of pilot symbols within the OFDM time- frequency plane must be sufficient to support accurate channel estimation over that space. [0005] If the window in which the pilot symbols are observed and utilized for channel estimation is small, the pilot density needs to be higher, and, generally, the power of the pilot symbols needs to be boosted above that of a data symbols. Higher pilot power Such is used for Frequency Division Duplex (FDD) systems, and for the case for the Time Division Duplex (TDD) mode in the proposed WINNER (Wireless World Initiative New Radio) air interface as shown in Fig. 1. Additional WINNER information appears in IST-4-027756 WINNER II, D3.16.7, Test Scenarios and Calibration Cases Issue 2," dated December 2006. [0006] Recently, it has been proposed that within the downlink transmission half of a TDD OFDM frame, pilot symbols are inserted in the first and last OFDM symbols, so that the channel response can be interpolated for the data symbol position lying between them. This pilot placement approach is shown in Fig. 2. (An OFDM symbol is taken as the overall collection of transmitted sub-carriers over one OFDM symbol time, where, as a simple non- limiting example, eight OFDM symbols are illustrated in the downlink half of the TDD OFDM frame of Fig. 2.)

[0007] Among the problems attending with this proposal is that (potentially significant) higher peak power results for the OFDM symbols carrying pilot symbols. In other words, the first and last downlink OFDM symbol times in each frame include four pilot symbols and the powers of those pilot symbols must be boosted much higher if pilot symbols from past and future frame are not utilized for channel estimation.

[0008] Further, if pilot symbols in the past and future OFDM frames are utilized, the pilot insertion in time is not regular, making channel estimation more complicated to implement. For the example shown in Fig. 2, the pilot symbols for the recurring OFDM frames are transmitted during the 1st, 8th, 17th and 24th OFDM symbol intervals. The non-regular occurrence of pilot symbols complicates interpolation-based channel estimation at the targeted receiver(s).

[0009] Still further, pilot symbols transmitted by all base stations are located in the same OFDM symbols (the first and last in a frame), because data frames are synchronized across the entire network in a TDD system. The pilot symbols from different base stations may interfere with each other, especially at cell borders, making channel estimation less accurate. The situation is worse when multiple antennas for spatial multiplexing are employed at the base station since every antenna requires a pilot. SUMMARY

[0010] According to teachings presented herein, pilot symbols are staggered in time, to better distribute them across the Orthogonal Frequency Division Multiplexing (OFDM) symbols within an ongoing series of transmitted OFDM frames. For example, a staggered insertion pattern can be defined to make the number of pilots transmitted in each OFDM symbol time equal, thereby equalizing the peak transmit power across time. Correspondingly, the data symbols for given users are staggered accordingly, so that channel estimation by those users can be achieved by interpolation, which is often more accurate than extrapolation.

[0011] In one embodiment, therefore, a method of transmission in a Time Division Duplex (TDD) OFDM system comprises inserting pilot symbols into OFDM frames according to a staggered pilot insertion pattern that spans more than one OFDM frame and evenly distributes pilot symbols across OFDM symbols within the OFDM frames. For example, the staggered pilot insertion pattern may be timed to place a set of staggered pilots within the downlink half of every successive TDD OFDM frame, thereby spanning the intervening uplink halves of those OFDM frames. Of course, longer insertion periods may be used, where the insertion period spans multiple TDD OFDM frames.

[0012] Regardless, the method further comprises transmitting user data within data blocks are bounded by the staggered pilot insertion pattern. Here, the "data blocks" comprise those data symbol positions that lie in the time-frequency plane between the inserted pilots. For example, the pilot insertion pattern may define paired sets of pilot symbols, each paired set comprising a first set of pilot symbols in one OFDM frame and a corresponding second set of pilot symbols in a later OFDM frame. In that case, a data block comprises those data symbol positions lying between one of the paired sets of pilot symbols. [0013] Thus, when viewing successive OFDM frames over time, inserting set(s) of staggered pilot symbols into the downlink portion of the OFDM frames forms a regularly repeating pattern of pilot symbols bounding the data symbols lying between them, and thereby allowing interpolation-based channel estimation for those intervening data symbols, provided the user data is made to span the OFDM symbol times between the inserted sets of pilots.

[0014] Further, in one or more embodiments, transmitting user data within data blocks bounded by the staggered pilot insertion pattern comprises forcing data for given users to time-wise span the OFDM frames spanned by the staggered pilot insertion pattern. Doing so allows the receiver targeted by a given data block to use the pilot symbols bounding that data block for interpolation-based channel estimation over the time-frequency domain of the data block.

[0015] Further, in one or more embodiments, the time-frequency plane of each OFDM frame may be subdivided into OFDM chunks, each OFDM chunk allocable to a different user. Thus, inserting pilot symbols into OFDM frames according to a staggered pilot insertion pattern that spans more than one OFDM frame may comprise inserting staggered pilot symbols on an OFDM chunk basis. That is, a staggered pilot insertion pattern may be sized according to OFDM chunk dimensions, and sets of staggered pilot symbols may be inserted into one or chunks within a given OFDM frame, and repeated according to a desired insertion interval in like (corresponding) chunks in succeeding OFDM frames. Thus, in at least one embodiment, the staggered pilot insertion pattern is defined on an OFDM chunk basis — i.e., dimensioned according to OFDM chunk dimensions — such that staggered pilot symbols recur within like chunks in successive (but not necessarily contiguous) OFDM frames at a spacing defined by an insertion period of the staggered pilot insertion pattern. [0016] Whether or not defined on a chunk basis, the staggered pilot insertion pattern may be defined using a diagonal distribution of pilot symbols within the time-frequency domain. A diagonal distribution may, for example, place pilot symbols at staggered, stairstep time-frequency positions within an OFDM chunk or overall OFDM frame. Alternatively, the staggered pilot insertion pattern may be defined using a Costas distribution within the time-frequency domain.

[0017] As a further aspect, the use of staggered pilot insertion patterns may be preconfigured or dynamically managed across two or more network transmitters, e.g. base stations. With this method, different staggered pilot insertion patterns are used at different base stations, to reduce pilot symbol interference between neighboring base stations. That is, by using different insertion patterns, pilot symbol overlap between base stations is reduced, if not eliminated.

[0018] One or more of the above aspects of pilot transmission may be implemented in a base station by appropriately configuring included processing circuits. Such processing circuits comprise hardware, software, or any combination thereof. As a non-limiting example, a base station or other network transmitter is configured for use in a wireless communication network and includes one or more microprocessor-based circuits that are configured or otherwise provisioned via stored computer program instructions. As such, the staggered pilot transmission method presented herein, and variations of that method, are carried out at the base station based on execution of the stored program instructions. Note that the base station processing circuits include, in one or more embodiments, medium access control processing circuits, which are configured to force user data to span the data blocks bounded by the staggered pilot insertion patterns.

[0019] Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Fig. 1 is a diagram of a known "superframe" OFDM structure that may be used in a Time Division Duplex (TDD) OFDM carrier transmission according to the WINNNER protocols, such as may be carried out by a base station in a wireless communication network.

[0021] Fig. 2 is a diagram illustrating known, regular pilot symbol placements for an OFDM signal. [0022] Fig. 3 is a diagram illustrating one embodiment of a pilot transmission method, wherein pilot symbols are inserted according to a staggered pilot insertion pattern that spans more than one OFDM frame, according to the desired insertion interval used for the pattern. [0023] Fig. 4 is a diagram illustrating another embodiment of a pilot transmission method, wherein pilot symbols are inserted according to a staggered pilot insertion pattern that spans more than one OFDM frame.

[0024] Fig. 5 is a diagram illustrating another embodiment of a pilot transmission method, wherein pilot symbols are inserted according to a staggered pilot insertion pattern that spans more than one OFDM frame.

[0025] Fig. 6 is a bock diagram illustrating a radio base station that includes processing circuit(s) configured to implement a pilot transmission method according to the teachings presented herein.

DETAILED DESCRIPTION

[0026] Fig. 3 is a simplified "signal" diagram showing successive, but not necessarily time-wise contiguous OFDM chunks 10 and 12 within a recurring series of OFDM frames. More particularly, downlink transmissions comprising first and second OFDM chunks 10 and 12 are shown separated in time, such as by an intervening uplink transmission. A set of staggered pilot symbols 14 is inserted at a defined insertion interval that spans more than one OFDM frame. While OFDM frames are not explicitly illustrated in Fig. 3, one may assume that OFDM chunk 10 lies in a first downlink OFDM frame and that OFDM chunk 12 lies in a later-transmitted, second downlink OFDM frame.

[0027] One sees that inserting the symbols 14 according to the staggered pilot insertion pattern defines a "bounded" data block 16, comprising the data symbol positions lying between, i.e., bounded by, the regularly recurring sets of pilot symbols 14. Note, however, that the sets of pilot symbols 14 may be spaced apart in the frequency domain, such that they extend beyond the frequency positions of the data block 16 — i.e., outside of the frequency span of the illustrated chunks 10 and 12. Thus, it should be understood that the set of pilot symbols 14 may span more than just the sub-carriers of the data block 16. Nonetheless, they may be used for interpolation-based channel estimation over the whole portion of the time-frequency plane bounded by the inserted sets of pilot symbols 14, and/or over selected portions, such as that portion associated with the data block 16 that is of interest here.

[0028] By forcing the data for a given user to span such data blocks, the targeted user may employ interpolation-based channel estimation of the time-frequency plane of the data block 16 based on the bounding sets of staggered pilots 12, for improved channel estimation (as compared to an extrapolation-only approach). Note that the pattern illustrated in Fig. 3 comprises a diagonal distribution, otherwise known as a "chirping " pattern. [0029] Fig. 4 illustrates pilot and data symbol placements according to another embodiment of the teachings presented herein. The illustration depicts two downlink OFDM frames, separated by one uplink OFDM frame, as is done in TDD-based OFDM systems. The pilot symbols, represented in the drawing by the cross-hatched tiles, are placed in horizontal scan lines according to a pattern that bridges the intervening uplink frame. More particularly, one sees that the pilot distribution comprises the time-frequency staggering of a regularly-spaced pilot pattern. The amount of staggering for each horizontal scan line is linearly proportional to the frequency of the scan line. (A horizontal scan line includes the OFDM symbol time positions taken along one "row" of the recurring OFDM symbols, i.e., taken over time along one sub-carrier frequency position.)

[0030] Forcing the downlink data for a given user to span the two downlink frames along one or more horizontal scan line allows that given user to perform interpolation-based channel estimation for the data. That is, the un-pattemed tiles represent a block of data symbols whose complex channel coefficients can be derived from the illustrated pilot symbols by interpolation. Interpolation in this sense contemplates two-dimensional interpolation across the time-frequency plane. Further, the diagonally hatched tiles represent user data symbols for which interpolation-based channel estimation can be performed using the pilots in one or the other of the two illustrated downlink frames, and the corresponding pilots in preceding or succeeding downlink frames (which are not illustrated). [0031] Note that the shifting of the horizontal scan line needs not form a chirp pattern. Other patterns such as the Costas array are also possible. More information regarding the use of Costas arrays appears in the paper, J. Guey, "Synchronization Signal Design for OFDM Based on Time-Frequency Hopping Patterns," Proc. IEEE ICC 2007, Glasgow, Scotland, June, 2007. Fig. 5 illustrates an example of Costas-based pilot distribution, for implementing pilot transmission according to a staggered pilot insertion pattern. [0032] As an additional point usefully illustrated by Fig. 5, it should be understood that "staggered" and "staggering" as discussed herein for pilot symbol insertion broadly connotes essentially any irregular positioning of pilots over a desired span of the time-frequency plane of an OFDM symbol. Thus, a basic arrangement of staggered pilot symbols may be defined — i.e., a set of pilot symbols to be placed at desired time-frequency locations within an OFDM frame, OFDM chunk, etc. — and that set of pilot symbols can be inserted at a desired insertion interval, to form regularly recurring patterns of inserted pilot symbols useful for interpolation-based channel estimation at the targeted receiver(s). [0033] Fig. 6 illustrates a radio base station 20, which is configured for pilot/data transmission according any one or more of the embodiments taught herein. The base station 20 includes transceiver circuits 22 for wireless transmitting downlink signals to users (i.e., mobile stations, terminals, or other wireless communication devices). The base station 20 further includes interface/control circuits 24 for interfacing with other entities in a wireless communication network, and for controlling base station operations. [0034] These circuits in particular include transmission processing circuits 26, for implementing pilot/data transmission according to the teachings presented herein. The processing circuits 26 comprise, for example, one or more microprocessor-based circuits, including memory or other storage elements for holding program code that defines the logical processing needed to implement pilot/data transmission as taught herein. [0035] A non-limiting examples of the advantages attending the pilot and data OFDM transmission techniques presented herein includes the improved distribution of pilot symbols across OFDM symbol times. In at least one embodiment, pilot symbols are not concentrated in one OFDM symbols; rather each OFDM symbol has the same number of pilot symbols. Therefore, even where pilot symbols are boosted, each downlink OFDM symbol has the same transmit power, which allows more efficient operation of the transmit power amplifiers. As a further advantage, the constant pilot insertion rate along horizontal scan lines permits reduced complexity channel estimation at the targeted receivers.

[0036] For example, mobile communication receivers operating in such a system may be advantageously configured to use Discrete Fourier Transform (DFT) based channel estimation solutions, such as those presented in the paper, J. Guey, D. Hui, and A. Hafeez, "Classical Channel Estimation for OFDM Based on Delay-Doppler Response," Proc. IEEE PIMRC 2007, Athens, Greece, Sep. 2007. Still further, the interference among pilot symbols from different base stations may be made less severe and more uniform, because each base station may employ a different variation of a regularly recurring pilot symbol pattern to avoid or randomize interference.

[0037] Thus, the foregoing description and the accompanying drawings represent non- limiting examples of the methods and apparatus taught herein. As such, the present invention is not limited by the foregoing description and accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.

Claims

CLAIMS What is claimed is:

1. A method of transmission in a Time Division Duplex (TDD) Orthogonal Frequency Division Multiplexing (OFDM) system comprising: inserting pilot symbols into OFDM frames according to a staggered pilot insertion pattern that spans more than one OFDM frame and evenly distributes pilot symbols across OFDM symbols within the OFDM frames; and transmitting user data within data blocks are bounded by the staggered pilot insertion pattern.

2. The method of claim 1 , wherein the staggered pilot insertion pattern defines paired sets of pilot symbols, each paired set comprising a first set of pilot symbols in one OFDM frame and a corresponding second set of pilot symbols in a later OFDM frame, and wherein each data block for transmitting user data comprises those data symbol positions lying between one of the paired sets of pilot symbols.

3. The method of claim 1 , wherein transmitting user data within data blocks bounded by the staggered pilot insertion pattern comprises forcing data for given users to time-wise span the OFDM frames spanned by the staggered pilot insertion pattern, such that the pilot symbols bounding each given data block enable interpolation-based channel estimation over the time-frequency domains of each given data block.

4. The method of claim 1 , wherein each OFDM frame comprises one or more OFDM chunks allocable to different users, and wherein inserting pilot symbols into OFDM frames according to a staggered pilot insertion pattern that spans more than one OFDM frame comprises inserting staggered pilot symbols on an OFDM chunk basis.

5. The method of claim 1 , further comprising using different staggered pilot insertion patterns at different base stations, to reduce pilot symbol interference between neighboring base stations.

6. The method of claim 1 , further comprising defining the staggered pilot insertion pattern using a diagonal distribution of pilot symbols within the time-frequency domain.

7. The method of claim 1 , further comprising defining the staggered pilot insertion pattern using a Costas distribution within the time-frequency domain.

8. The method of claim 1 , further comprising defining the staggered pilot insertion pattern on an OFDM chunk basis, such that staggered pilot symbols recur within like chunks in successive OFDM frames at a spacing defined by an insertion period of the staggered pilot insertion pattern.

9. The method of claim 1 , further comprising setting the frequency domain spacing used for inserting pilot symbols according to the staggered pilot insertion pattern based on propagation channel delay spread.

10. The method of claim 1 , further comprising dimensioning the staggered pilot insertion pattern, such that inserted sets of staggered pilot symbols span more than one OFDM chunk.

11. The method of claim 1 , wherein inserting pilot symbols into OFDM frames according to a staggered pilot insertion pattern that spans more than one OFDM frame and evenly distributes pilot symbols across OFDM symbols in the OFDM frames comprises inserting pilot symbols at regular intervals in selected horizontal scan lines, with a desired time- frequency insertion staggering between the selected horizontal scan lines.

12. A base station configured for operation in a Time Division Duplex (TDD) Orthogonal Frequency Division Multiplexing (OFDM) system, said base station comprising one or more processing circuits configured to: insert pilot symbols into OFDM frames according to a staggered pilot insertion pattern that spans more than one OFDM frame and evenly distributes pilot symbols across OFDM symbols within the OFDM frames; and transmit user data within data blocks are bounded by the staggered pilot insertion pattern.

13. The base station of claim 12, wherein the staggered pilot insertion pattern defines paired sets of pilot symbols, each paired set comprising a first set of pilot symbols in one OFDM frame and a corresponding second set of pilot symbols in a later OFDM frame, and wherein each data block for transmitting user data comprises those data symbol positions lying between one of the paired sets of pilot symbols.

14. The base station of claim 12, wherein the base station is configured to transmit the user data within the data blocks bounded by the staggered pilot insertion pattern based on forcing data for given users to time-wise span the OFDM frames spanned by the staggered pilot insertion pattern.

15. The base station of claim 14, wherein the one or more processing circuits of the base station include a medium access control processing circuit, which is configured to force the data for given users to time-wise span the OFDM frames spanned by the staggered pilot insertion pattern. 50710

16. The base station of claim 12, wherein each OFDM frame comprises one or more OFDM chunks allocabie to different users, and wherein the one or more processing circuits of the base station are configured to insert staggered pilot symbols on an OFDM chunk basis.

17. The base station of claim 12, wherein the one or more processing circuits of the base station are configured to define the staggered pilot insertion pattern using a diagonal distribution of pilot symbols within the time-frequency domain.

18. The base station of claim 12. wherein the one or more processing circuits of the base station are configured to define the staggered pilot insertion pattern using a Costas distribution within the time-frequency domain.

19. The base station of claim 12, wherein the one or more processing circuits of the base station are configured to insert the pilot symbols into the OFDM frames by inserting pilot symbols at regular intervals in selected horizontal scan lines according to a desired time- frequency insertion staggering between the selected horizontal scan lines.