EP1825611A1

EP1825611A1 - Channel estimation method based on pilot diversity

Info

Publication number: EP1825611A1
Application number: EP05821857A
Authority: EP
Inventors: Byoung-Gi Kim; Kun-Seok Kang; Do-Seob Ahn; Ho-Jin Lee
Original assignee: Electronics and Telecommunications Research Institute ETRI
Current assignee: Electronics and Telecommunications Research Institute ETRI
Priority date: 2004-12-13
Filing date: 2005-12-13
Publication date: 2007-08-29
Also published as: KR100581083B1; EP1825611A4; WO2006065053A1

Abstract

Provided is a channel estimation method based on pilot diversity. The method includes the steps of extracting pilot symbols of a secondary common control physical channel (S-CCPCH) which is despreaded, calculating a mean value of the pilot symbols of the S-CCPCH, calculating a power ratio of the pilot symbols of the S-CCPCH, those of a dedicated physical control channel (DPCCH), and those of a common pilot channel (CPICH), and assigning weights to the pilot symbols of the S-CCPCH, those of the DPCH, and those of the CPICH.

Description

CHANNEL ESTIMATION METHOD BASED ON PILOT

DIVERSITY

Technical Field

[1] The present invention relates to a method for estimating a channel in a communication system; and, more particularly, to a method for estimating a channel using an improved pilot diversity. Background Art

[2] A purpose of next generation satellite or mobile communication system is to provide various communication services to whomever, wherever and whenever.

[3] A proposal for connecting a global system for mobile communications (GSM) with a wideband code division multiple access (W-CDMA) was already adapted, and a few proposals based on the W-CDMA have been adopted as multiple access in an international mobile telecommunication (IMT)-2000 providing a multi-media service and a high quality audio service.

[4] Like terrestrial-universal mobile telecommunications systems (Terrestrial-UMTS),

European satellite-universal mobile telecommunications systems (S-UMTS) whose standardization is under way in European Telecommunications Standards Institute (ETSI) has adopted a W-CDMA multiple access technology characterized as an asynchronous mode between cells.

[5] A coherent demodulation method is used in uplink and downlink to improve a link capacity. A channel estimate is performed by using a pilot signal which is not modulated for the coherent demodulation.

[6] Channel estimation using a pilot symbol structure is adopted by W-CDMA standard. The channel estimation using the pilot symbol structure periodically performs the time-division multiplexing for pilot symbols, which are known to a transmitter and a receiver, and data symbols and transmits the multiplexed symbols.

[7] A channel change in a data symbol period is compensated with a channel estimation value in a pilot symbol period. The above channel estimation method is a method for estimating a channel using only pilot symbols of a dedicated physical control channel (DPCCH).

[8] Another method compensates data symbols for channel change by transferring pilot symbols, which is known to a transmitter and a receiver, using predetermined pilot symbol patterns. The above method estimates a channel using only a common pilot channel (CPICH).

[9] A demerit of the conventional channel estimation method is to incur more errors when a channel related with the channel estimation is under deep fading. [10]

Disclosure of Invention Technical Problem

[11] It is, therefore, an object of the present invention to provide a channel estimation method having a more improved performance than channel estimation in a receiver of a terminal having conventional pilot symbols of a CPICH or a DPCCH by combining pilot symbols of a CPICH, a DPCCH and a secondary common control physical chann el (S-CCPCH), and estimating a channel.

[12] Other objects and advantages of the invention will be understood by the following description and become more apparent from the embodiments in accordance with the present invention, which is set forth hereinafter. It will be also apparent that objects and aspects of the invention can be embodied easily by the means defined in the claims and combinations thereof.

Technical Solution

[13] In accordance with an aspect of the present invention, there is provided a method for estimating a channel using pilot diversity, the method including the steps of extracting pilot symbols of a secondary common control physical channel (S-CCPCH) which is despreaded, calculating a mean value of the pilot symbols of the S-CCPCH, calculating a power ratios of the pilot symbols of the S-CCPCH, those of a dedicated physical control channel (DPCCH), and those of a common pilot channel (CPICH), and assigning weights to the pilot symbols of the S-CCPCH, those of the DPCH, and those of the CPICH.

Advantageous Effects

[14] A channel estimation method in the present invention provides a more improved performance than channel estimation in a receiver of a terminal having conventional pilot symbols of a CPICH or a DPCCH by combining pilot symbols of a CPICH, a DPCCH and a secondary common control physical channel (S-CCPCH), and estimating a channel.

[15]

Brief Description of the Drawings

[16] The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

[17] Fig. 1 is a diagram showing a structure of a rake receiver to adopt a channel estimation method in accordance with an embodiment of the present invention;

[18] Fig. 2 is a diagram showing a frame structure of a DPCH used in an embodiment of the present invention; [19] Fig. 3 is a diagram showing a frame structure of a CPICH used in an embodiment of the present invention; [20] Fig. 4 is a diagram showing a frame structure of a S-CCPCH used in an embodiment of the present invention; [21] Fig. 5 is a diagram showing one finger out of fingers of the rake receiver described in Fig. 1; [22] Fig. 6 is a flow chart showing channel estimation process in accordance with an embodiment of the present invention; [23] Fig. 7 is a diagram showing a process combining pilot symbols of a S-CCPCH, those of a DPCH and those of a CPICH when performing the channel estimation in accordance with the flow chart illustrated in Fig. 6; and [24] Fig. 8 is a diagram showing a relative timing correlation of a channel estimation method that combines a CPICH, a DPCH and a S-CCPCH to estimate a channel in accordance with an embodiment of the present invention. [25]

Best Mode for Carrying Out the Invention [26] Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. [27] Fig. 1 is a diagram showing a structure of a rake receiver to adopt channel estimation method in accordance with an embodiment of the present invention. [28] The rake receiver includes a correlator 110 and an estimation unit 120 in each finger. A signal received through a multipath channel is divided by fingers of the rake receiver, and an ideal maximum ratio combining (MRC) method is performed for the divided signal through channel estimation. [29] Referring to Fig. 1, the rake receiver includes M fingers and a received signal R(t) is multiplied by a scrambling code and then descrambled in each finger. The de- scrambled signal is despreaded by a channelization code and channel-estimated in the estimation unit 120. An output value of each finger is combined by a combiner and then outputted to a deinterleaver. [30] As described above, the present invention uses the pilot symbols of a DPCH, those of a CPICH, and those of an S-CCPCH for the channel estimation. [31] Fig. 2 shows a diagram of a frame structure of the DPCH used in the present invention. [32] Referring to Fig. 2, the channel estimation is performed by using the pilot symbols that a control channel of the DPCCH and a data channel of the DPDCH are multiplexed in time-division and are transferred, and data symbols of the DPDCH is compensated based on the channel estimated result. A frame has 15 slots; each slot has 2560 chips; a chip rate is 3.84Mcps; the DPCH is a power control based channel.

[33] Fig. 3 provides a diagram of a frame structure of the CPICH used in an embodiment of the present invention.

[34] As shown in Fig. 3, a transmitter transmits the pilot symbols to a receiver according to a predetermined pattern for the channel estimation; the receiver, which has already known the predetermined pattern, estimates the channel by using the pilot symbols; and the CPICH is a non-power control based channel.

[35] Fig. 4 represents a diagram of a frame structure of the S-CCPCH used in an embodiment of the present invention.

[36] The present invention uses pilot symbols of the S-CCPCH for the channel estimation in the S-CCPCH shown in Fig. 4. The S-CCPCH is adopted in the present invention since a data rate of the S-CCPCH is similar to that of the DPCH.

[37] Fig. 5 illustrates a diagram of one finger of fingers in the rake receiver shown in

Fig. 1.

[38] Referring to Fig. 5, a signal received in a rake receiver is despreaded by a despreader 210, and then divided into a data signal and a pilot signal by the pilot symbols dividing unit not shown in drawings.

[39] The data signal and the pilot signal are inputted to a delay unit 221 and a channel estimation unit 223, respectively. The pilot signal inputted to the channel estimation unit 223 is pilot symbols of a DPCH, an S-CCPCH, and a CPICH. The channel estimation unit 223 outputs a channel estimation value as described below.

[40] The channel estimation value is multiplied by a delayed data signal made by delaying the data signal as long as a predetermined time in the delay unit 221, and the multiplied value is outputted to a combiner. Herein, the delay unit 221 and a multiplier play a role of compensating the data signal. Then, an output of the combiner is transmitted through a deinterleaver 230, a rate matching encoder 240, and a FEC decoder 250. As a result, the received data which was encoded in a transmitting terminal is decoded to original data and then outputted through the decoder 250.

[41] Fig. 6 is a flow chart showing a channel estimation process in accordance with the present invention.

[42] Referring to Fig. 6, pilot symbols extracting unit extracts pilot symbols of a despreaded S-CCPCH in step S610. And, the channel estimation unit 223 calculates a mean value of the pilot symbols of the S-CCPCH in step S620, wherein a method for calculating the mean value of the pilot symbols of the S-CCPCH follows a method for calculating a mean value of pilot symbols of the DPCH since a data rate of the S- CCPCH is the same as that the DPCH. [43] Subsequently, a power ratio of the pilot symbols of each channel is calculated in step S630, wherein the electrical power ratio of each channel is obtained by calculating electrical power ratios of the pilot symbols of the S-CCPCH and the DPCH to the pilot symbols of the CPICH.

[44] In step S640, each pilot symbols of the S-CCPCH, the DPCH and the CPICH is multiplied by a weight, wherein the weight is a value assigned with 0 to 1 according to a level of the received signal. Then, a channel estimation value is outputted by combining the pilot symbols of the S-CCPCH, those of the DPCH, and those of the CPICH with allotment of the weights in step S650.

[45] Meanwhile, since the pilot symbols of the S-CCPCH can be used as a frame synchronization word as well as used in the channel estimation, the pilot symbols of the S- CCPCH are not used in the channel estimation in case that the pilot symbol of the S- CCPCH is transferred to a transport channel of a forward access channel (FACH) or a paging channel (PCH) in the beginning stage and is used in the other channel estimation.

[46] Fig. 7 is a diagram showing a process combining the pilot symbols of the S-

CCPCH, those of the DPCH and those of a CPICH in the channel estimation according to the flow chart shown in Fig. 6.

[47] Referring to Fig. 7, channel estimation values of the CPICH, the DPCH, and S-

CCPCH are multiplied by the weights α, β and γ and then combined, to thereby output a final channel estimation value c(n, k, 1).

[48] Fig. 8 shows a relative timing correlation of the channel estimation method that combines the CPICH, the DPCH and the S-CCPCH for the channel estimation in accordance with the present invention.

[49] Hereinafter, the channel estimation in accordance with the present invention is compared with a channel estimation method based on only pilot symbols of the CPICH, and a channel estimation method combining pilot symbols of the DPICH and those of the CPICH.

[50]

[51] x(i)=α(i)+n(i) for i=l, 2, ..., N [equation 1]

[52]

[53] The equation 1 represents a channel estimation using N symbols of the CPICH in one slot after a multipath fading and a dispreading process in a rake receiver. Herein, α(i) means a channel gain to be estimated and n(i) means a noise including the interference of other cells. It is presumed that a mean value of n(i) is 0, and a variance of n(i) is σ n in case that there is no loss.

[54] A receiver of a terminal acknowledges a pilot pattern for pilot symbols of the

DPCCH in a similar manner to the equation 1. [55]

[56] y(])= λ(i)+α(j)+m(j) for j=l, 2, ..., M [equation 2]

[57]

[58] That is, the equation 2 shows a method for estimating a channel by combining pilot symbols of the DPCH and pilot symbols of the CPICH.

[59] Herein, M means the number of pilot symbols in one slot of the DPCH used in estimating a channel gain and λ(j)=(l/μ(j)) means a power ratio of the pilot symbols of the DPCH and those of the CPICH. It is presumed that m(j) is an additive white gaussian noise (AWGN) whose average value is 0 and variance is σ m . Since it is presumed that the channel gain is not changed during one slot of an estimation period, α(i) and λ(i) become α and λ respectively. [60]

[61] z(k)=λ ^k)X(Ic) α(k)+l(k) for k=l, 2, ..., K [equation 3]

[62] [63] The equation 3 is an equation extended from the equation 2. That is, the equation 3 shows a method for estimating a channel by combining pilot symbols of the DPCH, those of the CPICH, and those of the S-CCPCH. [64] Herein, K means the number of pilot symbols in one slot of the S-CCPCH used in estimating a channel gain and λ (k)=(l/μ (k)) means a power ratio of the pilot symbols of the S-CCPCH and those of the CPICH. It is presumed that l(k) is the

AWGN whose mean value is 0 and variance is σ² . k

[65] Since it is presumed that the channel gain is not changed during one slot of an estimation period, α(k) and λ (k) become α and λ respectively. [66] In addition, Since channelization codes used in the CPICH, the DPCH and the S-

CCPCH are different from each other, n(i), m(j) and l(k) are independent of each other.

It is possible to perform the above processes although the channels are overlapped at the same time. [67] Hereinafter, a channel estimation combining pilot symbols of the CPICH, those of the DPCH and those of the S-CCPCH is compared with a channel estimation using only pilot symbols of the CPICH, and a channel estimation combining pilot symbols of the CPICH and those of the DPCH.

[68] A vector of a signal received in a rake receiver of a terminal is as follows.

[69]

[70] z'=[x(l)x(2)...x(N)y(l)y(2)...y(M)z(l)z(2)...z(L)]^T

[71] [equation 4]

[72]

[73] where T denotes an operator of a transpose matrix.

[74] If λ, λ and α are known, a conditional probability density function is as follows.

[equation 5]

[76] [77] where α denotes a channel estimation value (case 1) using the CPICH, and λ denotes a channel estimation value (case 2) combining the DPCH and the CPICH, and λ denotes a channel estimation value (case 3) combining the CPICH, the DPCH and the S-CCPCH.

[78] N means the number of symbols of the CPICH; and M means the number of symbols of the DPCH; and K means the number of symbols of the S-CCPCH. [79] σ means a noise generated in a receiver in case of performing the channel ⁿ ₂ estimation by using only the CPICH; σ means a noise generated in the receiver in case of executing the channel estimation by combining the CPICH and the DPCH, and σ means a noise generated in the receiver in case of achieving the channel estimation k by combining the CPICH, the DPCH and the S-CCPCH.

[80] A cramer-rao lower bound (CRLB) is used to analyze performances of the above cases to thereby compare channel estimation values of the above cases. [81] Then, a performance analysis is performed as follows by comparing the case 1 with the case 2, and comparing the case 2 with the case 3 through the use of a Fisher information matrix,

[82] Firstly, the comparison of the case 1 and the case 2 is represented as follows. [83]

[84] [equation 6] [85] [86] An equation 7 is an inverse transformation of the equation 6, which is to calculate the variance when the pilot symbols of the CPICH and those of the DPCH are combined.

[87] Nσ: +Mλ²σ: λσ:

/ ^ι (λ,a) = NMa- a N

aN N

[equation 7]

[88]

[89] If λ is known, the outputted CRLB becomes as follows. [90]

CRI.B{ά \ λ) =

/U,αr), _: N ₊ M σ_n ^: μσ;_κ

[equation 8] [91]

[92] If both of λ and α are presumed, an equation 9 can be derived.

[93]

CRLR(ά \ λ) = l ' (Λ,αr), , = —

[equation 9] [94] [95] The channel estimation using only the pilot symbols of the CPICH in the case 1 is described as follows. [96]

[equation 10]

[97] [98] Accordingly, the equation 8 can be compared with the equation 9 as follows. [99] [100] σ:

N Mλ^~ N

[equation 11]

[101] [102] That is, it is noted that the performance of the channel estimation method combining the pilot symbols of the DPCH and those of the CPICH is superior to the method using only the pilot symbols of the CPICH.

[103]

[104] Subsequently, the case 2 is compared with the case 3 as described in an equation 12 by using the before-mentioned CRLB, so that it is noted that the performance of the inventive method is superior to those of other methods through the use of the Fisher information matrix.

[105]

[106]

^' Kλ^"a^z 2λλ^zaK σ; σ: iλ^SaK N MA Kλ^λ² o: σ: σ; σ;

[equation 12] [107]

[108] If λ is known, the outputted CRLB is such as in an equation 13.

[109] [HO]

CRLB{ά \ λ_i) =

[equation 13] [111] [112] Accordingly, comparing the equation 11 with the equation 13 is such as in an equation 14. [113] [114]

1

N M K Λ' /V

+ λlλ- μσ;,, m^σι σ- σ-

[equation 14] [115]

[116] As a result, it is noted that the channel estimation combining the CPICH, the DPCH and the S-CCPCH is superior to the channel estimation using only the CPICH and the channel estimation combining the CPICH and the DPCH.

[117]

[118] The conventional channel estimation methods incur many errors in case of deep fading.

[119] On the other hand, since the inventive channel estimation method using the S-

CCPCH to the conventional methods can obtain an improved pilot diversity gain by performing the channel estimation using other channel in case that once channel is not come up to a required level of a received signal, it is possible to implement an ideal maximum ratio combining method in a rake receiver.

[120] As above-mentioned, the method of the present invention can be embodied as a program and stored in recording media readable by a computer, e.g., CD-ROM, RAM, floppy disk, hard disk, magneto-optical disk, etc.

[121] The present application contains subject matter related to Korean patent application

No. 2004-0104747, filed in the Korean Patent Office on December 13, 2004, the entire contents of which being incorporated herein by reference.

[122] While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

[123]

Claims

[1] A method for estimating a channel using pilot diversity, which comprises the steps of: extracting pilot symbols of a secondary common control physical channel

(S-CCPCH) which is despreaded; calculating a mean value of the pilot symbols of the S-CCPCH; calculating a power ratio of the pilot symbols of the S-CCPCH, those of a dedicated physical control channel (DPCCH), and those of a common pilot channel (CPICH); and assigning weights to the pilot symbols of the S-CCPCH, those of the DPCH, and those of the CPICH. [2] The method as recited in claim 1, wherein the pilot symbols of the S-CCPCH are not used in the channel estimation in case that the pilot symbols of the S-CCPCH are transferred to a transport channel of a forward access channel (FACH) or a paging channel (PCH) in the beginning stage, and is used in case of the other channel estimation. [3] The method as recited in claim 1, wherein a formula of calculating the mean value of the pilot symbols of the S-CCPCH is identical to that of calculating a mean value of the pilot symbols of the DPCH. [4] The method as recited in claim 1, wherein, in the step of calculating the power ratios, power ratios of the pilot symbols of the DPCH and the S-CCPCH are derived based on the pilot symbols of the CPICH. [5] The method as recited in claim 1, wherein the weights are values assigned with 0 to 1 according to levels of a received signal in case that the received signal is inputted to a rake receiver.