Apparatus and method for estimating carrier-to-interference and noise ratio in a broadband wireless communication system

An apparatus and method for estimating the CINR of an uplink channel in a broadband wireless communication system are provided, in which tiles being subcarrier sets are separated from a feedback signal received on the uplink channel. All symbols included in the tiles of the feedback signal are correlated with each of codewords, the absolute values of the correlations are squared for the each codeword and summed, a codeword with a maximum sum from among the codewords is selected. Received power level and noise power level are calculated using all the symbols included in the tiles of the feedback signal correlated with the codeword with the maximum sum, and the CINR of the uplink channel is estimated using the received power and the noise power levels.

PRIORITY

This application claims priority under 35 U.S.C.§119 to an application filed in the Korean Intellectual Property Office on Jan. 27, 2006 and assigned Ser. No. 2006-8715, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an apparatus and method for estimating Carrier-to-Interference and Noise Ratio (CMNR) in a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for estimating the CINR of an uplink channel using an uplink fast feedback signal in a Base Station (BS) in a broadband wireless communication system. CINR represents a channel quality.

2. Description of the Related Art

In a broadband wireless communication system, a BS provides a high-speed packet data service by scheduling transmission of packet data and deciding transmission parameters for the packet data based on uplink fast feedback information representing a downlink channel quality. The BS receives uplink fast feedback signals from a plurality of Mobile Stations (MSs) and selects an MS having the best downlink channel quality from among the MSs according to the uplink fast feedback signals in each time slot. The BS then determines transmission parameters for the selected MS according to its downlink channel quality and sends packet data to the MS based on the transmission parameters. The transmission parameters include data rate, code rate, and modulation order. The uplink fast feedback information include at least one of Signal-to-Noise Ratio (SNR), Carrier-to-Interference Ratio (C/I), differential SNR of each band, fast Multiple Input Multiple Output (MIMO) feedback, and mode selection feedback.

For example, an Orthogonal Frequency Division Multiple Access (OFDMA) communication system has a physical channel designated for carrying the uplink fast feedback information. Thus, an MS sends the fast feedback information to the BS on the physical channel and the BS acquires uplink channel status information from the fast feedback channel even for a non-uplink traffic transmission period of the MS.

The fast feedback channel for carrying the fast feedback information is configured as illustrated inFIG. 1orFIG. 2, by way of example.

FIG. 1illustrates typical 3×3 frequency-time resources allocated for reception of fast feedback information in the BS.

Referring toFIG. 1, the feedback channel is composed of six subcarrier sets110called tiles and, each tile includes 3×3 subcarriers on the frequency-time domain. In each of tiles110, eight surrounding subcarriers carry modulation symbols and one center subcarrier carries a pilot symbol.

FIG. 2illustrates typical 4×3 frequency-time resources allocated for reception of fast feedback information in the BS.

Referring toFIG. 2, the feedback channel is composed of six subcarrier sets210called tiles, and each tile includes 4×3 subcarriers on the frequency-time domain. In each of tiles210, four corner subcarriers carry pilot symbols and the other eight subcarriers carry modulation symbols.

The BS can control the power of the uplink channel by estimating its CINR using the uplink fast feedback information. Without successful uplink power control, interference becomes severe between cells. Due to the resulting degradation of link performance or unstable communication status, Quality of Service (QoS) cannot be satisfied. As a consequence, the decrease of data rate leads to the decrease of cell throughput. Accordingly, there exists a need for a method that can reliably estimate CINR in the broadband wireless communication system.

To estimate CINR of the uplink channel using the uplink fast feedback information received from the MS, the BS first calculates the soft-decision values of symbols of the uplink fast feedback information. The BS correlates the soft-decision values with each codeword, squares the absolute values of the correlations, and sums the squares. Then the BS selects a codeword with the largest sum (hereinafter, referred to as a maximum codeword) from among given codewords and detects information data bits corresponding to the codeword. Each codeword is composed of orthogonal vectors having values as illustrated inFIG. 3.

FIG. 3illustrates typical orthogonal vectors used for modulation. Referring toFIG. 3, when the BS uses Quadrature Phase Shift Keying (QPSK), orthogonal vectors are formed using QPSK symbols,

After detecting the maximum codeword, the BS calculates the received power level or strength and noise power level of the received signal using the squared absolute correlation values of the received signal with respect to the maximum codeword and then calculates the CINR of the received signal based on the received power and noise power levels. For example, the BS calculates the received power level by averaging the squared absolute correlation values of subcarriers included in the tiles of the received signal with respect to the maximum codeword.

Also, the BS calculates the noise power strength by calculating the difference between every adjacent two modulation symbols correlated with the maximum codeword, squaring the absolute values of the differences for all six tiles, and averaging the squares. While the noise power strength is estimated based on the differences between adjacent correlated modulation symbols, it is to be clearly understood that correlated pilot symbols can be used instead of the correlated modulation symbols in estimation of the noise power strength.

Using the received power and noise power levels, the BS calculates the CINR according to Equation (1),

The correlated received signal is expressed as Equation (2),
Zm,k=Cm,k×Ym,k=Hm,k+Cm,k×Nm,k, 1≧m≧number of tiles, 1≧k≧number of FF symbols  . . . (2)
where number of FF symbols represents the number of fast feedback modulation symbols per tile, Cm,krepresents a code symbol in a fast feedback orthogonal vector, Hm,krepresents a channel coefficient, Nm,krepresents a noise component, and Ym,krepresents a received signal on a kthsubcarrier in an mthtile, expressed as Ym,k=Cm,kHm,k+Nm,k.

The noise power strength is computed by Equation (3),

As described above, the BS estimates the CINR using the uplink fast feedback information. Yet, it uses only modulation symbols or pilot symbols without fully utilizing all information of the subchannel of the MS, thereby decreasing the reliability of the CINR.

Accordingly, there is a need for a method that can increase the detection efficiency of fast feedback information and reliably estimate CINR in the BS.

SUMMARY OF THE INVENTION

An object of the present invention is to address at least the above described problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an object of the present invention is to provide an apparatus and method for efficiently estimating the CINR of an uplink channel in a broadband wireless communication system.

Moreover, the present invention provides an apparatus and method for estimating the CINR of an uplink fast feedback channel by fully utilizing information of a control subchannel included in the uplink fast feedback channel in a broadband wireless communication system.

In accordance with an aspect of the present invention, there is provided a method for estimating the CINR of an uplink channel in a broadband wireless communication system, in which tiles being subcarrier sets are separated from a feedback signal received on the uplink channel. All symbols included in the tiles of the feedback signal are correlated with each of codewords; the absolute values of the correlations are squared for each codeword and summed, a codeword with a maximum sum from among the codewords is selected. Received power and noise power strengths are calculated using all the symbols included in the tiles of the feedback signal correlated with the codeword with the maximum sum, and the CINR of the uplink channel is estimated using the received power and the noise power levels.

In accordance with another aspect of the present invention, there is provided an apparatus for estimating the CINR of an uplink channel in a broadband wireless communication system, in which a correlation calculator correlates all symbols included in a feedback signal received on a subchannel of the uplink channel with each of codewords and squares the absolute values of the correlations for each codeword. A detector sums the squared absolute correlations for each codeword and detects the feedback signal using a codeword with a maximum sum selected from among the codewords; and a CINR estimator estimates the CINR based on received power and noise power calculated using all the symbols included in the feedback signal correlated with the codeword with the maximum sum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a technique for estimating CINR by fully utilizing the information of a control subchannel included in an uplink fast feedback channel in a broadband wireless communication system. While the present invention is described in the context of an OFDMA broadband wireless communication system, it is to be appreciated that the present invention is also applicable to other multiple access schemes.

In broadband wireless communication system, noise projection method, correlation method, or the like, are used for CINR estimation. Herein, the CINR estimation is carried out by the noise projection method by way of example.

Referring toFIG. 4, the BS includes a Radio Frequency (RF) processor401, an Analog-to-Digital Converter (ADC)403, a Fast Fourier Transform (FFT) processor405, a non-coherent demodulator407, a channel decoder409, and a CINR estimator411.

RF processor401receives an uplink fast feedback signal from an MS through an antenna and downconverts the RF signal to a baseband signal. ADC403converts the analog signal received from RF processor401to a digital signal.

FFT processor405converts the time sample data received from ADC403to frequency data by FFT. Non-coherent demodulator407calculates soft-decision values of symbols received from FFT processor405.

Channel decoder409determines the reliability of the received uplink fast feedback information based on the soft-decision values. If it is determined that the uplink fast feedback information is reliable, channel decoder409decodes the soft-decision values at a predetermined coding rate and detects data by determining a codeword corresponding to the soft-decision values.

CINR estimator411estimates the received power and noise power levels of the uplink fast feedback channel using the codeword and estimates the CINR of the uplink channel based on the estimated received power and noise power levels.

The above-described BS receiver illustrated inFIG. 4has the detailed structure illustrated inFIG. 5, for CINR estimation.

FIG. 5is a detailed block diagram of non-coherent demodulator407, channel decoder409, and CINR estimator411. The following description will be made on the assumption that the uplink fast feedback channel is comprised of subcarrier sets, i.e. tiles each having 4×3 subcarriers in the frequency-time domain as illustrated inFIG. 2.

Referring toFIG. 5, non-coherent demodulator407includes a tile de-allocator501and correlators503to506. Channel decoder409includes a codeword arranger507, a detection decider513, and a detector515. CINR estimator411includes a received power calculator519, a noise power calculator521, a channel correlator523, and a CINR calculator525.

In non-coherent demodulator407, tile de-allocator501separates six tiles each having 4×3 subcarriers from FFT symbols received from FFT processor405illustrated inFIG. 4. Correlators503to506correlate the subcarriers of the tiles with each codeword and squares the absolute values of the correlations. The number of the correlators503to506is equal to the product of the number of tiles and the number of subcarriers per tile (i.e. eight modulation symbols and four pilot symbols).

In channel decoder409, codeword arranger507, which include adders509to511, sums the squared absolute correlation values of the subcarriers received from correlators503to506for each codeword, and averages the sums calculated for all codewords.

Detection decider513calculates the difference between the highest sum (MAX) and the average of the sums (AVG) and decides as to whether to detect the uplink fast feedback information by comparing the MAX−AVG difference with a predetermined threshold (Th).

For example, if the MAX−AVG difference is equal to or higher than the threshold ((MAX−AVG)≧Th), detection decider513determines that the information data of the codeword having MAX (referred to as maximum codeword) is reliable. Detector515detects the codeword corresponding to the uplink fast feedback information through a codeword detector517and provides the codeword to channel correlator523and received power calculator519.

If the MAX−AVG difference is less than the threshold ((MAX−AVG)<Th), detector515does not detect the uplink fast feedback signal, considering the reception environment of the uplink fast feedback signal is poor.

In CINR estimator411, received power calculator519calculates the received power level of the received signal by averaging the squared absolute correlation values of the modulation symbols and pilot symbols included in the six tiles of the received signal, which have been calculated with respect to the maximum codeword.

Channel correlator523calculates the difference between every adjacent two symbols in the six tiles of the received signal correlated with the maximum codeword. In other words, channel correlator523correlates every two adjacent correlated symbols with each other, inclusive of modulation symbols and pilot symbols, for example, in the manner illustrated inFIGS. 6A and 6B.

Referring toFIGS. 6A and 6B, channel correlator523sequentially calculates the difference between every adjacent two symbols of the uplink fast feedback signal correlated with the maximum codeword. InFIG. 6A, channel correlator523sequentially correlates between every adjacent two correlated symbols in a subcarrier set including 4×3 subcarriers. InFIG. 6B, channel correlator523sequentially correlates between every adjacent two correlated symbols in a subcarrier set including 3×3 subcarriers.

Noise power calculator521squares the absolute values of the differences for the six tiles and averages the squared absolute difference values, thereby calculating the noise power of the received signal.

CINR calculator525calculates the CINR of the received signal using the received power and the noise power levels according to Equation (4),

The correlated received signal is expressed as Equation (5),
Zm,k=Cm,k×Ym,k=Hm,k+Cm,k×Nm,k, 1≦m≦number of tiles, 1≦k≦(number of FF symbols+number of pilot symbols)  (5)
where Cm,krepresents a code symbol in a fast feedback orthogonal vector or a pilot symbol, Hm,krepresents a channel coefficient, Nm,krepresents a noise component, and Ym,krepresents a received signal on a kthsubcarrier in an mthtile, expressed as Ym,k=Cm,kHm,k+Nm,k.

The noise power is computed by Equation (6),

Referring toFIG. 7, the BS monitors reception of an uplink fast feedback signal from an MS within its service area in step701. Upon receipt of the uplink fast feedback signal, the BS FFT processes the received signal and separates the FFT signals into tiles in step703. For example, the BS separates six tiles of the uplink fast feedback channel from the FFT signals as illustrated inFIG. 2.

In step705, the BS calculates the squared absolute correlation values of modulation symbols on eight subcarriers and pilot symbols on four subcarriers in each of the tiles with respect to each codeword. Specifically, the correlation is the process of correlating each tile having 4×3 subcarriers carrying eight modulation symbols and four pilot symbols with 4×3 symbols including the symbols of an orthogonal vector corresponding to the tile in each codeword and pilot transmission symbols.

For each codeword, the BS sums the squared absolute correlation values of the six tiles and checks the maximum (MAX) of the sums for all codewords in step707.

In step709, the BS estimates the total power of the received signal by averaging the squared absolute correlation values of the received signal that have been calculated with respect to a codeword with the maximum sum, i.e. a maximum codeword.

In the mean time, the BS detects the uplink fast feedback signal to calculate its noise power level in step711. Whether to perform the detection is decided by comparing the difference between the maximum of the sums (MAX) and the average of the sums (AVG) with a predetermined threshold (Th). If the MAX−AVG difference is equal to or larger than the threshold ((MAX_AVG)≧Th), the uplink fast feedback signal is detected, considering that information data corresponding to the maximum codeword is reliable.

In step713, the BS calculates the difference between every adjacent two symbols in the received signal correlated with the maximum codeword and squares the absolute values of the differences. That is, the BS correlates between every adjacent two symbols in the correlated received signal.

The BS then averages the squared absolute differences for the six tiles, thereby calculating the noise power in step715and estimates the uplink CINR based on the total received power and the noise power in step717. The BS ends the procedure.

In the case where the uplink fast feedback channel is configured in tiles each having 4×3 subcarriers, the uplink CINR is estimated using all of eight available orthogonal vectors and four pilot symbols, as described above. In the case where the uplink fast feedback channel is configured in tiles each having 3×3 subcarriers, the uplink CINR can be estimated using eight available orthogonal vectors and one pilot symbol.

While the difference between every adjacent two symbols in the correlated received signal is calculated sequentially in the order shown inFIGS. 6A and 6Bin the above description, this operation can be performed in any other manner.

As is apparent from the above description, since CINR is estimated by utilizing all of the modulation symbols and pilot symbols of a control subchannel signal on an uplink fast feedback channel in a broadband wireless communication system according to the present invention, the processing time to calculate total received power is increased, channel information is accurately delivered, and system operation is stabilized. In addition, the present invention is applicable to any subchannel configuration irrespective of a tile structure or a CINR estimation scheme. Therefore, the system can be operated flexibly.