Apparatus and method for interference cancellation and synchronization maintenance over interference channel estimation in communication system based on full-duplex relay

An apparatus and method for interference cancellation and synchronization maintenance over interference channel estimation in a communication system are provided. An interference channel estimator fixes synchronization and estimates an interference signal. An interference signal controller eliminates the estimated interference signal from a received RF signal. A synchronization unit unfixes fixed synchronization and tracks synchronization when the interference signal cancellation is stable. A data buffer stores data from which the interference signal is canceled.

PRIORITY

The present application claims priority under 35 U.S.C.§119(a) to a Korean Patent Application filed in the Korean Intellectual Property Office on Jan. 14, 2008 and assigned Serial No. 10-2008-0003966, the contents of which are herein incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to an apparatus and method for interference cancellation and synchronization maintenance in a communication system. More particularly, the present invention relates to an apparatus and method for interference cancellation and synchronization maintenance over interference channel estimation in a communication system based on full-duplex relay.

BACKGROUND OF THE INVENTION

At present, the Institute of Electrical and Electronics Engineers (IEEE) 802.16j standardization group is in the process of standardization work for a Mobile Multihop Relay (MMR) technology. Advanced research institutes and enterprises of various global countries conducting research on 4th-Generation (4G) mobile communication are competitively promoting the development of a wireless multihop transmission technology.

Basically, a relay station (RS) demodulates and decodes data received from a transmitter, and then again encodes and modulates the data for transmission to a receiver. The RS enables high-speed data communication by reducing a path loss between wireless links, and enables communication even when the receiver is far away from the transmitter or is located in a shadow area.

In a conventional RS technology, in general, an RS performs transmission and/or reception using different time slots or different frequency resources. That is, the RS operates in a half-duplex. This is because a strong transmitted signal is added to a weak received signal, and the transmitted signal and the received signal interfere with one another if a transmit antenna and receive antenna of the RS are insufficiently isolated from each other. Because of such limitations, an RS may cause a waste of limited wireless resources and thus a deterioration of a system capacity. Also, in order to apply the RS technology to a conventional wireless communication system not taking an RS into consideration, it is required to change standards (such as a frame structure and so forth) for separation of transmission and reception resources.

A method for overcoming general limitations in the conventional RS technology is a full-duplex relay method for enabling transmission and reception by the same resources (e.g., time slot, frequency, and so forth) using interference cancellation. The full-duplex relay method is a method for enabling transmission and reception with the same resources by enabling an RS modulator/demodulator (modem) to estimate and cancel interference of a transmitted signal on a received signal using a digital signal processing technology. A full-duplex RS enables transmission and reception using the same resources. This increases an available resource of an RS, the flexibility of resource allocation, and system capacity. Also, because there is no need for resource separation for reception and transmission, the full-duplex RS can provide a relay service without a standard change of an existing system or through a minimum change thereof.

However, the full-duplex RS requires an interference cancellation technology for canceling interference caused by a transmitted signal of a transmitter in a receiver, and the interference cancellation basically requires estimating an interference channel of the transmitter. Also, because of interference of the transmitter, the full-duplex RS has to perform a process of synchronization considering interference with an existing mobile station (MS). That is, when the full-duplex RS intends to perform synchronization, if interference caused by a transmitted signal of a transmitter is basically generated because of a characteristic of the full-duplex method and thus a conventional synchronization process is performed, synchronization performance is deteriorated by not considering the interference. Therefore, the full-duplex method requires a process of synchronization considering the interference caused by the transmitted signal of the transmitter.

If a full-duplex RS is a Single Input Single Output (SISO) system using a single antenna for a transmitter and receiver, the full-duplex RS can estimate an interference channel of a transmitter using all transmitted signals as pilots. However, if the transmitter/receiver uses Multiple Input Multiple Output (MIMO) using a multiple antenna, orthogonality between transmit signals transmitted from transmit antennas is not kept, thus causing inter-antenna interference and deteriorating performance in estimating an interference channel. Therefore, in an environment with multiple antennas, the interference between antennas must be considered when estimating an interference channel of the transmitter.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is a primary aspect of the present invention to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, one aspect of the present invention is to provide an apparatus and method for interference cancellation and synchronization maintenance over interference channel estimation in a full-duplex relay station (RS).

The above aspects are achieved by providing an apparatus and method for interference cancellation and synchronization maintenance over interference channel estimation in a communication system based on full-duplex relay.

According to one aspect of the present invention, an apparatus for interference cancellation and synchronization maintenance over interference channel estimation in a communication system based on full-duplex relay is provided. The apparatus includes an interference channel estimator, an interference signal controller, a synchronization unit, and a data buffer. Upon receipt of a radio frequency (RF) signal, the interference channel estimator fixes synchronization and estimates an interference channel from a transmit modem to a receiver modem. The interference signal controller eliminates an estimated interference signal from the received RF signal using the interference channel. When the interference signal cancellation is stable, the synchronization unit unfixes fixed synchronization and tracks synchronization. The data buffer stores data from which the interference signal is canceled.

According to another aspect of the present invention, a method for interference cancellation and synchronization maintenance over interference channel estimation in a communication system based on full-duplex relay is provided. The method includes, upon receipt of a radio frequency (RF) signal, fixing synchronization and estimating an interference channel from a transmit modem to a receiver modem, eliminating an estimated interference signal from the received RF signal using the interference channel, unfixing fixed synchronization and tracking synchronization when the interference signal cancellation is stable, and storing data from which the interference signal is canceled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An apparatus and method for interference cancellation and synchronization maintenance over interference channel estimation in a communication system based on full-duplex relay according to an exemplary embodiment of the present invention are described below. In an exemplary embodiment of the present invention, a full-duplex relay station (RS) having a multiple antenna structure is described in detail below.

FIG. 1is a block diagram illustrating an RS for performing interference cancellation and synchronization over interference channel estimation in a communication system based on full-duplex relay according to an exemplary embodiment of the present invention.

As shown inFIG. 1, a full-duplex RS includes a radio frequency (RF) receive end101, an analog-to-digital (A/D) converter102, a receive modem100, a data buffer118, a transmit modem120, a digital-to-analog (D/A) converter115, an RF transmit end116, an interference channel estimator108, a channel estimator109, and an interference signal controller110. The receive modem100includes a synchronizer103, a Cyclic Prefix (CP) remover and Fast Fourier Transform (FFT) unit104, a data demodulator105, and a channel decoder106. The transmit modem120includes a channel encoder112, a data modulator113, and an Inverse Fast Fourier Transform (IFFT) & CP inserter114.

The RF receive end101converts an RF signal into a baseband signal and outputs the baseband signal to the A/D converter102. The A/D converter102converts a baseband signal into a digital signal and outputs the digital signal to the receive modem100.

The receive modem100demodulates (105) and decodes (106) data in a signal received through a multiple receive antenna, the RF receive end101, and the A/D converter102, and transmits the data to the data buffer118. If the received signal is an Orthogonal Frequency Division Multiplexing (OFDM) signal, an output signal from the A/D converter102is provided to the CP remover & FFT unit104. The CP remover & FFT unit104receives synchronization information from the synchronizer103, removes a CP from a signal received from the A/D converter102, and performs FFT operation. The synchronizer103finds a CP and a rear portion of a symbol from an output signal of the A/D converter102from which an interference signal affecting a receiver is eliminated, and performs symbol synchronization. According to an embodiment, the synchronizer103can also obtain a correlation coefficient for a preamble from an output signal of the A/D converter102and perform synchronization.

The transmit modem120encodes112and modulates113data of the data buffer118, and generates and transmits a transmitted signal to the D/A converter115. If the transmitted signal is an OFDM signal, the transmit modem120inserts a CP into a modulated signal through the IFFT & CP inserter114before output to the D/A converter115and performs IFFT operation. A baseband signal output from the D/A converter115is processed into an RF signal through the RF transmit end116, and is outputted to a multiple transmit antenna and forwarded to a user mobile station (MS). Also, the transmit modem120provides a transmitted signal to the interference channel estimator108and interference signal controller110.

The channel estimator109extracts a pilot signal from an output signal of the CP remover & FFT unit104, performs channel estimation, and provides a channel estimation value to the data demodulator105.

The interference channel estimator108receives a transmitted signal from the transmit modem120, receives a received signal from the receive modem100, estimates an interference channel from the transmit modem120to the receive modem100, and provides the estimated interference channel to the interference signal controller110. At this time, the interference channel estimator108performs interference channel estimation on a per subcarrier basis.

The interference channel estimation is described below. Although an interference channel of a full-duplex transmitter is almost unchanged over time, if unsynchronization takes place, it is shown as a change in phase rotation upon transmitter interference-channel estimation. Thus, at a time when several transmitter frames are used for estimating an interference channel of a transmitter as inFIG. 3, phase compensation of a channel estimation value by frame should be taken into consideration.

As described above, a full-duplex RS estimates and cancels an interference channel, thereby being able to eliminate an interference signal affecting a receiver. In other words, because the full-duplex RS is aware of all signals intended for transmission, all transmitted signals can use pilots for channel estimation. Accordingly, if unsynchronization occurs, upon channel estimation, it is shown as a change in phase rotation. However, the phase rotation can be prevented by eliminating an interference signal affecting a receiver.

An interference channel estimation method is described in detail below.

Input signals r[m,k] of the receive modem100can be expressed in Equation 1 below:

In Equation 1, the ‘s[m,k]’ denotes a signal vector of a size (Nt×1) including the total number (Nt) of transmission streams that a base station (BS) transmits through a kthsubcarrier at an mthsymbol, the ‘H[m,k]’ denotes a multiple antenna channel matrix of a size (Nr×Nt) from the BS to a receive modem of an RS, the ‘x[m,k]’ denotes a signal vector of a size (Nt(r)×1) including the total number (Nt(r)) of transmission streams that are outputs of a transmit modem of the RS, the ‘HI[m,k]’ denotes a multiple antenna interference channel of a size (Nr(r)×Nt(r)) from the transmit modem to the receive modem, the ‘m’ denotes a symbol index, the ‘k’ denotes a subcarrier index, the ‘τm’ denotes a time delay depending on a synchronization error, the ‘N’ denotes an FFT size, the

'ⅇ-j⁢2⁢⁢π⁢⁢k⁢⁢τmN'
denotes a degree of phase rotation by a synchronization error at the mthsymbol, and the ‘n[m,k]’ denotes a noise signal of the RS. Because it is based on an OFDM system, the assumption is that channels H[m, k] and HI[m,k] are constant at a given OFDM symbol and subcarrier.

The interference channel estimator108provides an estimation value ĤI[m,k] of an interference channel HI[m,k] from an input signal r[m,k] to the interference signal controller110. The transmit modem120provides an output signal x[m,k] to the interference signal controller110. The interference signal controller110generates an estimation value ĤI[m,k]·x[m,k] of an interference signal from the two input signals and provides the estimation value ĤI[m,k]·x[m,k] to the receive modem100. The receive modem100can eliminate the interference signal using Equation 2 below:

In Equation 2, the ‘y[m,k]’ denotes a received signal after interference cancellation. Other parameter description is referred to Equation 1. If estimation of an interference channel is accurate (that is, if

H^I⁡[m,k]=HI⁡[m,k]⁢ⅇ-j⁢2⁢⁢π⁢⁢k⁢⁢τmN),
the receive modem100can restore a transmitted signal s[m,k] without influence of an interference signal.

To estimate an interference channel HI[m,k], an exemplary embodiment of the present invention proposes two methods below:

First is a moving average and direct inversion interference channel estimation method; and

Second is a recursive interference channel estimation method.

The moving average and direct inversion interference channel estimation method is described below.

First, a received signal r[m,k] is preprocessed using a signal x[m,k] transmitted by a transmit modem120as in Equation 3 below:

In Equation 3, the ‘z[m,k]’ denotes a preprocessed input signal of a receive modem, the ‘r[m,k]’ denotes an input signal of the receive modem, the ‘(·)H’ denotes a Hermitian operation, and the x[m,k] denotes an output signal of a transmit modem. Other parameter description is referred to Equation 1.

Here, a moving average value, which is an accumulation of a preprocessed signal z[m,k] for ‘W’ number of symbols to obtain an average gain, can be obtained in Equation 4 below:

A description of the parameters is referred to Equations 1 and 3.

In Equation 4, the term x[m−j, k]xH[m−j,k] shows correlation between transmitted signals by antenna resulting from a MIMO structure. Under the assumption that the transmitted signals by antenna are independent from each other, correlation between antennas decreases, thus making it possible to estimate an interference channel of a transmit end. However, if an average duration is larger than time selectivity of a channel, performance is rather deteriorated.

Therefore, rather than removing correlation between transmitted signals by antenna using only an average gain, a correlation matrix is estimated and decorrelation is performed through direct inversion and thus, interference channel estimation can be expressed as in Equation 5 below:

In Equation 5, a description of the parameter is referred to Equations 1 and 3.

The recursive interference channel estimation method is described below.

In the moving average and direct inversion method, a complexity caused by matrix inversion increases if a MIMO size increases. Also, because there is an assumption that a channel does not change during an average duration to obtain an average gain, if a coherent time is less than the average duration because of a channel change by time selectivity, it is difficult to estimate a channel adaptively to a change of an average channel. In order to solve such a problem, an exemplary embodiment of the present invention proposes the recursive interference channel estimation method based on more adaptive weighted averaging according to a channel change without using the direct inversion method.

Weighted averaging is taken using a preprocessed signal z[m,k] of Equation 3, as in Equation 6 below:

In Equation 6, the ‘λ’ denotes a weight parameter, and other parameter description is referred to Equations 1 and 3.

If using the existing direct inversion method for a weighted averaging signal in Equation 6, a transmitter MIMO interference channel can be estimated as in Equation 7 below:

However, in the recursive method, without using the weighted averaging and direct inversion in Equation 7, a transmitter MIMO interference channel can be also estimated as in Equation 8 below:

In the recursive method, C[m,k] and Φ[m,k] are estimated, and thus a transmitter interference channel is estimated as expressed in Equation 8 below:

In Equation 8, initial values are set as

The interference signal controller110generates an interference signal estimation value for each subcarrier using an interference channel estimation value from the interference channel estimator108and a transmitted signal from the transmit modem120, and provides the interference signal estimation value to the receive modem100.

Here, the receive modem100eliminates an interference signal from a received signal using an interference signal estimation value, and then performs data demodulation and decoding.

FIG. 2is a flow diagram illustrating a process of synchronization maintenance over interference cancellation in a communication system based on full-duplex relay according to an exemplary embodiment of the present invention.

Referring toFIG. 2, in step200, an RS performs preset initial synchronization in a state of no transmission and, in step202, performs ranging. Here, upon a success of ranging, a propagation delay is calculated, thus being able to determine transmission timing.

Then, upon a start of transmission (or reception) in step204, the RS fixes a present synchronization state (that is, stops synchronization tracking and fixes synchronization at a predetermined time) in step205. Then, in step206, the RS estimates an interference channel HI[m,k] using a transmitted signal and a received signal. In order to estimate the interference channel HI[m,k], an exemplary embodiment of the present invention can use two methods:

First is a moving average and direct inversion interference channel estimation method (Equation 5); and

Second is a recursive interference channel estimation method (Equation 8).

Then, in step208, the RS removes an interference channel estimation value from a received signal, eliminating an interference signal.

Then, in step210, the RS checks if an error value is less or equal to a preset threshold value, thus determining if interference cancellation is stable. If the interference cancellation is not stable (i.e., if the error value is more than the threshold value), the RS returns to step206. If the interference cancellation is stable (i.e., if the error value is equal to or less than the threshold value), in step211, the RS unfixes fixed synchronization and, in step212, performs synchronization tracking and identifies an interference cancellation stable state. That is, after fixing synchronization until interference cancellation is stabilized, the RS estimates an interference channel and eliminates an interference signal.

Then, in step214, the RS identifies if there is any synchronization loss. As long as there is a synchronization loss, the RS stops transmission and performs initial synchronization of step200. If there is no synchronization loss, the RS returns to step212and again performs synchronization tracking and identifies an interference cancellation stable state.

After that, the RS terminates a process according to an exemplary embodiment of the present invention.

As described above, because there is interference by a transmitter in a full-duplex relay method, a synchronization process considering the interference has to be performed. That is, the synchronization process of the full-duplex relay method has to be performed in association with an interference cancellation technique.

As described above, an exemplary embodiment of the present invention has an advantage of being capable of maintaining stable synchronization by, together with interference channel estimation and elimination, performing a synchronization process in association with the interference channel estimation and elimination in a communication system based on full-duplex relay. Also, transmission/reception can be performed using the same wireless resources because of interference cancellation, thus being able to increase available resources, flexibility of resource allocation, and system capacity.