Method for implementing an equalizer of an OFDM baseband receiver

A method for implementing an equalizer of an orthogonal frequency division multiplexing (OFDM) baseband receiver is provided. The OFDM baseband receiver includes a channel estimation and tracking module for estimating a channel impulse response of an input signal of the equalizer. A conjugate of the channel impulse response is first calculated. The input signal and the conjugate of the channel impulse response are then multiplied to generate a product signal. The product signal is then taken as the output signal of the equalizer without dividing the product signal by a channel state information, wherein the channel state information represents a square of an absolute value of the channel impulse response.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an orthogonal frequency division multiplexing (OFDM) baseband receiver, and more particularly to an equalizer of an OFDM baseband receiver.

2. Description of the Related Art

FIG. 1is a block diagram of a portion of an OFDM baseband receiver100. The OFDM baseband receiver100includes a fast Fourier transformation (FFT) module102, a channel estimation and tracking module104, an equalizer106, a reciprocal circuit108, and a demapper110. When the OFDM baseband receiver100receives an OFDM signal, the OFDM signal is sampled and fed to the FFT module102to perform a fast Fourier transformation. The signal Skbefore the FFT is called a time domain signal, and the signal Ykafter the FFT is called a frequency domain signal. An OFDM signal is transmitted over 52 non-zero subcarriers, and the suffix k indicates the index of the subcarrier. Thus, signal Ykmeans the portion of the OFDM signal S transmitted over the k-th subcarrier.

Because signal Ykis transmitted over multiple subcarriers and suffers from various levels of channel distortion caused by multi-path fading channels, the signal Ykis delivered to the equalizer106to compensate for the channel distortion, otherwise inter-symbol interference (ISI) could damage the signal Yk. The channel estimation and tracking module104estimates a channel impulse response Hkof the signal Yk. The channel impulse response Hkrepresents the channel distortion level of signal Yk. Thus, the equalizer106can equalize signal Ykaccording to the channel impulse response Hkestimated by the channel estimation and tracking module104.

Ordinary equalizer106equalizes the signal Ykaccording to the following algorithm:

Xk=Yk×Conj⁡(Hk)Hk2;
wherein Ykis the input signal of the equalizer106, Conj(Hk) is the conjugate of channel impulse response Hkand |Hk|2which is the square of the absolute value of channel impulse response Hkis referred to a channel state information CSI. According to the algorithm, the equalizer106requires the inverse value of |Hk|2to derive the output signal Xk, and the reciprocal circuit108is thus created.

Because physically implementing a division for signal processing is difficult, a reciprocal circuit108is often implemented with a table which stores multiple exponents and mantissas of the inverse values corresponding to multiple |Hk|2values. When a |Hk|2value or a CSI value is calculated, the reciprocal circuit108first finds the approximation value closest to the CSI value in the table, and an inverse of the CSI approximation value is then found in the table. Thus, the reciprocal circuit108generates an approximation of the inverse of channel state information |Hk|2, or 1/CSI. The 1/CSI value is then delivered to the equalizer106, and the output signal Xkis generated by the equalizer106.

The approximation of 1/CSI is not very precise, however, due to the limited number of values stored in the table of the reciprocal circuit108. When the equalizer106uses the approximation to equalize the signal Yk, the error of 1/CSI further induces errors of the output signal Xk, and signal distortion results. If the number of values stored in the table of reciprocal circuit108is increased to improve the accuracy of 1/CSI, the reciprocal circuit108requires greater memory capacity to store the table, and additional hardware cost is incurred. Thus, a method for solving the problem is needed.

BRIEF SUMMARY OF THE INVENTION

A method for implementing an equalizer of an orthogonal frequency division multiplexing (OFDM) baseband receiver is provided. The OFDM baseband receiver includes a channel estimation and tracking module for estimating a channel impulse response of an input signal of the equalizer. A conjugate of the channel impulse response is first calculated. The input signal and the conjugate of the channel impulse response are then multiplied to generate a product signal. The product signal is then taken as the output signal of the equalizer without dividing the product signal by a channel state information, wherein the channel state information represents a square of an absolute value of the channel impulse response.

The invention also provides an OFDM baseband receiver. The OFDM baseband receiver comprises a channel estimation and tracking module for estimating a channel impulse response of an input signal and calculating a conjugate of the channel impulse response, and an equalizer coupled to the channel estimation module for multiplying the input signal and the conjugate of the channel impulse response to generate an output signal, and directly outputting the output signal without dividing the output signal by a channel state information, wherein the channel state information represents a square of an absolute value of the channel impulse response.

The invention also provides a maximal ratio combining (MRC)-OFDM baseband receiver. The MRC-OFDM baseband receiver receives an OFDM signal with a plurality of spatially correlated antennas to generate a plurality of input signals. The MRC-OFDM baseband receiver comprises a channel estimation and tracking module for estimating a plurality of channel impulse responses of the actual channel and calculating the conjugates of the channel impulse responses, and an equalizer coupled to the channel estimation module for respectively multiplying the input signals and the conjugates of the channel impulse responses to generate a plurality of product signals, adding the product signals to generate an output signal, and directly outputting the output signal without dividing it by a channel state information. The channel state information represents the sum of the squares of the absolute values of the channel impulse responses.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2is a block diagram of a portion of an OFDM baseband receiver200according to the invention. The OFDM baseband receiver200is roughly similar to the OFDM baseband receiver100and includes a fast Fourier transformation (FFT) module202, a channel estimation and tracking module204, an equalizer206, and a demapper210. However, the reciprocal circuit108does not exist in the OFDM baseband receiver200, and some modules of OFDM baseband receiver200require modification to suit this difference. For example, the equalizer206and demapper210are correspondingly amended.

Basically, the function of the FFT module202is identical to that of the FFT module102. When the OFDM baseband receiver200receives an OFDM signal, the OFDM signal is sampled and fed to the FFT module202to perform a fast Fourier transformation. After processed with FFT, signal Ykis then delivered to the equalizer206to compensate for the channel distortion.

The channel estimation and tracking module204is used to estimate a channel impulse response Hkof the signal Yk. The channel estimation and tracking module204includes a channel estimation module222and a channel tracking module224. The channel estimation module222estimates the channel impulse response Hk′ of the input signal Ykaccording to a preamble of the input signal Yk. Because the transmission of the signal Ykis continued, and the channel impulse response of the other part of the signal Ykmay be different from the channel impulse response Hk′ of the preamble of the signal Yk, the channel tracking module224is added to refine the channel impulse response of the signal Ykaccording to a channel tracking algorithm, wherein the channel tracking algorithm could be, for example, a RLS (recursive least square) tracking algorithm or a LMS (least mean square) tracking algorithm. The channel tracking module224receives an estimate Hk,HDgenerated by the demapper210to refine the channel impulse response Hk′ estimated by channel estimation module222. A refined channel impulse response Hkis then generated by the channel tracking module224for the equalizer206to equalize the signal Yk. Additionally, the channel tracking module224also calculates the conjugate of channel impulse response Hkand a channel state information CSI. The CSI value, |Hk|2, is a square of the absolute value of the channel impulse response Hk.

The function of the equalizer206is different from that of the equalizer106. Because the OFDM baseband receiver200does not include a reciprocal circuit, the equalizer206multiplies the input signal Ykand the conjugate of the channel impulse response Hkto generate a product signal Yk×Conj(Hk), and outputting the product signal directly without dividing the product signal by CSI. In other words, the equalizer206equalizes the input signal Ykaccording to the following algorithm:
Xk′=Yk×conj(Hk);
wherein Xk′ represents the output signal of the equalizer206, Ykrepresents the input signal of the equalizer206, Hkrepresents the channel impulse response estimated by the channel estimation and tracking module204, conj( ) is a conjugate function, and the suffix k represents the index of OFDM subcarrier. Thus, the output signal Xk′ of the equalizer206is CSI times larger than the output signal Xkof the ordinary equalizer106if CSI is larger than 1. That is to say, the output signal Xk′ of the equalizer206differs from the output signal Xkof the ordinary equalizer106by a multiplication factor of CSI.

The demapper210demodulates the output signal Xk′ of the equalizer206. Because an OFDM signal is mapped from one of multiple constellation points in a constellation according to the data content of the OFDM signal before it is transmitted, the OFDM baseband receiver200must recover the data content of the OFDM signal with the demapper210before it is further processed. Different modulation techniques has different constellation mapping. Ordinary modulation techniques used in OFDM systems are BPSK, QPSK, 16-QAM, and 64-QAM. The function of the demapper210will be detailed in the following paragraphs with an explanatory constellation mapping of 16-QAM modulation.

FIG. 3ais an explanatory constellation mapping300of an ordinary demapper110when 16-QAM is used as modulation technique. The 16-QAM modulation technique transforms every 4-bit data block of the OFDM signal to one of 16 constellation points. Each constellation point is a vector with different amplitude and phase. Thus, when a demapper110demodulates a signal Xk, the demapper110must find the constellation point closest to the signal Xkin the constellation300. For example, if the signal Xkhas an in-phase component X and a quadrature component Y, it can be marked in the constellation300as the point P, and the constellation point closest to the point P is the constellation point O. A method for finding the constellation point O closest to the point P is drawing a few decision boundaries B1˜B6to delimit the constellation points in the constellation300. When the signal Xkfalls into the region which represents the constellation point O and is delimited by the decision boundaries B6and B1, the vector X+iY of the constellation point O is considered as the actual value of the signal Xk, and the four bit data represented by the constellation point O is the demodulated data of the signal Xk.

However, because the signal Xk′ outputted from the equalizer206differs from the signal Xkof the ordinary equalizer106by a multiplication factor CSI, the demapper210of OFDM baseband receiver200requires corresponding adjustment.FIG. 3bshows an explanatory constellation mapping350of the demapper210according to the invention when 16-QAM is used as modulation technique. If the signal Xkhas an in-phase component X and a quadrature component Y, the corresponding signal Xk′ has an in-phase component X′ which equals X×CSI and a quadrature component Y′ which equals Y×CSI. Thus, the signal Xk′ of the equalizer206can be marked in the constellation350as the point P′, which is different from the point P ofFIG. 3a. If the demapper210wants to demap the signal Xk′ to an accurate 4-bit data block, the distances between each constellation point of the constellation350and the origin point (i.e. the amplitudes of the constellation points in the constellation350) also requires adjustment according to the multiplication factor CSI generated by the channel estimation and tracking module204, as shown inFIG. 3b. This can be achieved by adjusting the locations of the decision boundaries B1′˜B6′ of the constellation350according to the multiplication factor CSI, as shown inFIG. 3b. Thus, the demapper210can accurately find the constellation point O′ nearest to the point P′ according to the decision boundaries B1′·B6′ and then obtains the 4-bit value represented by the constellation point O′ to demodulate the signal Xk′.

FIG. 3aandFIG. 3bonly show the concept of demodulation process of the demappers110and210. Practically, the demapper210only calculates some functions of the channel state information and the signal Xk′ to demodulate the signal Xk'. The demapper210includes a soft demapper212and a hard demapper214. The soft demapper212first adjusts the locations of the decision boundaries B1′˜B6′ according to the multiplication factor CSI as described. A few boundary values representing the location of the decision boundaries can then be determined. The soft demapper then calculates a few soft decision values SDkaccording to the signal Xk′ and the boundary values, wherein the soft decision values SDkrepresent the distance between the point P′ and the decision boundaries B1′˜B6′. The hard demapper214can then determine the constellation point O′ according to the signs of soft decision values SDk, and obtains the demodulated 4-bit data represented by the constellation point O′. The demodulated data is delivered to a deinterleaver for further processing. Because the vector of the constellation point O′ is decided as the actual value of signal Xk, it can be used to derive an estimate of channel impulse response Hk, HD, and the estimate of channel impulse response Hk, HDis fed back to channel estimation and tracking module204to adjust the estimation of channel impulse response Hk.

FIG. 4is a block diagram of a portion of a maximal ratio combining (MRC)-OFDM baseband receiver400according to the invention. The MRC-OFDM baseband receiver400is roughly similar to the OFDM baseband receiver200. However, the OFDM baseband receiver400has multiple spatially correlated antennas to receive an OFDM signal, and the multiple antennas generate multiple received signals. Because the signals received by different antennas have been transmitted through different paths before reception, the path gains of signals fade independently. If the multiple received signals are combined to generate a single signal, the distortion of the combined signal is reduced. Accordingly, the MRC-OFDM baseband receiver400does not have a reciprocal circuit for calculating the inverse of channel state information.

The MRC-OFDM baseband receiver400includes a FFT module402, a channel estimation and tracking module404, an equalizer406, and a demapper410. Although there are only two input signals Sk1and Sk2inFIG. 4, there can be arbitrary number of input signals in the MRC-OFDM baseband receiver400, and the two input signals Sk1and Sk2are only explanatory.

The two input signals Sk1and Sk2are subject to a FFT module402to generate the two frequency domain signals Yk1and Yk2. The channel estimation and tracking module404is then used to estimate the channel impulse response Hk1and Hk2of the signals Yk1and Yk2. The channel estimation module422estimates the channel impulse responses Hk1′, and Hk2′ of the input signals Yk1and Yk2according to the preambles of the input signals Yk1and Yk2. The channel tracking module424refines the channel impulse response of the signal s Yk1and Yk2according to a channel tracking algorithm, wherein the channel tracking algorithm could be, for example, a RLS (recursive least square) tracking algorithm or a LMS (least mean square) tracking algorithm. The channel tracking module424receives Hk,HD1and Hk,HD2generated by the demapper410to refine the channel impulse response Hk1′, and Hk2′, estimated by channel estimation module422and generate the channel impulse response Hk1and Hk2. The channel tracking module424additionally calculates the conjugates Conj(Hk1) and Conj(Hk2) and a channel state information CSI′. The CSI′ value, |Hk1|2+|Hk2|2, is a summation of the squares of the absolute values of the channel impulse responses Hk1and Hk2.

The function of the equalizer406is different from that of the equalizer206. The equalizer406respectively multiplies the input signals Yk1and Yk2and the conjugates of the channel impulse responses Conj(Hk1) and Conj(Hk2) to generate multiple product signals Yk1×Conj(Hk1) and Yk2×Conj(Hk2). Because there is no reciprocal circuit in the MRC-OFDM baseband receiver400, the product signals are then added to generate an output signal Xk″ which is directly outputted by the equalizer406without being divided by the channel state information CSI′. In other words, the equalizer406equalizes the multiple input signals Yk1˜Yknaccording to the following algorithm:
Xk=Yk1×conj(Hk1)+Yk2×conj(Hk2)+ . . . +Yk1×conj(Hk1)+ . . . +Ykn×conj(Hkn);
wherein Xkrepresents the output signal of the equalizer, Ykirepresents the input signal of the equalizer, Hkirepresents the channel impulse response estimated by the channel estimation and tracking module, conj( ) is a conjugate function, the suffix k represents the index of OFDM subcarrier, i represent the index among the multiple input signals, and n is the number of the input signals or “2” in the example of MRC-OFDM baseband receiver400.

The demapper410is roughly similar to the demapper210ofFIG. 2. Because the output signal Xk″ of the equalizer406differs from the ordinary output signal of the ordinary equalizer by a multiplication factor CSI′, the constellation mapping process of the demapper410requires correspondingly adjustment, as described inFIG. 3b. The demapper410first adjusts the distances between each constellation points of the constellation350and the origin point (i.e. the amplitudes of the constellation points in the constellation350) in the constellation350according to the multiplication factor of the channel state information CSI′. The constellation point O′ which is the most approximate to the output signal Xk″ in the constellation350is then found, and the data represented by the constellation point O′ is then determined. Thus, the output signal Xk″ is demodulated, and the data represented by the constellation point O′ is delivered to a deinterleaver.

In reality, the demapper410only calculates some functions of the channel state information and the signal Xk″ to demodulate the signal Xk″. The demapper410includes a soft demapper412and a hard demapper414. The soft demapper412first adjusts the locations of the decision boundaries B1′˜B6′ in the constellation350ofFIG. 3baccording to the multiplication factor CSI′. A few boundary values representing the location of the decision boundaries can then be determined. The soft demapper412then calculates a few soft decision values SDkaccording to signal Xk″ and the boundary values, wherein the soft decision values SDkrepresent the distances between the point P′ and the decision boundaries B1′˜B6′. The hard demapper414can then determine the constellation point O′ according to the signs of soft decision values SDk, and obtains the demodulated data represented by the constellation point O′. The demodulated data is delivered to a deinterleaver for further processing. Because the vector of the constellation point O′ is decided to be the actual value of signal Xk″, it can be used to derive the estimates Hk, HD1and Hk,HD2of the channel impulse responses Hk1and Hk2, and the estimates Hk, HD1and Hk, HD2are fed back to the channel estimation and tracking module404to adjust the estimation of the channel impulse responses Hk1and Hk2.

The invention provides a method for implementing an equalizer of an OFDM baseband receiver without a reciprocal circuit. The output signal of the equalizer is not divided by a channel state information. Because the output signal of the equalizer is not multiplied by the inverse of the channel state information, the error induced from the inaccuracy of the inverse of the channel state information does not exist in the output signal of the equalizer, and the performance of the OFDM baseband receiver is improved. Additionally, because no reciprocal circuit is needed, the hardware cost for implementing the reciprocal circuit is saved.