KR100881904B1 - Extracting the phase of an ofdm signal sample - Google Patents

Abstract

The disclosed embodiments relate to developing circuits present in a typical OFDM receiver to find complex phases corresponding to the input signal without implementing additional expensive circuitry or using a relatively slow inverse tangent look-up table. The magnitude of the complex number is normalized 106 through a closed loop and processed (108, 110) to yield an output 112 that is proportional to the phase of the complex number.

Description

Method and apparatus for extracting phase of an ODF signal sample TECHNICAL FIELD

The present invention is directed to orthogonal frequency division multiplexed (OFDM) signal processing.

This section is intended to introduce the reader to various technical aspects that may pertain to various aspects of the invention described and / or claimed below. This review is believed to be beneficial in providing the reader with background information to better understand the various aspects of the present invention. Thus, it should be understood that these descriptions are to be read in this light, not as poets of the prior art.

Wireless LAN (WLAN) is a flexible data communication system implemented as a replacement or extension of a wired LAN in a building or campus. Using electromagnetic waves, WLANs transmit and receive data in the air while minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility and, through a simplified configuration, enable mobility LANs. Some industries that benefit from increased productivity using portable terminals (eg, notebook computers) for transmitting and receiving real-time information are digital home networking, health-protection, retail, manufacturing and warehousing.

Manufacturers of WLANs have certain transmission techniques to choose from when designing WLANs. Some exemplary techniques are multicarrier systems, spread spectrum systems, narrowband systems, and infrared systems. Although each system has gains and losses, it has been found that one particular type of multicarrier transmission system, OFDM, is particularly useful for WLAN communication.

OFDM is a reliable technique for efficiently transmitting data on a channel. The technique uses a plurality of sub-carrier frequencies within the channel bandwidth to transmit data. These sub-carriers separate and isolate sub-carrier frequency spectra, which may waste part of the channel bandwidth, and thus avoid conventional inter-carrier inteference (ICI). For optimal bandwidth efficiency compared to FDM). In contrast, although the frequency spectra of the OFDM sub-carriers significantly overlap within the OFDM channel bandwidth, OFDM allows analysis and recovery of information that has been modulated with each sub-carrier.

Transmission of data over a channel via an OFDM signal also provides several other advantages over many conventional transmission techniques. Some of these advantages are immunity to multipath delay spread and frequency selective fading, efficient spectrum usage, simplified sub-channel equalization, and good interference characteristics.

In processing an OFDM signal, it is often desirable to determine the phase of a given complex number corresponding to the input signal. One example of such a complex number is a frequency domain subcarrier value. The ability to determine the phase of a complex number representing an input signal is useful for many purposes, such as tuning the received data signal to ensure the maximum amount of integrity in the received data compared to the transmitted data. . Traditionally, determining the phase of a complex number first requires finding the tangent of the complex number's phase.

Tangent = imaginary / real

After determining the complex tangents, an inverse tangent look-up table can be used to determine the complex angle phase angles corresponding to the input signal. Depending on the accuracy required, such inverse tangent look-up tables can be very large and expensive to implement in hardware. It is desirable to have a method and apparatus that can determine the phase of a complex number corresponding to an input signal without accessing an inverse tangent look-up table.

The disclosed embodiments relate to developing circuits that exist within a typical OFDM receiver to find additional complex phases or find complex phases corresponding to an input signal without using a relatively slow inverse tangent look-up table. The magnitude of the complex number corresponding to the input signal is normalized, which has the effect of extracting the exponent of the complex portion of the sample. The exponent passes through a closed circuit that includes a numerically controlled oscillator (NCO). The input to the closed loop can remain constant long enough to allow the loop to converge on the phase of any input sample for a predetermined number of clock cycles. After convergence, the NCO's output will be a signal proportional to the complex phase corresponding to the input signal. Additional mathematical processing may be required to convert the NCO output to the required phase value.

1 is a block diagram of an exemplary OFDM receiver.

FIG. 2 illustrates the placement of a training sequence, user data, and pilot signals within an OFDM symbol frame. FIG.

3 is a block diagram of a circuit for extracting a complex number of phases corresponding to an input signal.

4 illustrates another embodiment of a circuit for normalizing a complex number corresponding to an input signal.

5 is a process flow diagram illustrating operation of one embodiment of the present invention.

The features and advantages of the present invention will become more apparent from the following description given by way of example.

Referring to FIG. 1, the first element of a typical OFDM receiver 10 is an RF receiver 12. Various RF receivers 12 exist in the art and are well known. In general, however, the RF receiver 12 includes an antenna 14, a low noise amplifier (LNA) 16, an RF band pass filter 18, an automatic gain control (AGC) circuit 20, an RF mixer 22, an RF carrier wave. A frequency local oscillator 24, and an IF band pass filter 26.

Through the antenna 14, the RF receiver 12 couples the RF OFDM-modulated carrier after passing through the channel. Then, by mixing the RF OFDM-modulated carrier with the receiver carrier at frequency f _cr generated by the RF local oscillator 24, the RF receiver 12 modulates the RF OFDM-modulation to obtain the received IF OFDM signal. Downconverted carriers. The frequency difference between the receiver carrier and the transmitter carrier contributes to the carrier frequency offset (delta f _c ).

The received IF OFDM signal is coupled to mixer 28 and mixer 30 to be mixed with an in-phase IF signal and a 90 ° phase shifted (quadrature) IF signal to yield in-phase and quadrature OFDM signals, respectively. The in-phase IF signal entering the mixer 28 is produced by the IF local oscillator 32. A 90 ° phase shifted IF signal entering the mixer 30 is derived from the in-phase IF signal of the IF local oscillator 32 by passing through a 90 ° phase shifter 34 before providing the IF signal in phase to the mixer 30. do.

The in-phase and quadrature OFDM signals then pass through each of the A / D converters ADCs 36 and 38 which are digitized at a sampling rate f _{ck_r} determined by the clock circuit 40. ADCs 36 and 38 calculate digital samples that form each of an in-phase and quadrature discrete-time OFDM signal. The difference between the sampling rate of the receiver and the sampling rate of the transmitter is the sampling rate offset (delta f _ck = f _{ck_r} − f _{ck_t} ).

Unfiltered in-phase and quadrature discrete-time OFDM signals from ADCs 36 and 38 are then passed through digital low pass filters 42 and 44, respectively. The outputs of the digital low pass filters 42, 44 are respectively filtered in-phase and quadrature samples of the received OFDM signal. Thus, the received OFDM signal is converted into phase (q _i ) and quadrature (p _i ) samples representing each of the real and imaginary elements of the complex-valued OFDM signal (r _i = q _i + jp _i ). Such in-phase and quadrature (real and imaginary) samples of the received OFDM signal are then passed to FFT 46. Note that in some conventional implementations of the receiver 10, the A / D conversion is done before the IF mixing process. In such implementations, the mixing process involves the use of digital mixers and digital frequency synthesizers. Also note that in many conventional implementations of receiver 10, D / A conversion is performed after filtering.

FFT 46 performs a Fast Fourier Transform (FFT) of the received OFDM signal to recover the sequence of frequency domain sub-symbols used to modulate the sub-carriers during each OFDM symbol interval. FFT 46 then passes the sequence of these sub-symbols to decoder 48.

Decoder 48 recovers the transmitted data bits from the sequence of frequency domain sub-symbols passed from FFT 46. This recovery is performed by decoding the frequency domain sub-symbols to obtain a data bit stream that should ideally match the data bit stream entering the OFDM transmitter. This decoding process may include, for example, soft Viterbi decoding and / or Reed-Solomon decoding to recover data from blocks and / or convolutionally encoded sub-symbols.

2, an exemplary OFDM symbol frame 50 of the present invention is shown. The symbol frame 50 is a training sequence or symbol 52 containing a known transmission value for each subcarrier in an OFDM symbol, and a predetermined number of cyclic prefixes 54 and user data. And 56 pairs. For example, the proposed ETSI-BRAN HIPERLAN / 2 (Europe) and IEEE802.11a (US) wireless LAN standards, incorporated herein by reference, include 64 known values or subsymbols (eg, 52). The non-zero values and twelve zero values are selected from the selected training symbols of the training sequence (e.g., "training symbol C" of the proposed ETSI standard and "long OFDM training symbol of the proposed IEEE standard". (long OFDM training symbol) "). User data 56 has a predetermined number of pilots 58 and also contains known transmission values embedded on a predetermined subcarrier. For example, the proposed ETSI And the IEEE standard has four pilots located at bin or subcarriers ± 7 and ± 21. The present invention is directed to the proposed ETSI-BRAN HIPERLAN / 2 (Europe) and IEEE802.11a (US) WLAN standards. Although described as operating in a receiver according to Obtaining, implementing the teachings of the present invention in other OFDM systems is considered within the capacity range of a skilled person in the art.

3 is a block diagram of a circuit for extracting a complex number of phases corresponding to an input signal. In processing an OFDM signal, it is often desirable to determine a complex number of phases, such as the output from bandpass filter 26 (FIG. 1). The complex number whose phase is required to be known is identified by reference numeral 60 in FIG. The complex number can generally be expressed as

| a | e ^{j * phi}

In this expression, | a | is the magnitude of the complex number and φ (phi) is the phase angle of the complex number in radians.

In order to extract this phase angle, the complex representation must be mathematically processed to determine the angle φ. The first step in this mathematical process is to extract the exponent from the complex representation of the sample. Extraction of the index can be performed by normalizing the size of the sample to one. Normalizing the magnitude of the complex number ensures constant gain closed-loop operation, which also ensures loop convergence over a fixed number of clock cycles. Normalizing the magnitude of the complex number allows sampling of the loop output after a predetermined number of clocks (e.g., after it is known that the loop has converged), and thus a relatively expensive and potentially incredible loop-lock indicator. This eliminates the need to implement a potential unreliable loop-lock indicator.

In order to normalize the complex numbers, the magnitude of the complex | a | is squared by a squaring circuit (62), which is present in most OFDM receivers (i.e. need not be specifically included). Square circuit 64 squares the entire complex value (both real and imaginary elements). If no additional square circuit exists in the OFDM receiver in which the invention is implemented, square circuit 62 can be used to square the complex number as well as determine the square of the magnitude of the complex number. An inversion circuit 66 (also generally present in most OFDM receivers) is used to invert the output of the squared circuit 62. The outputs of square circuit 64 and inverting circuit 66 are combined by a multiplier 68. The output of multiplier 68 is equal to the normalized value of the square of the complex number, which can be expressed as e ^{2j * phi} .

Now that the magnitude of the complex number is normalized, the second step in determining the phase angle of the complex number includes sending the output of the multiplier 68 through a closed loop. In general, a second order carrier tracking loop implemented in an OFDM receiver (ie, not necessarily added in particular) is useful for this purpose.

One of ordinary skill in the art understands that the output of multiplier 68 should be provided to the tracking loop (starting with derotator 70) for a predetermined number of clock cycles to ensure the stability of the loop. will be. The exact number of clock cycles required to ensure loop stabilization of a given OFDM receiver configuration can be readily determined by one of ordinary skill in the art. In addition, the determination of the number of clock cycles required for loop stabilization is not an important part of the present invention.

Continuing to describe a complex number processing by a closed loop, such as a carrier tracking loop of a typical OFDM receiver, the output from multiplier 68 enters revert 70 and the output of revert 70 passes to phase detector 72. do. The output of the phase detector 72 is delivered to an integrated gain amplifier 74 and a proportional gain amplifier 76. The output of the integrated gain amplifier 74 enters integrator 78. The output of integrator 78 is coupled with the output of proportional gain amplifier 76 by summing circuit 80. The output of the summation circuit 80 enters the numerically controlled oscillator NCO 82, which enters a sign / cosine look-up table 84. The output of the sine / cosine look-up table 84 is provided as feedback to the revert 70.

In the described embodiment, the output of the NCO 82 is equal to twice the phase of the complex number corresponding to the input signal. The division into two of the output of the NCO 82 using a divider circuit 88 results in a signal 86 that is equivalent to a complex phase angle φ. Accordingly, the present invention determines the complex phase angle corresponding to the input signal without the use of cumbersome and expensive inverse tangent look-up tables.

4 is another embodiment of a circuit for normalizing complex numbers. In the embodiment shown in FIG. 4, the complex number 60 is inverted by the inversion circuit 90. The complex conjugate of the inverse of the sample is determined by the complex conjugate circuit 92. The multiplier 94 multiplies the output of the complex conjugate circuit 92 by a complex number (both real and imaginary). The output of multiplier 94, which is the normalized value of complex number 60, may enter the closed loop circuit shown in FIG. 3, beginning with revert 70. Signal processing may then continue as described with respect to FIG. 3.

5 is a process flow diagram illustrating the operation of another embodiment of the present invention. The entire process is referred to by reference numeral 100. The process starts at 102. A complex number corresponding to the input signal is obtained (104). This complex number may represent a portion of the OFDM signal that has been received by the OFDM receiver. The complex number is normalized (106). Normalization can be performed using any known method, including hardware methods, software methods, and methods in which hardware and software are combined. Examples of circuits that can be used for normalization of complex numbers are shown and described with respect to FIGS. 3 (circuit entering revert 70) and FIG. As noted above, normalization of the complex number corresponding to the input signal ensures that loop convergence can be guaranteed within a predetermined number of receiver clock cycles.

The normalized complex number then enters a closed circuit (108). An example of such a loop is a carrier tracking loop that generally exists within an OFDM receiver. Input to the closed loop continues 110 for a predetermined number of clock cycles. As mentioned above, the purpose of waiting for a predetermined number of clock cycles is to allow loop convergence around a value proportional to the phase of the complex number. The loop output is divided by 2 to yield a complex phase. The method of the present invention obviates the need to implement an expensive and time consuming inverse tangent look-up table to determine the complex phase corresponding to the input signal. The process of FIG. 5 ends at 114.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular form disclosed. Rather, the invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims that follow.

The present invention is applicable to orthogonal frequency division multiplexed (OFDM) signal processing, and in particular, to a method for extracting the phase of an OFDM signal sample.

Claims (20)

A method of determining a complex phase corresponding to an input signal, Normalizing the complex number 106 to obtain a normalized complex number; Processing 108 the normalized complex number through a closed loop and outputting a signal proportional to the phase of the complex number, wherein the processing comprises detecting the phase of the normalized complex number and detecting the phase of the complex number Processing a normalized complex number comprising controlling a signal proportional to the phase based on the step of; And Determining (110, 112) a complex number phase from a signal proportional to the complex number phase. The method of claim 1, Receiving (104) an orthogonal frequency division multiplex (OFDM) signal, wherein the complex number corresponds to at least a portion of an OFDM signal. The method of claim 1, Processing the normalized complex number comprises waiting (110) for loop convergence, wherein the phase of the complex number corresponding to the input signal is determined. The method of claim 3, Wherein said waiting step (110) is performed for a predetermined number of clock cycles. The method of claim 1, And determining the phase comprises dividing (112) a signal proportional to the phase of the complex number into a single number to yield a complex number of phases. The method of claim 4, wherein Determining a phase of a complex number corresponding to the input signal, wherein the number is two. The method of claim 1, Normalizing the complex number (106), Inverting the complex number to obtain an inverted complex number (90); Determining a complex conjugate of inverted complex numbers (92); And And multiplying (94) the complex conjugate of the inverted complex number by the complex number. The method of claim 1, Normalizing the complex number (106), Square 62 the magnitude of the complex number to yield a squared complex size; Inverting the squared complex magnitude to produce an inverted squared complex magnitude 66; Square 64 a complex number to obtain a squared complex number; And Multiplying (68) the inverted squared complex magnitude with the squared complex number. A device for determining the phase of a complex number, Circuits 62-68 for normalizing complex numbers to output normalized complex numbers; And A closed loop circuit 70-84 that receives a normalized complex number and produces an output proportional to the phase of the complex number, Wherein the closed loop circuit comprises a phase detector for detecting the normalized complex number phase and an oscillator for generating an output based on the output of the phase detector. The method of claim 9, And the device is included in an orthogonal frequency division multiplex (OFDM) receiver. The method of claim 9, Wherein the output (86) proportional to the complex number phase is twice the phase of the complex number. The method of claim 9, Wherein the normalized complex number is provided to a closed loop circuit for a predetermined time. The method of claim 12, And said predetermined time corresponds to a predetermined number of clock cycles. The method of claim 9, The circuit for normalizing the complex number, A circuit 90 adapted to invert the complex number to obtain an inverted complex number; Circuit 92 adapted to determine a complex conjugate of inverted complex numbers; And And a circuit (94) adapted to multiply the complex conjugate of the inverted complex by the complex number. The method of claim 9, The circuit for normalizing the complex number, Circuit 62 adapted to square the magnitude of the complex number to yield a squared complex magnitude; Circuitry 66 adapted to invert the squared complex magnitude to yield an inverted squared complex magnitude; Circuit 64 adapted to square a complex number to obtain a squared complex number; And And a circuit (68) adapted to multiply the inverted squared complex magnitude by the squared complex number. An orthogonal frequency division multiplex (OFDM) receiver, Circuitry for receiving a transmitted OFDM signal and converting at least a portion of the transmitted OFDM signal into a complex number; A circuit (62-68) for normalizing the complex number to output a normalized complex number; And A closed loop circuit 70-84 that receives the normalized complex number and produces an output proportional to the phase of the complex number, Wherein the closed loop circuit comprises a phase detector for detecting the normalized complex phase and an oscillator for generating an output based on the output of the phase detector. The method of claim 16, Orthogonal frequency division multiplexing (OFDM) receiver with an output proportional to the complex number phase being twice the phase of the complex number. The method of claim 16, And the normalized complex number is provided to the closed loop circuit for a predetermined number of clock cycles. The method of claim 16, The circuit for normalizing the complex number, A circuit 90 adapted to invert the complex number to obtain an inverted complex number; Circuit 92 adapted to determine a complex conjugate of inverted complex numbers; And An orthogonal frequency division multiplexing (OFDM) receiver comprising circuitry (94) adapted to multiply the complex conjugate of an inverted complex number by the complex number. The method of claim 16, The circuit for normalizing the complex number, Circuitry (62) adapted to square the magnitude of the complex number to yield a squared complex magnitude; Circuitry 66 adapted to invert the squared complex magnitude to yield an inverted squared complex magnitude; Circuitry 64 adapted to square the complex number to obtain a squared complex number; And An orthogonal frequency division multiplexing (OFDM) receiver comprising circuitry adapted to multiply the inverted squared complex number by the squared complex number.

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