Source: http://www.google.com/patents/US7433298?dq=%223do%22+dna
Timestamp: 2017-07-28 01:55:57
Document Index: 80800137

Matched Legal Cases: ['§ 119', 'Application No. 60', 'art 16', 'art 16', 'art 11', 'art 11']

Patent US7433298 - Compensation for residual frequency offset, phase noise and I/Q imbalance in ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsSignal processing methods and apparatus which incorporate I/Q imbalance compensation based on most likely estimates of the I/Q imbalance between the I and Q components of a baseband signal are disclosed. In accordance with at least one disclosed embodiment of the invention, most likely estimates of the...http://www.google.com/patents/US7433298?utm_source=gb-gplus-sharePatent US7433298 - Compensation for residual frequency offset, phase noise and I/Q imbalance in OFDM modulated communicationsAdvanced Patent SearchTry the new Google Patents, with machine-classified Google Scholar results, and Japanese and South Korean patents.Publication numberUS7433298 B1Publication typeGrantApplication numberUS 10/316,806Publication dateOct 7, 2008Filing dateDec 10, 2002Priority dateAug 19, 2002Fee statusPaidAlso published asUS7643405, US7881237, US8488442, US8792325Publication number10316806, 316806, US 7433298 B1, US 7433298B1, US-B1-7433298, US7433298 B1, US7433298B1InventorsRavi NarasimhanOriginal AssigneeMarvell International Ltd.Export CitationBiBTeX, EndNote, RefManPatent Citations (15), Non-Patent Citations (5), Referenced by (22), Classifications (17), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetCompensation for residual frequency offset, phase noise and I/Q imbalance in OFDM modulated communications
US 7433298 B1Abstract
Signal processing methods and apparatus which incorporate I/Q imbalance compensation based on most likely estimates of the I/Q imbalance between the I and Q components of a baseband signal are disclosed. In accordance with at least one disclosed embodiment of the invention, most likely estimates of the common phase error may also be used to compensate the initial channel estimates to further improve receiver performance. Though applicable to any multicarrier OFDM communications system, the disclosed methods and apparatus may be conveniently implemented in IEEE 802.11a, IEEE 802.11g, or 802.16a compliant wireless communications systems to reduce the effects of imbalanced I/Q components of baseband signals recovered from inbound RF signals, as well as counter residual frequency offset and phase noise potentially present in such baseband signals.
1. An orthogonal frequency division modulation (OFDM) baseband processor, comprising:
a channel estimator to form a plurality of initial channel estimates responsive to at least one training symbol in a preamble of a packet borne across a baseband signal, the packet comprising a series of OFDM symbols;
an analog-to-digital converter (ADC) to recover digital in-phase(I) and quadrature-phase(Q) components of the baseband signal; and
an I/Q compensation unit responsive to said channel estimator and said ADC to compensate the I and Q components for I/Q imbalance using a maximum likelihood estimate of I/Q imbalance between the I and Q components, wherein the maximum likelihood estimate of the I/Q imbalance is based on at least one of the plurality of initial channel estimates.
2. The baseband processor of claim 1, wherein the maximum likelihood estimate of the I/Q imbalance comprises an I/Q gain imbalance estimate and an I/Q phase imbalance estimate.
3. The baseband processor of claim 1, wherein the packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
4. The baseband processor of claim 1, further comprising:
a common phase error calculation unit to obtain a maximum likelihood estimate of common phase error of the baseband signal; and
a channel estimate compensation unit responsive to said common phase error calculation unit and said channel estimator to compensate the initial channel estimates using the maximum likelihood estimate of the common phase error.
5. The baseband processor of claim 4, wherein the packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
an radio frequency (RF) receiver unit capable of receiving an RF signal and recovering a baseband signal from the RF signal, the baseband signal bearing a packet comprising a series of orthogonal frequency division modulation (OFDM) symbols; and
a channel estimator to form a plurality of initial channel estimates responsive to at least one training symbol in a preamble of the baseband signal packet;
7. The transceiver of claim 6, wherein the maximum likelihood estimate of the I/Q imbalance comprises an I/Q gain imbalance estimate and an I/Q phase imbalance estimate.
8. The transceiver of claim 6, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
10. The transceiver of claim 9, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
11. A network interface apparatus, comprising:
an analog-to-digital converter (ADC) to recover digital in-phase(I) and quadrature-phase(Q) components of the baseband signal;
an I/Q compensation unit responsive to said channel estimator and said ADC to compensate the I and Q components for I/Q imbalance using a maximum likelihood estimate of I/Q imbalance between the I and Q components, wherein the maximum likelihood estimate of the I/Q imbalance is based on at least one of the plurality of initial channel estimates; and
a symbol demodulator responsive to said I/Q compensation unit to recover inbound data defined by at least one of the OFDM symbols in the baseband signal packet from the I/Q imbalance compensated I and Q components; and
a network interface responsive to said symbol demodulator to receive the inbound data.
a network interface responsive to said symbol demodulator to receive the inbound data and selectively transmit the inbound data to said information processor.
13. An orthogonal frequency division modulation (OFDM) baseband processor, comprising:
means for forming a plurality of initial channel estimates responsive to at least one training symbol in a preamble of a packet borne across a baseband signal, the packet comprising a series of OFDM symbols;
means for recovering digital in-phase(I) and quadrature-phase(Q) components of the baseband signal; and
means for compensating the I and Q components for I/Q imbalance using a maximum likelihood estimate of I/Q imbalance between the I and Q components, wherein the maximum likelihood estimate of the I/Q imbalance is based on at least one of the plurality of initial channel estimates.
14. The baseband processor of claim 13, wherein the maximum likelihood estimate of the I/Q imbalance comprises an I/Q gain imbalance estimate and an I/Q phase imbalance estimate.
15. The baseband processor of claim 13, wherein the packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
16. The baseband processor of claim 13, further comprising:
means for obtaining a maximum likelihood estimate of common phase error of the baseband signal; and
means for compensating the initial channel estimates using the maximum likelihood estimate of the common phase error.
17. The baseband processor of claim 16, wherein the packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
means for receiving an radio frequency (RF) signal and recovering a baseband signal from the RF signal, the baseband signal bearing a packet comprising a series of orthogonal frequency division modulation (OFDM) symbols; and
means for forming a plurality of initial channel estimates responsive to at least one training symbol in a preamble of the baseband signal packet;
19. The transceiver of claim 18, wherein the maximum likelihood estimate of the I/Q imbalance comprises an I/Q gain imbalance estimate and an I/Q phase imbalance estimate.
20. The transceiver of claim 18, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
21. The transceiver of claim 18, further comprising:
22. The transceiver of claim 21, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
23. A network interface apparatus, comprising:
means for recovering digital in-phase(I) and quadrature-phase(Q) components of the baseband signal;
means for compensating the I and Q components for I/Q imbalance using a maximum likelihood estimate of I/Q imbalance between the I and Q components, wherein the maximum likelihood estimate of the I/Q imbalance is based on at least one of the plurality of initial channel estimates; and
means for recovering inbound data defined by at least one of the OFDM symbols in the baseband signal packet from the I/Q imbalance compensated I and Q components; and
means for receiving the inbound data.
means for receiving the inbound data and selectively transmitting the inbound data to said information processing means.
25. A method for processing a baseband signal, the signal bearing a packet comprising a series of orthogonal frequency division modulation (OFDM) symbols, the method comprising:
forming a plurality of initial channel estimates responsive to at least one training symbol in a preamble of the baseband signal packet;
recovering digital in-phase(I) and quadrature-phase(Q) components of the baseband signal; and
compensating the I and Q components for I/Q imbalance using a maximum likelihood estimate of I/Q imbalance between the I and Q components, wherein the maximum likelihood estimate of the I/Q imbalance is based on at least one of the plurality of initial channel estimates.
26. The method of claim 25, wherein the maximum likelihood estimate of the I/Q imbalance comprises an I/Q gain imbalance estimate and an I/Q phase imbalance estimate.
27. The method of claim 25, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
obtaining a maximum likelihood estimate of common phase error of the baseband signal; and
compensating the initial channel estimates using the maximum likelihood estimate of the common phase error.
29. The method of claim 28, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
receiving an radio frequency (RF) signal; and
recovering the baseband signal from the RF signal.
31. The method of claim 25, further comprising recovering inbound data defined by at least one of the OFDM symbols in the baseband signal packet from the I/Q imbalance compensated I and Q components.
32. A computer readable storage medium having instructions for execution by an information processor to perform the following steps in support of processing a baseband signal, the signal bearing a packet comprising a series of orthogonal frequency division modulation (OFDM) symbols, the steps comprising:
33. The computer readable storage medium of claim 32, wherein the maximum likelihood estimate of the I/Q imbalance comprises an I/Q gain imbalance estimate and an I/Q phase imbalance estimate.
34. The computer readable storage medium of claim 32, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
35. The computer readable storage medium of claim 32, the steps further comprising:
36. The computer readable storage medium of claim 35, wherein the baseband signal packet is formatted in accordance with at least one of the IEEE 802.11a, IEEE 802.11g, and IEEE 802.16a standards.
37. The computer readable storage medium of claim 32, the steps further comprising:
38. The computer readable storage medium of claim 32, the steps further comprising recovering inbound data defined by at least one of the OFDM symbols in the baseband signal packet from the I/Q imbalance compensated I and Q components.
39. The OFDM baseband processor of claim 1 further comprising:
a common phase error calculation unit that determines at least one of a maximum likelihood estimate of gain imbalance and a maximum likelihood estimate of phase imbalance of said baseband signal,
wherein said I/Q compensation unit compensates for said I/Q imbalance based on said at least one of a maximum likelihood estimate of gain imbalance and a maximum likelihood estimate of phase imbalance.
40. The OFDM baseband processor of claim 39 wherein said at least one of a maximum likelihood estimate of gain imbalance and a maximum likelihood estimate of phase imbalance is feedback to said I/Q compensation unit.
41. The OFDM baseband processor of claim 1 wherein said initial channel estimates are feedback to said I/Q compensation unit.
42. The OFDM baseband processor of claim 1 further comprising a common phase error calculation unit that determines said I/Q imbalance.
43. The OFDM baseband processor of claim 4 wherein said common phase error is feedforward to said channel estimate compensation unit.
44. The OFDM baseband processor of claim 4 wherein said channel estimate compensation unit generates compensated channel estimates that are feedforward to said channel estimator,
wherein said channel estimator generates channel estimates based on said compensated channel estimates.
45. The OFDM baseband processor of claim 44 wherein said channel estimator generates channel estimates based on OFDM symbol bearing subcarriers.
46. The OFDM baseband processor of claim 4 wherein said common phase error calculation unit determines said I/Q imbalance based on symbol bearing subcarriers, and
wherein said channel estimator generates channel estimates based on said OFDM symbol bearing subcarriers.
47. An orthogonal frequency division modulation (OFDM) baseband processor comprising:
a channel estimator that determines a plurality of initial channel estimates based on a training symbol in a physical layer convergence procedure (PLCP) preamble of a protocol data unit (PPDU) frame of a baseband signal;
an analog-to-digital converter (ADC) to recover digital in-phase(I) and quadrature-phase(Q) components of said baseband signal; and
an I/Q compensation unit that compensates for imbalance between said I and Q components using a maximum likelihood estimate of I/Q imbalance,
wherein said maximum likelihood estimate of I/Q imbalance is based on at least one of said plurality of initial channel estimates.
48. The OFDM baseband processor of claim 47 wherein said training symbol includes long and short PLCP preamble symbols.
49. The OFDM baseband processor of claim 47 wherein said baseband signal includes a series of OFDM data symbols, and
wherein said maximum likelihood estimate of I/Q imbalance is based on a measured I/Q imbalance of said PLCP preamble.
50. The OFDM baseband processor of claim 47 wherein said PLCP preamble is received prior to a header and OFDM data of said PPDU frame.
51. The OFDM baseband processor of claim 47 wherein said PLCP preamble comprises:
a first symbol; and
a second symbol that is longer than said first symbol.
52. The OFDM baseband processor of claim 51 wherein said first symbol includes carrier frequency information, and
wherein said channel estimator determines channel estimates based on said second symbol.
53. An orthogonal frequency division modulation (OFDM) baseband processor comprising:
wherein said maximum likelihood estimate of I/Q imbalance is based on at least one of said plurality of initial channel estimates, and
wherein said training symbol comprises at least one of channel offset information, frequency offset information, and carrier frequency information.
54. The OFDM baseband processor of claim 53 wherein said preamble includes long and short PLCP preamble symbols.
55. The OFDM baseband processor of claim 53 wherein said PLCP preamble is received prior to a header and OFDM data of said PPDU frame.
56. The OFDM baseband processor of claim 53 wherein said PLCP preamble comprises:
57. The OFDM baseband processor of claim 56 wherein said first symbol includes carrier frequency information, and
58. The OFDM baseband processor of claim 47 wherein said channel estimator determines said plurality of initial channel estimates of said training symbol.
59. The OFDM baseband processor of claim 47 wherein said I/Q imbalance includes a maximum likelihood estimate of gain imbalance between I and Q components of said training symbol and a maximum likelihood estimate of phase imbalance between I and Q components of said training symbol.
60. The OFDM baseband processor of claim 47 wherein said PPDU frame includes said PLCP preamble and M OFDM symbols, where M is an integer value greater than or equal to 1,
wherein said I/Q imbalance is of said PLCP preamble, and
wherein said I/Q compensation unit compensates for imbalance between I and Q components of a first of said M OFDM symbols based on said I/Q imbalance of said PLCP preamble.
61. The OFDM baseband processor of claim 60 wherein said I/Q compensation unit compensates for imbalance between I and Q components of another one of said M OFDM symbols based on at least one of said I/Q imbalance of said PLCP preamble and an I/Q imbalance of said first of said M OFDM symbols.
62. The OFDM baseband processor of claim 61 wherein said I/Q imbalance of said PLCP preamble includes a maximum likelihood estimate of gain imbalance and a maximum likelihood estimate of phase imbalance between I and Q components of said PLCP preamble, and
wherein said I/Q imbalance of said first of said M OFDM symbols includes a maximum likelihood estimate of gain imbalance and a maximum likelihood estimate of phase imbalance between I and Q components of said first of said M OFDM symbols.
This application claims priority benefit under 35 U.S.C. § 119(e)(1) to U.S. Provisional Application No. 60/404,655, filed on Aug. 19, 2002, entitled “Compensation for Residual Frequency Offset, Phase Noise and I/Q Imbalance for OFDM”, which is incorporated herein fully by reference.
The invention generally relates to symbol modulated communication techniques, and more particularly, to a method and apparatus which improves reception performance of OFDM symbol modulated signals through compensating for at least one of residual frequency offset, phase noise and I/Q imbalance in a received baseband signal.
The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11a (1999) and Draft IEEE Standard 802.11g (2002) High Rate PHY Supplements to the ANSI/IEEE Standard 802.11 1999 edition, collectively incorporated herein fully by reference, has recently been commercialized with the promise of 54 Mbps effective bandwidth, making it a strong competitor to traditional wired Ethernet and the more ubiquitous “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard.
FIG. 4 is a more detailed block diagram of the transmitter unit shown in FIG. 1.
Block 404 in FIG. 1 also represents mapping or symbol modulating the data. The encoded and interleaved binary serial input data is divided into groups of bits, each group sized according to the selected modulation (1, 2, 4 or 6 bits). For example, 64-QAM modulation maps 6-bit quantities onto the constellation. The same procedures can be extended to higher rate encoding beyond the 802.11a/g standards, such as 256-QAM as proposed in e.g. IEEE 802.16a, in which case each group of 8 bits of the serial data is mapped onto one complex number (I+jQ) corresponding to a location on the 256-QAM constellation. The output values are multiplied by a normalization factor, depending on the base modulation mode (for 64-QAM, it is 1/√{square root over (42)}) to achieve the same average power for all mappings.
x ( t ) = 1 N ∑ k = 0 N - 1 X k ⅇ j 2 nki NT ( 1 ) where Xk are the frequency-domain data symbols. In other words, the N values Xk represent the respective values of the discretely-varying (e.g. QPSK or QAM) signals modulating the OFDM carriers.
Before describing the receiver 150 of the transceiver 100, examine more closely the structure of the data unit frame and how it is designed to assist the receiver 150 in perceiving and decoding inbound OFDM packets or frames. FIG. 5 is a block diagram illustrating the structure of a PLCP protocol data unit (PPDU) frame, in accordance with the IEEE 802.11a standard, and is similar to the 20 Mbps+rate PPDU frame format for IEEE 802.11g. In particular, this frame structure is a part of the IEEE 802.11a physical layer extension to the basic 802.11 protocol. The 802.11a extension defines requirements for a PHY operating in the 5.0 GHz unlicensed frequency bands and data rates ranging from 6 Mbps to 54 Mbps.
Under this protocol, the PPDU (PLCP protocol data unit) frame consists of a PLCP preamble, and signal and data fields as illustrated in FIG. 5. The receiver 150 uses the PLCP preamble to acquire the incoming OFDM signal and synchronize the baseband processor 120. The PLCP header contains information about the PSDU (PLCP service data unit containing data of interest) from the sending OFDM PHY. The PLCP preamble and the signal field are always transmitted at 6 Mbps, binary phase shift keying (BPSK), modulated using convolutional encoding rate R=½.
The PLCP preamble 502 is used to acquire the incoming signal and train and synchronize the receiver 150. The PLCP preamble consists of 12 symbols, 10 of which are short symbols, and 2 long symbols. The short symbols are used to train the receiver's AGC (not shown) and obtain a coarse estimate of the carrier frequency and the channel. The long symbols are used to fine-tune the frequency and channel estimates. Twelve sub-carriers are used for the short symbols and 52 for the long symbols. The training of an 802.11a compliant OFDM receiver, such as receiver 150, is accomplished in 16 μsec. This is calculated as 10 short symbols times 0.8 μsec each, plus 2 long training symbols at 3.2 μsec each, plus the guard interval. See e.g. IEEE standard 802.11a (1999) section 17.3.3. These training symbols, as noted above, provide for initial channel and frequency offset estimation, but do not compensate for other factors, such as sampling frequency jitter.
It should be noted that, unlike conventional OFDM baseband processors, the baseband processor 120 utilizes adaptive common phase error compensated channel estimates in the OFDM demodulation and decoding process. In particular, a common phase error and I/Q imbalance calculation unit 240 is provided after the guard subcarrier discard block 235 to calculate the most likely estimate of the I/Q imbalance αML and the most likely estimate of the common phase error Λ0,ML using the channel estimates Ĥk1 . . . Ĥk52 initially derived from the pilot subcarriers by the channel estimator 265, as well as the OFDM symbol bearing subcarriers Yk1 . . . Yk52 themselves on a per symbol basis. In turn, the imbalance estimate αML is used to derive εML and θML for use in the I/Q imbalance compensation performed by the I/Q imbalance compensation unit 220, and the common phase error Λ0,ML estimate is multiplicatively applied to the channel estimates Ĥk1 . . . Ĥk52 by the channel estimate compensation unit 245. The resulting compensated channel estimates, {tilde over (H)}k1 . . . {tilde over (H)}k52 minus those specified for the pilot subcarriers which are unneeded for demodulation and Viterbi decoding, are provided here to the Viterbi decoder 260 to provide more accurate recovery of the most likely sequence of transmitted data from the received OFDM symbol(s). These compensated channel estimates {tilde over (H)}k1 . . . {tilde over (H)}k52 are also provided to the channel estimator 265 to refine the channel estimates for subsequent OFDM symbol(s), if any, in the frame (adaptive channel estimation using common phase error compensation). Details as to calculating αML and Λ0,ML will be discussed below with reference to equations (6)-(11), as will performance of common phase error compensation of the channel estimates with reference to e.g. equation (12) discussed below.
Obtaining the most likely estimates of the common phase error and I/Q imbalance, as well as channel estimate and I/Q imbalance compensation consistent with the present invention, will now be discussed. Recalling equation (1), the transmitted signal x(t) is convolved with a multi-path channel with impulse response h(t). At the receiver (such as receiver 160 of the transceiver 100 shown in FIG. 1), residual frequency offset and phase noise contribute to a multiplicative distortion ejφ(t). Let y(t)=jφ(t)[h(t)*x(t)], where * denotes convolution. The in-phase (I) and quadrature-phase (Q) components of y(t) are distorted by a gain imbalance of ε and a phase imbalance of θ. Finally, white Gaussian noise v(t) is added to form the received baseband signal z(t):
z ( t ) = y ( t ) [ cos ( θ / 2 ) + j ɛ 2 sin ( θ / 2 ) ] + y * ( t ) [ ɛ 2 cos ( θ / 2 ) - j sin ( θ / 2 ) ] + v ( t ) ( 2 ) For |θ|<<1 and |ε|<<1,
Let φn, yn, zn, vn denote the discrete-time versions of φ(t), y(t), z(t), v(t), respectively, sampled at the rate 1/Ts. Let Yk, Zk, Vk denote the N-point FFT's of yn, zn, vn, respectively. Also, let α=(ε−jθ)/2 which represents the I/Q imbalance in the frequency domain, and Λk, phase noise and residual frequency offset, denote the FFT of ejφn (the residual frequency offset and phase noise in the frequency domain). The FFT output Zk is given by
Z k ≈ Y k + Y - k * α + V k = 1 N Λ 0 H k X k + 1 N Λ 0 * H - k * X - k * α + W k ( 4 ) where
H k = ∫ h ( τ ) ⅇ - j 2 nk τ NTs ⅆ τ is the FFT of the channel impulse response and Wk represents intercarrier interference and noise (also known as Additive White Gaussian Noise or AWGN). The common phase error (CPE) is given by Λo. Let Ak=HkXk/N. Therefore,
Z k≈Λ0 A k+Λ0 *αA k *+W k (5)
Λ o , ML = c 1 r 2 - c 2 * r 1 c 1 2 -  c 2  2 and ( 6 ) α ML = c 1 r 1 - c 2 r 2 c 1 r 2 * - c 2 r 1 * , ( 7 ) c 1 = ∑ i = 1 M (  A ^ k i  2 +  A ^ - k i  2 ) , ( 8 ) c 2 = 2 ∑ t = 1 M A ^ ki A ^ - ki , ( 9 ) r 1 = ∑ i = 1 M ( Z k i A ^ - k i + Z - k i A ^ k i , and ( 10 ) r 2 = ∑ i = 1 M ( Z k i A ^ k i * + Z - k i A ^ - k i * ) . ( 11 ) where
Λ 0 min , α [ ∑ i = 1 M [  Z ki - Λ o A ki - Λ o * αA - ki *  2 +  Z - ki - Λ o A - ki - k 1 Λ o * αA ki *  2 ] ] , based on a likelihood function for equation (5) listed above for pilot subcarriers at ±k1, . . . , ±kM distributed according to multidimensional Gaussian distribution. To find Λ0,ML and αML from this expression, this expression is differentiated with respect to α and Λ0, the results are set to 0 and solved for these variables.
εML=2 (αML) (13)
θML=−2 (αML) (14)
Let In, Qn, denote the I and Q components of the output of the analog to digital converter, namely ADC 210 in FIG. 2 which define the nth OFDM symbol in the inbound PLCP frame. The I/Q imbalance is compensated by the I/Q imbalance compensation unit 220 by forming Ĩn, {tilde over (Q)}n, where
I ~ n = ( 1 - ɛ ML , n - 1 2 ) I n + θ ML , n - 1 2 Q N ( 15 ) Q ~ n = θ ML , n - 1 2 I n + ( 1 + ɛ ML , n - 1 2 ) Q n ( 16 ) Thus, in this embodiment, the values for εML and θML for the previous symbol (n−1) are used to compensate the I/Q imbalance in the nth or succeeding OFDM symbol. In turn, the imbalance compensated signal components Ĩn, {tilde over (Q)}n are provided to the input of the FFT 230 to improve symbol demodulation performance, and ultimately OFDM receiver performance.
{tilde over (H)}k=Λ0,MLĤk;Ĥk=Ĥk (12)
In other words, the channel estimate for demodulating next OFDM symbol is the CPE compensated version of the channel estimates for the present OFDM symbol, with HkINIT (or the initial channel estimates) being used for the first OFDM symbol in the received PLCP frame. In an alternative embodiment, also not shown in FIG. 2, historical analysis of Λ0,ML may be employed for common phase error compensation, similarly to αML as previously discussed.
FIG. 3 illustrates a calculation flow diagram for obtaining αML, Λ0,ML for the four pilot subcarriers in the IEEE 802.11a/g standards consistent with the embodiment shown in FIG. 2. In particular, this calculation flow diagram represents an implementation of equations (6)-(11) undertaken by the common phase error and I/Q imbalance calculation unit 240 in terms of complex convolution 305, multiplier 310, adder 315, subtractor 320, and division 325 units. It should be understood that FIG. 3 merely illustrates certain calculations and is not a particularized hardware schematic, in whole or in part, of the common phase error and I/Q imbalance calculation unit 240. In fact, the illustrated calculations can be conveniently implemented in a variety of ways, as would be understood by those skilled in the art, including programmable hardware, e.g. a DSP or microprocessor core with appropriate embedded software, or dedicated custom hardware, such as provided by discrete logic and/or an ASIC, could be used in whole or in part to provide the desired functionality. As such, various arrangements consistent with the calculation flow diagram of FIG. 3 or equations (6)-(11) may be used without departing from the scope of the present invention.
FIG. 6 illustrates I/Q imbalance and common phase error compensation according to an alternative embodiment of the invention. In this embodiment, processing begins at step 610 when the beginning of a new PLCP frame is recognized. During the preamble (result of query 610 is yes) of this frame, the initial channel estimates HkINIT are formed from the pilot sub-carriers based on known preamble information consistent with 802.11a/g standards (step 615), and the channel estimates Ĥk and εML and θML are initialized. Then, until the end of the current frame is reached (step 625), the I/Q components for the current OFDM symbol in the frame are recovered (step 630), compensated for I/Q imbalance based on εML and θML calculated with reference to the previous symbol (or initial values if at the beginning symbol of the header or payload) (step 640) whilst the current Λ0,ML and αML values are calculated (step 635), and the channel estimates are updated (step 638). Thereafter, as shown in FIG. 6, the I/Q imbalance compensated I and Q components obtained in step 640 are then converted into the frequency domain (step 642), demapped from the constellation(step 645), Viterbi decoded (step 650), and descrambled (step 655) as is known in the art. Note that one or more of the illustrated processing steps shown in the flowchart of FIG. 6 may be carried out by discrete or combinational logic, as well as through an information processor, such as a general purpose microprocessor or microcontroller, or a specific-purpose processor such as a digital signal processor programmed in accordance with the functions and general sequence so described. Of course, any variety and combination of logic and/or instruction programming consistent with FIG. 6 may be used, such as the substitution of any functionally equivalent steps or inclusion of additional steps or operations, without departing from the scope of the present invention.
α ML = ∑ i = 1 M [ ( Z k i - A ^ k i ) A ^ - k i + ( Z - k i - A ^ - k i ) A ^ k i ] ∑ i = 1 M [  A ^ - k i  2 +  A ^ k i  2 ] , ( 17 ) where Âk=Ĥk{circumflex over (X)}k|N. Here, the common phase error Λo,ML is deemed to be negligible and so no adaptive compensation of the channel estimates accounting for Λo,ML need occur. In comparison to the previously described embodiments, this results in a less complex I/Q imbalance calculation unit (which only needs to calculate αML per equation (17), as well as εML and θML in light thereof, as presented in equations (13) and (14) if, for example, an I/Q imbalance compensation unit such as unit 220 (FIG. 2) is employed. However, this potentially result in reduced receiver performance in comparison with previously described embodiments, particularly where effective data throughput approaches 802.11a/g maximum rates or orthogonality of the sub-carriers is substantially comprised by ambient noise.
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