This invention relates to methods for demodulating orthogonal frequency division multiplexing (OFDM) modulated signals. In particular, this invention relates to methods for in-phase (I) and quadrature phase (Q) imbalance compensation in OFDM systems.
In many communications systems, data is often converted into a passband signal, e.g., centered around a carrier frequency, before transmission. One reason for converting the original signal into a passband signal is that the conversion allows multiple channels of data to be transferred over a single transmission medium, e.g. by using several different carrier signals. A common example is radio broadcasts.
In many systems, the passband signal is first converted to its baseband, i.e. is centered around zero frequency as opposed to the carrier frequency, before further signal processing takes place. The generation of the baseband signal is in many cases done with analog devices before any analog-to-digital (A/D) conversion takes place. The baseband signal normally comprises an in-phase (I) component and a quadrature (Q) component.
The I and Q components of a baseband signal are often processed separately, e.g., in parallel. As part of the process to obtain a baseband signal, the passband signal is copied and multiplied by a cos(2πfct) signal to generate the I component. The same passband signal is copied and multiplied by a sin(2πfct) signal to generate the Q component. In principle, the in-phase cos(2πfct) and quadrature sin(2πfct) components should have exactly π/2 phase shift and the same amplitude. However, in reality, it is very difficult and costly to achieve a highly accurate π/2 phase shift and equal amplitude using analog devices. Consequently, the resultant in-phase and quadrature components generally have imbalance in amplitude and/or phase (I/Q imbalance), which causes signal quality degradation in the subsequent signal processing by receivers of the signal.
I/Q imbalance is a well-known problem in receiver design of many communication systems, such as in OFDM systems. Therefore, many I/Q imbalance compensation devices are known in the art. Unfortunately some of these devices can be very complex in their design. Complex designs are often harder to implement in hardware, and have higher processing overhead than simple designs.
Therefore, it is desirable to provide methods for I/Q imbalance compensation for the demodulation of an OFDM modulated signal.
An object of this invention is to provide methods for I/Q imbalance compensation in the time domain.
Another object of this invention is to provide methods for I/Q imbalance compensation, where I/Q imbalance can be reduced to within 0.1 dB.
Yet another object of this invention is to provide methods for I/Q imbalance compensation, where pilot information is not needed to compensate for I/Q imbalance.
Briefly, the present invention relates to methods for calculating an I/Q imbalance compensated signal from a received signal, comprising the steps of: removing DC from the received signal; calculating an autocorrelation matrix of I/Q signal vector of the received signal; estimating I/Q imbalance compensation values, K1 and K2, as a function of an amplitude imbalance, g, and a phase imbalance, θ; and calculating an I/Q compensated signal as a function of the estimated K1 and K2.
An advantage of this invention is that I/Q imbalance compensation is performed in the time domain.
Another advantage of this invention is that I/Q imbalance can be reduced to within 0.1 dB.
Yet another advantage of this invention is that pilot information is not needed to compensate for I/Q imbalance.
The foregoing and other objects, aspects, and advantages of the invention will be better understood from the following detailed description of the preferred embodiment of the invention when taken in conjunction with the accompanying drawings in which:
a illustrates a process flow for an embodiment of the present invention for I/Q imbalance compensation
b illustrates a fixed point design for calculating autocorrelation matrix values for a first calibration mode.
c illustrates a fixed point design for estimating I/Q imbalance parameters.
d illustrates a fixed point design for I/Q imbalance compensation based on I/Q imbalance parameters.
The digital signal may be down sampled by 4 with a sampling offset compensation. Next, the signal can be adjusted by DC compensation, where DC is estimated and removed. The signal is then adjusted by IQ imbalance compensation 104, and a frequency offset compensation. After compensation, a fast-Fourier transform (FFT) can be applied to the signal to convert the signal from the time domain to the frequency domain. The focus of the present invention is on DC compensation and I/Q imbalance compensation.
The I/Q imbalance model 102 can be used to model the I/Q imbalance.
The real radio frequency (RF) signal just before a down-conversion can be written as
r(t)=y(t)ej+2πf
The local oscillator (LO) with I/Q imbalance can be given by
{tilde over (x)}
LO(t)=cos(2πfct)−jg sin(2πfct+θ) (2)
where g denotes the amplitude imbalance, fc denotes the carrier frequency, and θ denotes the phase imbalance. Furthermore, we can define the complex I/Q imbalance parameters as
The local oscillator with I/Q imbalance, Equation (2), can be rewritten using Equation (3) and Equation (4) as—
{tilde over (x)}
LO(t)=K1e−2πf
Therefore, the received base band signal with the I/Q imbalance can be given by
Y(t)=LP{r(t){tilde over (x)}LO(t)}=K1y(t)+K2y*(t) (6)
Also, by considering the I and Q components of Y(t) and y(t),
Y
I(t)=yI(t) (7)
Y
Q(t)=g Cos(θ)yQ(t)−g sin(θ)yI(t) (8)
And including a frequency offset of,
Δ
the I and Q components of Y(t) and y(t), Equation (7) and Equation (8) respectively, can be
Y
I(t)=cos(Δ
Y
Q(t)=g cos(Δ
Equation (10) and Equation (11) can be rewritten as the following,
Furthermore, the following autocorrelation matrix can be defined,
R
Z″Z″
=R
ZZ
A(T)AT(t) (14)
where,
Furthermore, the autocorrelation matrix values can be denoted,
R[0]=σZ2 (16)
R[1]=R[2]=−g sin(θ)σZ2 (17)
R[3]=−g2σZ2 (18)
According to Equation (3), Equation (4), and Equation (15), K1 and K2 can be estimated according to Equation (6). Thus, an I/Q imbalance compensated signal can be achieved
where (∥K1∥2−∥K2∥2) can be denoted as K12.
a illustrates a process flow for an embodiment of the present invention for I/Q imbalance compensation. Referring to
Calibration Mode 1
b illustrates a fixed point design for calculating autocorrelation matrix values for a first calibration mode. For this circuit, autocorrelation matrix values R[0], R[1], and R[3] are calculated based on an input signal.
c illustrates a fixed point design for estimating I/Q imbalance parameters. Here, the calculated autocorrelation matrix values are inputted, and the I/Q imbalance parameters are calculated based on the autocorrelation matrix values.
d illustrates a fixed point design for I/Q imbalance compensation based on the I/Q imbalance parameters. Here, the I/Q imbalance parameters are inputted along with the input signal, and the input signal is I/Q imbalance compensated based on the I/Q imbalance parameters. In performing this calculation, a sqrt_appr module for calculating a square root is used.
For the first calibration mode, K1 and K12 can be set to an initial value of 1; and K2 can be set to an initial value of 0. After DC calibration (i.e. about 2*8192 samples), K1, K2 and K12 can be outputted at the end of every 8192 samples. The K1, K2 and K12 values may converge after processing a 100 to 1000 times.
Calibration Mode 2
The initial values for K1, K2 and K12 are based on a tuner calibration value, which can be obtained from Mode 1. After DC calibration (i.e. about 2*8192 samples), K1, K2 and K12 values can be outputted after every 8192 samples are processed.
Time Control
The output time for the calculated K1, K2, and K12 values should be after the coarse time synchronization has finished. In other words, during the coarse time synchronization period, the K1, K2, and K12 values should be constant to improve performance.
While the present invention has been described with reference to certain preferred embodiments or methods, it is to be understood that the present invention is not limited to such specific embodiments or methods. Rather, it is the inventor's contention that the invention be understood and construed in its broadest meaning as reflected by the following claims. Thus, these claims are to be understood as incorporating not only the preferred methods described herein but all those other and further alterations and modifications as would be apparent to those of ordinary skilled in the art.
This application claims priority from a provisional patent application entitled “Effects of IQ Imbalance and Compensation in DVB-H” filed on Nov. 6, 2007 and having an Application No. 60/985,971. Said application is incorporated herein by reference.
Number | Date | Country | |
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60985971 | Nov 2007 | US |