Abstract:
A system for determining a time delay between an in-phase signal component and a quadrature-phase signal component includes an in-phase signal start time determination module coupled to an in-phase delay module, the in-phase signal start time determination module and the in-phase delay module configured to receive an in-phase signal component of a received signal. The in-phase signal start time determination module is configured to receive a reference signal. The system also includes a quadrature-phase signal start time determination module coupled to a quadrature-phase delay module, the quadrature-phase signal start time determination module and the quadrature-phase delay module configured to receive a quadrature-phase signal component of a received signal. The quadrature-phase signal start time determination module is configured to receive a reference signal, wherein the in-phase delay module is configured to develop an in-phase delay signal and the quadrature-phase delay module is configured to develop a quadrature-phase delay signal.

Description:
BACKGROUND 
     Many communications systems employ communication protocols in which data to be communicated is converted to a system that includes in-phase (I) and quadrature-phase (Q) components. The in-phase and the quadrature-phase components are shifted in phase by an angular amount, such as, for example, 90 degrees. The data signal is impressed on the in-phase and the quadrature-phase signal components by a transmitter and recovered by a receiver. One example of a communication system that employs in-phase and quadrature-phase components is what is referred to as a multiple input multiple output (MIMO) communication system that uses orthogonal frequency division multiplexing (OFDM) in a multiple-antenna arrangement and that complies with communication standard IEEE 802.11n. 
     In any communication system that uses in-phase and quadrature-phase signal components, the quality of the signal transmission is dependent upon the relationship between the in-phase and the quadrature-phase components. For example, differences in the time delay between the in-phase and the quadrature-phase components can result in serious performance degradation to communication systems if not well compensated. The differences in the time delay between the in-phase and the quadrature-phase components, also referred to as I/Q time delay, is caused, at least in part, due to the different transmission delay of the I and Q signals. The main culprits giving rise to I/Q time delay are different circuit length between the I and Q channels, channel fading, and phase noise, which are all inevitable in real communication systems. 
     Therefore, it would be desirable to have a way to determine and compensate the delay between the in-phase and the quadrature-phase components of a communication signal. 
     SUMMARY 
     An embodiment of a system for determining a time delay between an in-phase signal component and a quadrature-phase signal component includes an in-phase signal start time determination module coupled to an in-phase delay module. The in-phase signal start time determination module and the in-phase delay module are configured to receive an in-phase signal component of a received signal. The in-phase signal start time determination module is configured to receive a reference signal. The system also includes a quadrature-phase signal start time determination module coupled to a quadrature-phase delay module. The quadrature-phase signal start time determination module and the quadrature-phase delay module are configured to receive a quadrature-phase signal component of a received signal. The quadrature-phase signal start time determination module is configured to receive a reference signal. The in-phase signal start time determination module is configured to develop an in-phase start time signal and the quadrature-phase signal start time determination module is configured to develop a quadrature-phase start time signal, wherein the in-phase delay module develops an in-phase delay signal representative of a delay of the in-phase signal component of the received signal and wherein the quadrature-phase delay module develops a quadrature-phase delay signal representative of a delay of the quadrature-phase signal component of the received signal. 
     Other embodiments and methods of the invention will be discussed with reference to the figures and to the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be described by way of example, in the description of exemplary embodiments, with particular reference to the accompanying figures. 
         FIG. 1  is a schematic diagram illustrating a basic communication system. 
         FIG. 2  is a schematic diagram illustrating the basic components of the transmitter of  FIG. 1 . 
         FIG. 3  is a schematic diagram illustrating the basic components of the receiver of  FIG. 1 . 
         FIG. 4  is a schematic diagram illustrating a portion of the receive baseband module of  FIG. 3 . 
         FIG. 5  is a schematic diagram illustrating an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement. 
         FIG. 6  is a schematic diagram illustrating an alternative embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement. 
         FIG. 7  is a schematic diagram illustrating an example of the operation of the start time determination module described above in  FIGS. 5 and 6 . 
         FIG. 8  is a flowchart showing the operation of an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement of  FIG. 5 . 
         FIG. 9  is a flowchart showing the operation of an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement of  FIG. 6 . 
         FIG. 10  is a flowchart showing the operation of an embodiment of the start time determination module described in  FIG. 7 . 
         FIG. 11  is a plot illustrating a signal space diagram of a signal received in a conventional receiver. 
         FIG. 12  is a plot illustrating a signal space diagram of a signal received in a receiver including the system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation. 
     
    
    
     DETAILED DESCRIPTION 
     The system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation can be implemented on a measurement instrument or can be implemented in a receiver of a communication device for compensating for the delay between an in-phase signal component and a quadrature-phase signal component, also referred to as I/Q time delay. As will be described below, the system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation uses a stored preamble of a standard communication signal or use a unique test signal to perform the I/Q signal delay measurement. The system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation are independent of system architecture and do not have any dependency on the structure of system. The delay of the in-phase and the quadrature-phase components of a received signal is obtained by comparing the received signal with a reference signal. The I/Q time delay is obtained from the difference between the delay of the in-phase and the quadrature-phase components. 
       FIG. 1  is a schematic diagram illustrating a basic communication system. The communication system includes a transmitter  110 , a channel  120  and a receiver  130 . The transmitter  110  can be any transmitter and can be located in, for example, a portable cellular communication device, a personal computer (PC), a personal digital assistant (PDA), a portable game player, a wireless local area network (LAN) device, or any other communication device. The receiver  130  can be any receiver adapted to receive the transmissions from the transmitter  110  and can be located in, for example, a portable cellular communication device, a personal computer (PC), a personal digital assistant (PDA), a portable game player, a wireless local area network (LAN) device, or any other communication device. Further, the transmitter  110  and the receiver  130  can be incorporated into a transceiver and the transceiver can communicate with another transceiver according to the principles described below. The channel  120  represents the transmission environment between the transmitter and the receiver, and can be any wired or wireless communication channel. 
       FIG. 2  is a schematic diagram illustrating the basic components of the transmitter  110  of  FIG. 1 . The transmitter  110  includes a transmit baseband module  210 , a modulator  220  and a transmit radio frequency (RF) module  230 . The transmit baseband module  210  performs baseband signal processing and provides a transmit signal to the modulator  220 . In an embodiment, the transmit signal provided by the transmit baseband module  210  includes an in-phase component and a quadrature-phase component. The modulator  220  modulates the baseband signal and optionally upconverts the transmit signal to an intermediate frequency (IF) or to an RF signal level. The modulator provides the modulated signal to the transmit RF module  230 . The transmit RF module  230  generally includes one or more amplification stages to amplify the modulated signal for transmission over the channel  120  ( FIG. 1 ). 
       FIG. 3  is a schematic diagram illustrating the basic components of the receiver  130  of  FIG. 1 . The receiver  130  includes a receive RF module  310 , a demodulator  320  and a receive baseband module  330 . The receive RF module  310  receives and filters the transmit signal received over the channel  120  ( FIG. 1 ). The receive RF module  310  provides the received signal to the demodulator  320 . The demodulator  320  demodulates the received signal to recover the information signal. The information signal includes an in-phase component, I rx (t) and a quadrature-phase component Q rx (t). The in-phase component, I rx (t) and the quadrature-phase component, Q rx (t), are provided to the receive baseband module  330  where the information contained in the in-phase component and the quadrature-phase component are recovered to baseband level. Time delay between the in-phase component and the quadrature-phase component mainly arises in the interface between the receive RF module  310  and the receive baseband module  330 . 
       FIG. 4  is a schematic diagram illustrating a portion of the receive baseband module  330  of  FIG. 3 . The receive baseband module  330  includes a synchronizer  410  and a baseband demodulator  420 . The synchronizer  410  correlates the in-phase and the quadrature-phase components and provides a correlated receive signal to the baseband demodulator  420 . The baseband demodulator  420  recovers the original transmit information. 
       FIG. 5  is a schematic diagram illustrating an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement. In an embodiment, the system and method for in-phase/quadrature-phase (I/Q) time delay measurement can be implemented in a synchronizer, such as the synchronizer  410  of  FIG. 4 . The embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement  500  shown in  FIG. 5  is implemented in the synchronizer  410  of  FIG. 4 . However, other implementations and embodiments are possible. 
     The system and method for in-phase/quadrature-phase (I/Q) time delay measurement  500  includes a delay module  514  and a delay module  516 . In this example, the delay module  514  operates on the in-phase component and the delay module  516  operates on the quadrature-phase component. However, this is arbitrary. 
     The in-phase component, I rx (t), of the receive signal is supplied via connection  502  to the delay module  514  and to a start time determination module  512 . In accordance with an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation, a reference signal comprising an in-phase component, I tx (t) and a quadrature-phase component, Q tx (t), of the transmit signal is supplied to the start time determination module  512  via connection  506 . The reference signal on connection  506  can be a preamble of the transmit signal stored in the baseband module  330  of the receiver  130 , or can be a unique test signal. In an embodiment, the reference signal is a preamble of a communications message, where the preamble, and possibly other portions of the message, is defined by the applicable communication standard. In such an embodiment, the format of the reference signal is known at the receiver end. The reference signal can be stored in a memory associated with the transmitter  110  ( FIG. 1 ) or can be generated in real-time. In an embodiment in which the reference signal is a unique test signal, the receiver  130  ( FIG. 1 ) operates in test mode and the test signal would be defined for such test mode. 
     The output of the start time determination module  512  is a signal, t I0 , representing the start time of the in-phase component of the received signal, I rx (t). The signal, t I0  is provided to the delay module  514  via connection  524 . The delay module  514  delays the signal I rx (t) by the amount t I0 , and provides the output signal, I rx  (t−t I0 ) on connection  526 . The signal on connection  526  represents the receive signal, I rx (t) delayed by an amount of time corresponding to the actual start time of the reference signal I tx (t). 
     The quadrature-phase component, Q rx (t), of the receive signal is supplied via connection  504  to the delay module  516  and to a start time determination module  518 . In accordance with an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation, a reference signal comprising an in-phase component, I tx (t) and a quadrature-phase component, Q tx (t), of the transmit signal is supplied to the start time determination module  518  via connection  508 . The reference signal on connection  508  can be a preamble of the transmit signal stored in the baseband module  330  of the receiver  130 , or can be a unique test signal, as described above. 
     The output of the start time determination module  518  is a signal, t Q0 , representing the start time of the quadrature-phase component of the received signal, Q rx (t). The signal, t Q0  is provided to the delay module  516  via connection  522 . The delay module  516  delays the signal Q rx (t) by the amount t Q0 , and provides the output signal, Q rx  (t−t Q0 ) on connection  528 . In an embodiment, the delay module  514  delays its output by one period time. The signal on connection  528  represents the receive signal, Q rx (t) delayed by an amount of time corresponding to the actual start time of the reference signal Q tx (t). 
     The signal, I rx  (t−t I0 ) on connection  526  and the signal, Q rx  (t−t Q0 ) on connection  528  are provided to a real-to-complex conversion module  532 . The real-to-complex conversion module  532  converts the signals on connections  526  and  528  to a complex signal having the form I rx  (t−t I0 )+jQ rx  (t−t Q0 ) on connection  534 . 
     In a traditional synchronizer, the start time of the in-phase and quadrature-phase signals is assumed to be the same and the delay between the I and Q signals cannot be measured. As described above, by using the reference signals I tx (t) and Q tx (t), the start times of the in-phase component and the quadrature-phase component are estimated separately and I/Q time delay can be obtained using the following equation.
 
 t   IQ   =t   I0   −t   Q0    Eq. (1)
 
       FIG. 6  is a schematic diagram illustrating an alternative embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement. In an embodiment, the system and method for in-phase/quadrature-phase (I/Q) time delay measurement can be implemented using a conventional synchronizer, such that the I/Q delay measurement results can be used for compensation of I/Q time delay in a baseband demodulator, such as the baseband demodulator  420  of  FIG. 4 . However, other implementations and embodiments are possible. 
     In the embodiment shown in  FIG. 6 , the in-phase component, I rx (t), of the receive signal is supplied to a synchronizer  606  via connection  602  and the quadrature-phase component, Q rx (t), of the receive signal is supplied to a synchronizer  606  via connection  604 . The output of the synchronizer  606  is the complex term I rx (t−t 0 )+jQ rx (t−t 0 ). However, the term I rx (t−t 0 )+jQ rx (t−t 0 ) fails to account for any time delay between the in-phase component and the quadrature-phase component. The output of the synchronizer  606  is supplied via connection  618  to a baseband demodulator  628 . 
     The in-phase component, I rx (t), of the receive signal is also supplied to a start time determination module  608  via connection  602  and the quadrature-phase component, Q rx (t), of the receive signal is supplied to a start time determination module  612  via connection  604 . The start time determination module  608  is similar to the start time determination module  512  of  FIG. 5  and the start time determination module  612  is similar to the start time determination module  518  of  FIG. 5 . 
     In accordance with an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation, a reference signal comprising an in-phase component, I tx (t) and a quadrature-phase component, Q tx (t), of the transmit signal is supplied to the start time determination module  608  and the start time determination module  612  via connection  616 . 
     The output of the start time determination module  608  is a signal, t I0 , representing the start time of the in-phase component of the receive signal, I rx (t). 
     The output of the start time determination module  612  is a signal, t Q0 , representing the start time of the quadrature-phase component of the receive signal, Q rx (t). The signal t I0  on connection  622  and the signal t Q0  on connection  624  are supplied to a subtractor  626 . The output of the subtractor  626  is a signal, t IQ , representing the net delay of the in-phase and the quadrature-phase receive signals. The signal t IQ  is supplied via connection  632  to the baseband demodulator  628 . The signal t IQ  is used to compensate for the I/Q signal delay by altering the term I rx (t−t 0 )+jQ rx (t−t 0 ) to account for I/Q signal delay. 
       FIG. 7  is a schematic diagram illustrating an example of the operation of the start time determination module described above in  FIGS. 5 and 6 . The start time determination module  700  includes a cross correlator  712  and a cross correlator  714 . The receive signal, I rx (t) or the receive signal Q rx (t), is supplied to the cross correlator  712  and the cross correlator  714  via connection  702 . 
     The reference signal, I rx (t) is supplied to the cross correlator  712  via connection  706  and the reference signal, Q tx (t), is supplied to the cross correlator  714  via connection  708 . The cross correlator  712  calculates and outputs the value of [I tx (t−t 0 )*I rx (t)] or [I tx (t−t 0 )*Q rx (t)]. The cross correlator  714  calculates and outputs the value of [Q tx (t−t 0 )*I rx (t)] or [Q tx (t−t 0 )*Q rx (t)] for different values of t 0 . 
     The output of the cross correlator  712  is supplied via connection  716  to the absolute value module  722 . The output of the cross correlator  714  is supplied via connection  718  to the absolute value module  724 . The absolute value modules  722  and  724  provide as an output the absolute values of their inputs. 
     The output of the absolute value module  722  is provided to the adder  732  via connection  726  and the output of the absolute value module  724  is provided to the adder  732  via connection  728 . The output of the adder on connection  734  is supplied to a module  736  that determines the maximum magnitude of the signals t I0  and t Q0 . In block  736 , the value of to that makes (abs[I tx (t−t 0 )*I rx (t)]+abs[Q tx (t−t 0 )*I rx (t)]) or (abs[I tx (t−t 0 )*Q rx (t)]+abs[Q tx (t−t 0 )*Q rx (t)]) achieve its maximum is selected as its output. 
       FIG. 8  is a flowchart showing the operation of an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement of  FIG. 5 . 
     In block  802 , the receive signals I rx (t) and Q rx (t) are provided to the delay modules  514  and  516  and the start time determination modules  512  and  518  of  FIG. 5 . In block  804 , the reference signals I tx (t) and Q tx (t) are provided to the start time determination modules  512  and  518  of  FIG. 5 . 
     In block  806 , the start time, t, for the in-phase (t I0 ) and quadrature-phase (t Q0 ) components is found. In block  808 , the delay of the in-phase and quadrature-phase components is found as t IQ =t I0 −t Q0 . 
       FIG. 9  is a flowchart showing the operation of an embodiment of the system and method for in-phase/quadrature-phase (I/Q) time delay measurement of  FIG. 6 . 
     In block  902 , the delay times t I0  and t Q0  are determined by the start time determination modules  608  and  612 , respectively. In block  904 , the delay t I0  is subtracted from the delay t Q0  to obtain the delay t IQ . In block  906 , the delay t IQ  is applied to the baseband demodulator to compensate for the I/Q time delay. 
       FIG. 10  is a flowchart showing the operation of an embodiment of the start time determination module described in  FIG. 7 . In block  1002 , depending on whether the in-phase or the quadrature-phase signal is being processed, either the receive signal I rx (t) or the receive signal Q rx (t) is provided to the cross correlators of  FIG. 7 . In block  1004 , the reference signals I tx (t) and Q tx (t) are provided to the cross correlators  712  and  714 , respectively, of  FIG. 7 . In block  1006 , the absolute values of the output of the cross correlators is provided to an adder. In block  1008 , the absolute values are added. In block  1012 , the combined absolute value is used to find t I0  or t Q0 . 
       FIG. 11  is a plot illustrating a signal space diagram of a signal received in a conventional receiver. 
       FIG. 12  is a plot illustrating a signal space diagram of a signal received in a receiver including the system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation. As illustrated by the comparison of  FIGS. 11 and 12 , the EVM (Error Vector Magnitude) improves after I/Q time delay compensation has been performed based on the measurement results using the system and method for in-phase/quadrature-phase (I/Q) time delay measurement and compensation described above. 
     The foregoing detailed description has been given for understanding exemplary implementations of the invention and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the art without departing from the scope of the appended claims and their equivalents.