Abstract:
A radio-frequency receiver employing antenna diversity is provided with a plurality of antennas and a local oscillator. Individual error signals are derived from each of respective ones of signals received from the plurality of antennas. The error signals are thereafter combined in such a way as to arrive at a combined error signal having greater reliability than any one of the individual error signals taken alone. Finally, an automatic frequency control circuit is controlled using the combined error signal. Since the effect of fading on the input signal to the automatic frequency control circuit is reduced, performance of the automatic frequency control circuit is increased and the quality of radio reception is improved.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to radio receivers and more particularly to automatic frequency control in diversity radio receivers. 
     Automatic frequency control is commonly used in radio receivers to, insofar as possible, keep the radio receiver locked on a frequency desired to be received despite imperfect component stability that would otherwise result in frequency drift. In one known arrangement, for example, a received carrier frequency is mixed with a local replica of the carrier frequency produced by a local oscillator to yield a baseband signal. The frequency error of the baseband signal is measured and the error signal is used to adjust the frequency in the local oscillator to more closely coincide with the actual carrier frequency, resulting in better reception. 
     Since radio communications exhibit a well-known fading channel characteristic, when fading is severe, it becomes difficult to obtain a reliable error signal in order to adjust the frequency of the local oscillator using an automatic frequency control circuit. Perversely, effective automatic frequency control is most needed during such fading in order to improve reception quality. 
     Space diversity whereby reception is effected using a plurality of spaced-apart antennas is known to reduce the effects of fading. No application has been made, however, of space diversity to automatic frequency control. 
     What is needed then, is an automatic frequency control arrangement that is more effective in the presence of fading. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a radio-frequency receiver employing antenna diversity is provided with a plurality of antennas and a local oscillator. Individual error signals are derived from each of respective ones of signals received from the plurality of antennas. The error signals are thereafter combined in such a way as to arrive at a combined error signal having greater reliability than any one of the individual error signals taken alone. Finally, an automatic frequency control circuit is controlled using the combined error signal. Since the effect of fading on the input signal to the automatic frequency control circuit is reduced, performance of the automatic frequency control circuit is increased and the quality of radio reception is improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will be readily apparent to one of ordinary skill in the art from the following written description, read in conjunction with the drawings, in which: 
     FIG. 1 is a block diagram of the essential portion of a radio receiver with automatic frequency control according to the present invention; and 
     FIG. 2 is a block diagram of the data detector and phase error estimator of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, the space diversity receiver of the present invention is illustrated as having two antennas 11 and 13; however, any number of antennas may be used consistent with the principles of the present invention. The radio frequency signal picked up on the first antenna 11 is designated as f carr1  and a radio frequency signal picked up by the second antenna 13 is designated as f carr2 . The respective radio frequency signals are input to mixers 15 and 17. Frequency synthesizer 33, which includes a local oscillator, produces a local replica of the carrier frequency f ref  which is also input to each of the mixers 15 and 17. The radio frequency signals and the reference frequency are beat together in the mixers to recover respective baseband information signals s 1  (t) and s 2  (t). These baseband information signals are used by a data detector and phase error estimator 23 to detect the data originally transmitted and to produce an error signal for use by an automatic frequency control circuit 31. The automatic frequency control circuit 31 controls the reference frequency produced by the frequency synthesizer 33. 
     For purposes of explaining the principle of the present invention, it will be assumed that transmissions to be received by the receiver of FIG. 1 employ digital angle modulation. In particular, for purposes of illustration, differential phase shift keying will be assumed, although the principles of the present invention are applicable to all kinds of digital modulation. 
     Since the Earth&#39;s atmosphere is a very non-ideal channel medium for radio frequency transmissions, the radio frequency signals f carr1  and f carr2  and hence the information signals s 1  (t) and s 2  (t) will be subjected to time-varying phase shifts resulting in phase errors. These phase errors are estimated by the data detector and phase error estimator 23 and output as Ph err1  and Ph err2 . Radio frequency signals are also subject to fading as previously explained. Since one of the information signals s 1  (t) and s 2  (t) will often be received more clearly than the other of the information signals at a given time, a weighted combination of the corresponding phase error signals Ph err1  and Ph err2  is formed using respective amplifiers 25 and 27 and a summer 29 to form a weighted sum Ph err .tot of the respective phase error signals for input to the automatic frequency control circuit 31. Appropriate weights for each of the phase error signals are calculated by a processor 21 and input to their respective amplifiers 25 and 27 to set the gains K1 and K2 of the respective amplifiers. 
     A signal measuring unit 19 performs signal measuring with respect to the information signals s 1  (t) and s 2  (t) and provides measurement results to the processor 21 for use in determining the weights to be applied to the respective phase error signals. The signal measuring unit 19 may also be a part of the data detector and phase error estimator 23. Different types of signal measuring may be employed. Possible types include measuring signal amplitude, signal energy (proportional to the square of the signal amplitude) and signal quality in terms of a signal-to-noise ratio, for example. Other possible measures of signal quality are a signal-to-interference ratio and a bit error rate, the latter utilizing detected data. The signal measuring unit 19 essentially quantifies how well each of the information signals s 1  (t) and s 2  (t) is received, i.e., it measures the reliability of each of the information signals received. How well the information signal is received will determine how much weight that signal is given for purposes of automatic frequency control. For example, if one of the received information signals is very weak compared to the other due to fading, the strong information signal will dominate control of the automatic frequency control circuit. Thus, a weighted sum is formed by measuring the reliability of information signals received at the antennas and applying a larger weight to the phase error signal representing an information signal having a greater reliability and applying a smaller weight to the phase error signal representing an information signal having lesser reliability. 
     The data detector and phase error estimator 23 of FIG. 1 is shown in greater detail in FIG. 2, wherein T represents one symbol time in the information stream and is taken as a sampling interval. First, the information signals s 1  (t) and s 2  (t) containing both amplitude and phase information are input to phase detectors 35 and 37 to produce signals φ 1  (t) and φ 2  (t) containing phase information only with respect to the phase of the reference frequency, f ref . At sampling times t=n×T for n=1,2 . . . , the phase signals φ 1  (t) and φ 2  (t) are sampled in respective sampling units 39 and 41 to produce sampled data phase signals φ 1  (n×T) and φ 2  (n×T). Previous sampled data phase signals φ 1  ((n-l)×T) and φ 2  (n-l)×T) are delayed by one sample time T by delay elements 43 and 45 and then subtracted from the present sampled data phase signal φ 1  (n×T) and φ 2  (n×T). In differential phase shift keying, the resulting phase differences ΔΦ 1  (n×I) and ΔΦ 2  (n×T) represent the coded information and are input to a decision element 51. In particular, according to differential phase shift keying, if -90°&lt;ΔΦ&lt;+90° then DATA n  0; else if +90°&lt;ΔΦ&lt;+270° then DATA n  =1. 
     The decision element 51 uses the two different phase difference signals to produce a more reliable data output than would result by using only a single phase difference input. A weighted combination of the different phase difference signals may be formed in like manner as the weighted combination of the individual phase error signals in FIG. 1, or a &#34;best one&#34; of the phase difference signals according to measurements performed by the signal measurement unit 19 may be chosen on which to base a decision. If an odd number of antennas are used to produce an odd number of phase difference signals, then the decision element 51 may be such as to form a majority decision. Each of the foregoing alternatives has its relative merits. Other methods of forming a decision may also be suitable. The enumerated alternatives are therefore intended to be exemplary only. 
     The resulting data decision is encoded in a phase coder 53 to produce what would have been the original phase information Ph info . This phase information is subtracted from the respective phase difference signals in summers 55 and 57 to produce the respective phase error signals. In accordance with differential phase shift keying, if DATA n  32 0 then Ph info  =0° and Ph err  =ΔΦ; else if DATA n  1 then Ph info  =180° and Ph err  ΔΦ-180°. 
     The automatic frequency control circuit 31 of FIG. 1 need not be of any particular construction but rather may be of any of the types commonly employed in modern radio frequency receivers. As is the case with any automatic frequency control circuit, the performance of the circuit will be no better than the reliability of the signal driving the automatic frequency control loop. By driving the automatic frequency control loop using a weighted combination of error signals derived from different input signals of a diversity receiver and by calculating appropriate weights according to how well the different signals are received, a more reliable signal is produced to drive the automatic frequency control loop. Especially during fading when automatic frequency control is most needed, performance of the automatic frequency controller may be improved. 
     It will be appreciated by those of ordinary skill in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.