Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-01-14
2002-05-14
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C455S136000
Reexamination Certificate
active
06389085
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communications systems and in particular to a spatial diversity receiving system for receiving a digitally modulated signal.
BACKGROUND OF THE INVENTION
There are several digital modulation techniques, including BPSK, QAM, (sometimes referred to as QASK), QPSK, which can be demodulated by a delay and multiply operation known as differential detection. In point-to-point and point-to-multi-point communications systems, particularly mobile radio systems, it is known that the medium's channel properties vary over time. These channel variations cause the receiver signal to fade in and out or be subject to intermittent outages. Examples of media or communications channels exhibiting fading properties include electromagnetic signals propagating through the Earth's atmosphere, or undersea acoustic signals.
Signal fading in communications channels is a natural phenomenon that limits the range of separation between the transmitter and receiver. Many very practical and useful channels exhibit fading. The fading in these channels is usually caused by the receiver being linked to the transmitter by more than one propagation path, the lengths of which vary with time. Such channels usually exhibit filing properties terminated Rayleigh fadfing or Rician fading. Fading can also be caused by time varying absorption. For example, the absorption properties of an atmospheric radio channel depend on the moisture content of the atmosphere, which changes over time.
The reception over a channel subject to fading, can be improved by incorporating diversity to reduce the fading and intermittent channel effects that interfere with communications over the channel. There are several diversity options including multiple transmissions using the same communication frequency with each transmission separated in time from the other, which may be termed time diversity; multiple carrier frequencies requiring several different communication frequencies, which may be termed frequency diversity; and multiple antennas or receiving elements with each antenna or receiving element physically separate from the other, which may be termed spatial diversity; etc.
It is well known that the reception of a signal that propagates through a fading or intermittent channel can be improved through the use of spatial diversity. Spatial diversity systems utilize multiple, physically separated, receiving elements in a way that mitigates the fading or intermittent outages experienced by each of the receiving elements. A strong persistent signal, with a much smaller degree of fading, is obtained by properly processing the signals from each of the receiving elements and then properly combining these signals. The strength and persistence of the combined signal depends on the number of receiving elements and the techniques used to process and combine the signals. There is a variety of signal processing and combining techniques that can be used.
One approach to reducing signal fading is to use some form of diversity to receive multiple signals at the receiver and then to combine these signals in a constructive way. A diversity system could involve frequency diversity which is implemented by transmitting identical information on two or more separate carriers that are separated enough in frequency for the fading of each carrier to be uncorrelated with the others. It could involve sending the identical information on the same carrier but at two or more different times with enough separation in time for the fading at each time to be uncorrelated (time diversity). It could also involve sending the information only once, on one carrier and receiving it on two or more separate antennas physically separated with enough distance between them for the fading at each antenna in the receiver to be uncorrelated with the others (spatial diversity).
In all diversity systems the multiple signals have to be combined, which usually involves processing or conditioning each of the multiple signals and then summing or selecting these processed signals. There are different methods for combining the signals, each offering different trade offs between performance and implementation complexity.
Each of the three types of diversity has its advantages and disadvantages. However all three types require a diversity combiner. Spatial diversity has the strong advantage of requiring minimal bandwidth to transmit the information. It has the disadvantage in radio frequency communication of requiring more high frequency circuitry than the others, in particular multiple antennas and associated radio frequency (RF) electronics. Spatial diversity is particularly attractive when the carrier frequency is sufficiently high for the antennas to be implemented as printed circuits. At these frequencies the cost of implementing spatial diversity is the cost of the electronics associated with each antenna.
The theory of spatial diversity and its ability to mitigate fading is well known. To reach the theoretical performance limit requires knowledge of the amplitude and phase of the carrier, which changes with time and can only be estimated. The challenge in achieving near optimal performance is in the implementation of an amplitude and phase estimator and the implementation of the phase and amplitude corrector. There are many techniques for estimating and correcting amplitude and phase and for combining the corrected signals. The various techniques have different degrees of compromise between performance and ease of implementation. Examples of prior art that fall into this category include:
U.S. Pat. No. 4,386,435 to Ulmer providing a space diversity receiver includes an IF band combiner amplifier to sum the IF signals where one signal has a phase corrector which adjusts the phase in response to received signal characteristics. A similar approach is also used in U.S. Pat. Nos. 4,326,294 and 4,710,975 both to Okamoto et al.
U.S. Pat. No. 5,530,925 to Garner combines signals from two physically diverse antennas after down conversion to an intermediate frequency. U.S. Pat. No. 4,498,885 to Namiki combines two spatially diverse signals relying on controlled phase shifting of one of the signals to cancel the effects of an interference wave.
In U.S. Pat. No. 5,203,025 to Anvari et al. the relative phase and amplitude of the IF stage of a spatially diverse receiver are used to determine a signal combining strategy of either direct summation, or inversion then summation, for recovering the modulating signal. A related approach is employed by Karabinis in U.S. Pat. No. 4,373,210 which selects from two spatially diverse signals based on relative signal to interference ratios. The receiver in U.S. Pat. No. 4,079,318 to Kinoshita combines signals from spatially diverse antennas relying on phase control at the intermediate frequency stage.
There are several techniques for combining the signals from multiple antennas to mitigate the effects of fading. These techniques can be logically divided into two categories: post-detection combining and pre-detection combining. The post-detection combiners essentially require an entire receiver for each antenna but the decision and control circuits remain common. While these circuits can achieve near optimum performance, they are expensive solutions. Pre-detection schemes can also be very complicated and expensive. To achieve near optimum performance requires circuits that estimate the phase of the carrier received at each antenna as well as voltage controlled phase shifter circuits to perform in phase alignment or correction. The purpose of these circuits is to align the phase of the carriers received on each antenna. The implementation cost of such systems is quite high.
A simpler method that yields suboptimum performance can be described as a frequency stacked IF combiner. In this method the carriers from each antenna are translated to separate IF frequencies, with the separation between IF carriers being multiples of the bit rate. The multiplicity of frequency stacked IF signals is summed without regar
Bayard Emmanuel
Blake Cassels & Graydon LLP
Leier Terry L.
Pham Chi
Wavecom Electronics Inc.
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