System and method for diversity reception of signals.

This invention relates to the field of mobile wireless receivers, and more particularly to a method of diversity reception for achieving excellent voice communication quality in situations where the strength of the electromagnetic field reaching the receiver is subject to large scale fluctuations.

BACKGROUND

As is well known, mobile land communications and the like suffer from an undesirable phenomena called Rayleigh fading in which there are wide fluctuations in the strength of the received electromagnetic wave signals due to the influence on wave propagation of building structures and the relatively short 1 -4 meter height of the antennas on vehicles. Various diversity reception methods have been used to ameliorate these fluctuations in the received electromagnetic signal. These include such techniques as automatic -switching among several antennas, automatic selection from among several' receivers, and the combining of signals from several receivers. See, for example, the discussion of basic diversity classifications and combining methods in the book Microwave Mobile Communications, William C. Jakes, Jr., editor, published by John Wiley & Sons, Inc. (New York 1974), particularly at pages 309 -321.

OBJECTS OF THE INVENTION

Looking at the various diversity techniques mentioned above, automatic selection from among several receiving antennas (automatic antenna selection) has the disadvantage that at the time the antennas are switched a great deal of noise is generated in the receiver. The technique of combining signals from several receivers has the disadvantage that if this combining is done before demodulation, it is necessary to very carefully combine the phases of the received waves, which requires extremely complex devices. As for automatic selection from among several receivers, even if it has the advantage that the implementing construction is relatively simple, it has the disadvantage of requiring several receivers, only one of which is fully employed at any given time. Accordingly, the other receivers are relatively idle.

Therefore, in order to eliminate the disadvantages of the above-mentioned techniques, an object of the present invention is to provide a method of diversity reception that in addition to being based on a method of automatic selection also incorporates the advantages of the method of combining signals from several receivers.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a diversity reception of signals is accomplished by first and second receivers which are respectively adapted to recover and output first and second recovered baseband signals. The first and second receivers have first and second wave detectors, respectively, for generating output voltages corresponding to the strength of a modulated electromagnetic signal wave reaching the respective receiver. Then, a comparator means detects the difference in output voltages between the first and second wave detectors, when the difference exceeds a predetermined value, and an output detection signal is given. An adder circuit is adapted to receive the first and second recovered baseband signals and to combine them in correlation.

First and second switching circuits are selectively capable of cutting off the first or second recovered signal, which signals are being input to the operational adder circuit. A third switching circuit is capable of adjusting the gain of the adder circuit, by reducing it by one half. When the comparator means gives an output detection signal, the first and second switching circuits are operated to cut off only the recovered baseband signal input from the input to the adder circuit which is then being received from the receiver having the lowest output voltage. When the comparator does not give the output detection signal, the first and second switching circuits are controlled so that both the first and second recovered baseband signals are input to the adder circuit. At the same time, the third switching circuit is controlled to reduce the gain of the adder circuit by one half. This causes the desired diversity output signal to be produced as the output of the adder circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Figure 1 is a simplified diagrammatic - schematic of a multireceiver embodiment utilizing my invention;</li><li>Figure 2 shows, for relative comparison, the amplitude of a signal S recovered by the embodiment of Figure 1 and the amplitude of the noise N accompanying it, each as a function of-time;</li><li>Figure 3 is a functional block diagram showing a prior art method of automatic selection between multiple receivers;</li><li>Figure 4 shows, as a function of time, typical corresponding voltage outputs of the wave envelope detectors of the prior art method of Figure 3;</li><li>Figure 5 is a modification of Figure 4 made by tracing only the larger of the two voltage outputs for each point in time; and</li><li>Figure 6 shows for relative comparison the amplitude of a signal S as recovered by the prior art automatic receiver switching method of Figure 3 and the amplitude of the noise N accompanying it, each as a function of time.</li></ul>

PRIOR ART RECEIVER SWITCHING AND ITS DISADVANTAGES

The block diagram of Figure 3 shows a prior art diversity method in which there is automatic selection between multiple receivers. For example, receivers 1 and 2 respectively have spaced apart receiving antennas 11 and 12. If the wavelength of the received signal is X , then the antennas are preferably spaced apart a distance X/4, or greater, so that their respective received signals will be virtually uncorrelated. This assures independent fading so that the improvement from diversity reception will be great.

The signals picked up by antennas 11 and 12 are respectively input to FM receivers 1 and 2 where they undergo the usual detection to recover the low frequency baseband signal. For determining signal strength, each of the receivers 1 and 2 contains a detector 1a, 2a for detecting the wave envelope of the received signal. Each wave envelope detector 1a, 2a generates an output voltage r,, r₂ proportional to the strength of the electromagnetic wave picked up and input from its respective antenna 11, 12.

A comparator 3 receives as input the respective envelope detector output voltages r,, r, of receivers 1 and. 2, regularly determines which is greater, and then by means of switch 4 automatically selects the greater signal. The signal recovered from the receiver, which is selected by means of switch 4, is amplified by a low frequency baseband amplifier 5 and is output to a speaker 6.

Figure 4 shows, as a function of time, typical corresponding voltage outputs r,, r₂ of the wave envelope detectors 1a and 2a of the prior art method of Figure 3. Assuming that the receiving system is a mobile one carried by a vehicle, the antennas 11 and 12 will be moving across the landscape at some transportation speed, causing the envelope detector output voltages r,, r2 to each vary irregularly with time. Note that during the time interval to - t, the voltage r, &gt; r₂, indicating that the electromagnetic signal received by antenna 11 is greater, while during the interval t, -t2 the voltage r2 &gt; r,, indicating that the signal received by antenna 12 has become greater. Therefore, during the interval to -t, receiver 1 is selected by switch 4, and afterward, during the interval t, -t₂, receiver 2 is selelc- ted, etc.

Figure 5 is a modification of Figure 4 made by tracing the envelope of only the instantaneously greater of the two voltage outputs r,, r2 for each point in time. Note that because of the automatic switching between receivers the large fluctuations of received electromagnetic signal have largely disappeared. For relative comparison, Figure 6 shows the amplitude of a demodulated signal S as recovered by the prior art (Fig. 3) receiver switching method and the amplitude of the noise N accompanying it, each as a function of time. Because of the automatic switching of switch 4, during the interval to -t, the signal-to-noise ratio (S/N) is that of receiver 1 while during the interval t, -t2 the S/N is that of receiver 2, etc.

As stated above, the prior art method of automatic switching between receiver 1 and receiver 2 according to which has the stronger received signal can ameliorate the undesirable effects of Rayleigh fading. However, with this method only one of the receivers is selected at any time. Therefore, the remaining receivers are not usefully employed, which is especially a problem when a multiplicity of receivers are used together. The result is an undesirable cost/performance ratio.

EXAMPLE OF AN EMBODIMENT OF THE INVENTION

Figure 1 is a simplified diagrammatic - schematic of a multireceiver embodiment utilizing my invention to eliminate the above-mentioned disadvantages of receiver switching. In Figure 1 the antennas 11, 12, the receivers 1, 2, the wave envelope detectors 1 a, 2a, and detector outputs r,, r₂, are the same as for Figure 3's prior art method of automatic switching between multiple receivers.

However, in the embodiment utilizing my invention there are multiple comparators and other novel circuitry. A comparator 21 compares the signals r, and r_o'. By means of the voltage divider formed by resistors R, and R₂, the signal r; is set to have the reduced value:<maths id="math0001" num=""><img file="EP0206568A2_D0001.tif" /></maths>

The output of comparator 21 is arranged to output a "1" when<maths id="math0002" num=""><img file="EP0206568A2_D0002.tif" /></maths>and to output a "0" when<maths id="math0003" num=""><img file="EP0206568A2_D0003.tif" /></maths>

As is typical with digital signals, the "1" output by comparator 21 can be selected to be represented by some convenient voltage, for example 5 volts, while the "0" output can be 0 volts.

Similarly, a comparator 22 compares the signals r₂ and r,'. By means of the voltage divider formed by resistors R₃ and R₄, the signal r,' is set to have the reduced value:<maths id="math0004" num=""><img file="EP0206568A2_D0004.tif" /></maths>The output of comparator 22 is arranged to output a "1" "when<maths id="math0005" num=""><img file="EP0206568A2_D0005.tif" /></maths>and to output a "0" when<maths id="math0006" num=""><img file="EP0206568A2_D0006.tif" /></maths>

Switch SW1, shown diagrammatically for simplification, is adapted to be controlled by the output of comparator 21. When comparator 21 outputs a "1", switch SW1 becomes CLOSED and the recovered output signal S, of receiver 1 is input through input resistor R_s to an operational adder 23, having an output 101 which is coupled to a low frequency baseband amplifier (not shown) and an output speaker (also not shown) similar to amplifier 5 and speaker 6 in Figure 3. When comparator 22 outputs a "1", switch SW2 becomes CLOSED and provides the recovered output signal S₂ of receiver 2 to operational adder 23 through input resistor R₆.

The AND gate 31 is arranged so that if it simultaneously receives a "1" from both comparator 21 and comparator 22 it outputs a "1" "to CLOSE switch SW3. Operational adder 23 can readily be formed from a differential-input-single- output-type operational amplifier, with the (+) input set to ground and the (-) input receiving signals which are to be added via input resistors R_s and R₆, as well as a negative feedback via feedback resistors R, and R_e. As is well known, such an operational adder reverses the sign and multiplies each input by a factor <chemistry id="chem0001" num="0001"><img file="EP0206568A2_D0007.tif" /></chemistry>where R_f is the amplifier's feedback resistor and R is the input resistor for the input branch concerned.

Thus, when switch SW1 is CLOSED and SW2 is OPEN, switch SW3 will also be OPEN and only recovered signal S, of receiver 1 will be input to operational adder 23, where it is then ouput multiplied by the factor <chemistry id="chem0002" num="0002"><img file="EP0206568A2_D0008.tif" /></chemistry>

Similarly, when switch SW1 is OPEN and SW2 is CLOSED, switch SW3 will also be OPEN and only recovered signal S₂ of receiver 2 will be input to operational adder 23, where it is then output multiplied by the factor <chemistry id="chem0003" num="0003"><img file="EP0206568A2_D0009.tif" /></chemistry>

Finally, there is the frequent case where both switches SW1, SW2 are simultaneously CLOSED, in which case switch SW3 is also CLOSED, shorting out resistor R₇. Recovered signal S, of receiver 1 will then appear in the output of operational adder 23 multiplied by the factor R₈/R₅ along with recovered signal S₂ of receiver 2 multiplied by the factor R₈/R₆. If resistors R_s, R₆, R,, and R_s are all chosen to be the same value, the output signal at terminal 101 (i.e. the diversity reception output signal) will be 2 S, when switch SW1 is CLOSED and switch SW2 is OPEN. When switch SW1 is OPEN and switch SW2 is CLOSED, the ouput signal at 101 will be 2 S₂. And if both switches SW1, SW2 are CLOSED, then switch SW3 will also be closed and the combined output signal at 101 will be S, + S2.

Let us now consider the signal-to-noise ratio - (S/N) of the receiver system for an FM receiver. For certain electromagnetic waves signals, namely those at the threshold of a detection level and above, if the recovered signal S is kept uniform the noise accompanying, it has the characteristic of being inversely proportional to the strength of the received electromagnetic signal. If we call N, and N₂ the noises respectively accompanying the recovered signals of receiver 1 and 2, and if we assume that either switch SW1 or SW2 is OPEN, then at the output terminal 101 the signal-to-noise ration (S/N) will be that of the single selected receiver with the stronger received electromagnetic signal, namely S₂/N₂ or S,/N,.

But when both switches SW1, SW2 are simultaneously CLOSED, that is when the received electromagnetic signals input to the two receivers are roughly comparable, then the signal to noise ratio at output terminal 101 will be:<maths id="math0007" num=""><img file="EP0206568A2_D0010.tif" /></maths>

That is to say, the received signals S, and S₂ being correlated are summed by the operational adder 23 as S = S, + S₂. However, as the recovered noises N, and N₂ are uncorrelated, the effect of an input of both noise signals to the operational adder 23 is a combined noise N having amplitude given by the formula<maths id="math0008" num=""><img file="EP0206568A2_D0011.tif" /></maths>

For example, if we consider the case where the signal strengths reaching the two receivers are equal, then S, = S_z, and N,² = N₂² so the combined S/N computes to be<maths id="math0009" num=""><img file="EP0206568A2_D0012.tif" /></maths>which means an improvement only by the factor √2 , or 3 dB in terms of decibels.

Let us now consider the output at terminal 101 in the case where<maths id="math0010" num=""><img file="EP0206568A2_D0013.tif" /></maths>That is, the electromagnetic signal reaching receiver 1 is √3 times larger than that reaching receiver 2. Since the noise in the recovered signal is inversely proportional to the strength of the received electromagnetic wave, the recovered noise at receiver 2 as compared with the recovered noise in receiver 1 is given by N₂ = √3 N,.

Then if we assume that both switches SW1 and SW2 are CLOSED, the signal-to-noise ratio at the output terminal 101 is given by<maths id="math0011" num=""><img file="EP0206568A2_D0014.tif" /></maths>This is clearly a marginal case since if the signal strength reaching receiver 2 is any further reduced, i.e.<maths id="math0012" num=""><img file="EP0206568A2_D0015.tif" /></maths>with both SW1 and SW2 CLOSED the combined S/N .output at terminal 101 will become worse than the ratio S,/N, for receiver 1 alone. This explains why it is necessary to automatically OPEN switch SW2 when the signal at receiver 2 falls into this low range.

Note also that when just one receiver is being input to operational adder 23 (e.g., switch SW1 is CLOSED, SW2 OPEN), the adder gain is 2, but when an additional receiver is switched in (e.g., SW1 remains CLOSED, SW2 becomes CLOSED), switch SW3 must automatically be closed to drop the adder's gain to 1 so the new combined recovered signal S, + S₂ at terminal 101 maintains an approximately equivalent amplitude when compared to the terminal's output for the just prior one- receiver signal (viz., 2 S,). That is, should switch SW3 be left OPEN when switches SW1 and SW2 are simultaneously CLOSED, the recovered signal S output level at terminal 101 will jump to double what it was when one of the switches SW1-, SW2 was OPEN. Assuming that the fading phenomena has a relatively short typical period, this would cause the received signal level S to repeatedly fluctuate, contributing a large component to the perceived noise.

Referring to Table 1, the relationship between signals r, and r₂ and the position of switches SW1, SW2 and SW3 is shown, from which it is clear that the illustrated embodiment utilizing my invention has three distinct states. <tables id="tabl0001" num="0001"><img file="EP0206568A2_D0016.tif" /></tables>

For relative comparison, Figure 2 shows the amplitude of a signal S recovered by the embodiment of Figure 1 by utilizing my invention and shows the amplitude of the noise N accompanying it, each as a function of time. When this is compared with the unsophisticated S/N performance of the prior art post-detection selection switching of Figure 6, one finds that there is an improvement that can be as large as 3 dB.

Therefore, as explained above, my very effective method for diversity reception of signals can be utilized by a receiver system having very simple structure. In addition to two separate receivers, one provides an adder dircuit for combining signals recovered from the two receivers, means for cutting off the weaker received signal when the difference in the amplitudes of the electromagnetic signals reaching two receivers becomes great, and means for adjusting the gain of the adder circuit to maintain a uniform received signal level when switching to adding together two received signals.

While the principles of the invention have been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention. Therefore, the claims are to be construed to cover all equivalent structures.