SINGLE-SIDEBAND COMMUNICATION SYSTEM

This invention relates to a single-sideband (SSB) communication system and, more specifically, to apparatus for carrying out a demodulating process of SSB signals, in which a frequency detector and an amplitude limiter can be employed.

It is desirable in radio communication that degradations caused by multi-path fading are completely removed. Therefore, modulation methods which can counter the effects of fading must be adopted. The immediate solution would be to use some form of angle modulation, such as frequency modulation (FM) or phase modulation (PM). However, the necessary FM transmission bandwidth B is approximately twice the sum of the peak frequency deviation AF and the highest modulating frequency f , that is B = 2(AF + f). This relation is called Carson's rule, and indicates that the FM transmission bandwidth has to be much wider than the information itself. Even if the frequency deviation is limited to zero, the necessary FM bandwidth is twice as wide as the information bandwidth, namely, 2f .

Using SSB modulation, voice signals can be inherently transmitted in a bandwidth comparable to the information bandwidth. Therefore, an SSB modulation method is very useful from the view-point of a considerable saving of bandwidth. However, in SSB, the information is contained in the signal envelope. For the purpose of maintaining high quality of demodulated voice signals, an automatic gain control (AGC) circuit fast enough to follow fading must be provided. Furthermore, regeneration of the carrier is necessary for demodulation of the SSB signal. Carrier recovery is a cumbersome operation for mobile radios.

There are many transmitters exploying the SSB modulation method, but SSB signals cannot be demodulated without using a carrier regenerated in the receiver and also cannot remove amplitude variations using an amplitude limiter. In a technical paper entitled "Information in the zero crossings of bandpass signals", Bell System Technical Journal, vol. 56, no. 4, p.p. 487-510, April 1977, the author of the paper, B. F. Logan, pointed out that the signal can be reconstructed after amplitude limiting if the signal is expressed by only real simple zeros and that a full-carrier SSB signal belongs to the signal expressed by only real simple zeros.

An object of the invention is to provide an SSB communication system in which the necessary radio-frequency bandwidth is comparable to the information bandwidth and in which an amplitude limiter in the receiver portion of the system can be safely used to remove amplitude degradations caused by fading. Besides removing amplitude degradations, the system of the invention provides for demodulation of SSB signals by the use of a frequency detector and an equalizer.

According to the invention there is provided a receiver for a single-sideband (SSB) communication system in which a transmitted SSB signal is composed of both a single-sideband carrier-suppressed signal and a carrier signal, the receiver being characterised by an amplitude limiter for removing amplitude variations in the received SSB signal; a frequency detector for demodulating the SSB signal at the output of the amplitude limiter; an equaliser connected to the output of the frequency detector to compensate the frequency response of the frequency-detected signal; and a lineariser connected to the output of the equaliser to cancel out distortions contained in the demodulated signal.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which

• Figure 1 is a block schematic diagram of an SSB communication system according to the invention:
• Figure 2 is a block schematic diagram of a lineariser forming part of the system of Figure 1;
• Figure 3 is a block schematic diagram of another form of lineariser;
• Figure 4 is a block schematic diagram of a further form of lineariser;
• Figure 5 is a block schematic diagram of a selective amplification circuit for carrier component amplification;
• Figure 6 is a block schematic diagram of a pre-distortion circuit in a transmitter portion of the system; and
• Figure 7 is a block diagram of a diversity receiver circuit of an SSB communication system.

The principle of the present invention is based upon the fact that signals expressed only in real simple zeros can be fed into an amplitude limiter. When an SSB signal modulated by a band-limited baseband signal is accompanied by a carrier, whose level is higher than that of the SSB signal, this SSB signal is called a full-carrier SSB.

The theoretical analysis of the present invention is first presented. A full carrier SSB signal can be expressed bywhere m is a modulation index and g(t) is a real information signal extending the frequency range, for instance, of between 300 Hz and 3.0 kHz. g(t) (= H(g(t))) is a Hilbert transform of ^g(t). w_c (= 2πf_c) is the angular frequency of a sinusoidal carrier signal. Equation (1) can be rearranged as,where

When both m and g(t) are constrained such thatS(t) can be expressed only in real simple zeros. Therefore, S(t) can be safely applied to the amplitude limlter.

Following amplitude limiting In the receiver, a frequency detector (frequency discriminator) performs the operation,Then,where the prime symbol implies differentiation.

In a frequency response equalization process, an integrator is used, the output of the same is given as,When the constant can be omitted without the loss of generality, the first term on the right hand side of Eq. (3) corresponds to the information signal. Other terms are the higher order products of g(t), which should be cancelled out by the use of linearizer circuit.

The relation between the integrator and - 6 dB/octave response equalizer will be clarified as follows:

• It is possible to compensate the frequency response with a - 6 dB/octave response equalizer, whose function can be explained mathematically,which can be easily reduced by using the relation between the differentiation of sinusoidal functions and Hilbert transforms, because g(t) is Fourier transformable. Thus, the demodulated signal is equalized as,

The Hilbert transform of v₁(t) can be written as,which corresponds to Eq. (4) except for the coefficient of - 1. Here, the following relations are utilized,

Next, the following equation is useful for explaining how to fabricate a linearizer by using the existing devices, that Is,Substituting Eqs. (3) and (5) into Eq. (6), that equation becomes,which indicates that the second and third order products of g(t) are completely deleted.

The Hilbert transform of u(t) is given as

As the human ears cannot distinguish between g(t) and ^g(t), the Hilbert transformer after the lineariser is not necessary when voice signals are applied. However, when data signals, such as MODEM signals, are applied, the Hilbert transformer is necessary to prevent the generation of distortion. This is because the difference between g (t) and g(t) is apparent for data signals.

Figure 1 shows a block diagram of a system according to the present invention. The system comprises a signal source 1, a local oscillator 2, an SSB modulator 3, an adder 4, an up-converter 5 for converting the frequency of the output of the adder 4 to radio-frequency, an RF (radio frequency) amplifier 6 and a transmitting antenna 7, together forming a transmitter portion 8 of the system.

The system further comprises a receiving antenna 9, a down-converter 10 for converting the frequency from RF to IF_' (intermediate frequency), an IF amplifier 11, an amplitude limiter 12 for removing amplitude variations of a received SSB signal, a frequency detector 13, a frequency-response equaliser 14, a lineariser 15 for cancelling distortions contained in the demodulated signal, an AF (audio frequency) amplifier 16, and an output terminal 17, together forming a receiver portion 18 of the system.

In the transmitter portion 8 of the system, voice signals in a frequency range of, for example 300Hz to 3.0kHz are provided from the signal source 1. The voice signals are fed to the SSB modulator 3, which is also fed with an IF carrier signal produced by a local oscillator 2. Thus, a single-sideband suppressed-carrier signal is generated at the output of the SSB modulator 3 by using, for example, the phasing method.

The output of the SSB modulator 3 is fed to the adder 4, where it is combined with the carrier (the signal from the local oscillator 2). The output of the adder 4 feeds the up-converter 5 which converts the frequency to the RF region. The RF amplifier 6 amplifies the RF signal, which is radiated through the air by the transmitting antenna 7.

In the receiver portion 18 of the system, the signal received by the receiving antenna 9 feeds the down-converter 10, which converts the frequency from RF to IF. Then, in order to compensate for the radio transmission path loss, the IF signal is amplified in the IF amplifier 11 up to the level where the demodulation process can be performed. The output of the IF amplifier 11 feeds the amplitude limiter 12, which removes amplitude variations caused by fading. If the level of the received carrier is lower than that of the SSB signal, the carrier component must be amplified to satisfy the conditions for a full-carrier SSB signal, before the signal is applied to the amplitude limiter 12. The output of the amplitude limiter 12 is applied to the frequency detector 13 to provide demodulated voice signals which, in turn, feed the equaliser 14 which compensates the frequency response in the bandwidth of interest. There are two possible ways of compensating, namely an integrator and a -6 dB/octave response equaliser, which are, for convenience, referrred to herein as 14a and 14b, respectively.. The output of the equaliser 14 is fed to the lineariser 15 in order to cancel out distortion generated in the demodulation process. The output of the lineariser 15 is fed through the AF amplifier 16 to the output terminal 16.

Figure 2 is a block diagram of the lineariser 15, when the frequency detector 13 and the integrator 14a are employed. The lineariser comprises an input terminal 21, a Hilbert transformer 22, a multiplier 23, a delay circuit 24, a subtractor 25, and an output terminal 26.

Equation (3) is useful for explaining how to cancel out the second order product of g(t). The apparatus shown in Figure 2 compensates for the second order product of g(t). The input terminal 21 is connected to the output of the frequency detector 13. The signal at the input terminal 21 which feeds the Hilbert transformer 22 is denoted by (t) m²g (t) ĝ (t) + 0(m³). The output of the Hilbert transformer 22 is multiplied by the signal derived from the input terminal 21, so that the second-order product of - m²g (t) (t) + O (m³) is obtained at the output of the multiplier 23. The signals at the output of the delay circuit 24 and the multiplier 23 are applied to the subtractor 25 to delete the second-order product of g(t). The delay circuit 24 can then be adjusted so that the distortion at the output terminal 26 is minimised. This procedure can be performed, for example, by using a spectrum analyser.

Figure 3 shows another form of the lineariser 15, which comprises an input terminal 31, a Hilbert transformer 32, a substractor 33, an adder 34, a multiplier 35, a delay circuit 36, an adder 37, an output terminal 38 and an attenuator 39 for halving the signal level.

The lineariser depicted in Figure 3 is applicable when the frequency detector 13 is accompanied by a -6 dB/octave response equaliser 14b. The operation of the lineariser is governed by equation (4).

The signal at the input terminal 31, which is the output of the frequency equaliser 14, is applied to the Hilbert transformer 32, the subtractor 33, the adder 34 and the delay circuit 36. The output of the Hilbert transformer 32 is applied to the subtractor 33 and the adder 34. The product signal which is derived from the multiplier 35 is m²(g(t)-(t)) (g(t)+(t))+O(m³). This signal is fed through the attenator 39 to the adder 37 where it is combined with the output of the delay circuit 36. The delay time should be adjusted in order to cancel out the second-order product of g(t). The delay circuit 36 is useful to compensate the total delay time arising from the Hilbert transformer 32, the subtractor 33, the adder 34 and the multiplier 35. Then the signal at the output of the adder 37 is mg(t) + O(m³) which is free from the second-order product of g(t).

Figure 4 shows another form of the lineariser 15, which comprises an input terminal 41, a delay circuit 42, a Hilbert transformer 43, a cuber 44, multipliers 45 and 46, attenuators 47 and 48, a summation circuit 49 and an output terminal 50.

The signal at the input terminal 41, which is the output of the frequency detector 13, is denoted by v_l(t) as formulated in equation (3). The input signal is distributed to the delay circuit 42, the Hilbert transformer 43, the cuber 44 and the multiplier 45. The output of the Hilbert transformer is then ₁(t), which feeds the multipliers 45 and 46. The signals of the output of the multipliers 45 and 46 are v₁(t)₁(t) and v₁(t)v₁²(t), respectively. The output of the multiplier 46 is connected to the attenuator 48, the attenuation factor of which is 1/2. The cuber 44 provides the cube of v_l(t), and this is fed to the attenuator 47 which has an attenuation factor of 1/6. The signals derived from the delay circuit 42, the multiplier 45 and the attenuators 47 and 48 are gathered at the summation circuit 49, the rule of which, as shown in the drawing, is governed by equation (6). The delay time of the delay circuit 42 should be adjusted to cancel out the second and third order products of g(t). Then, the signal at the output terminal is mg(t) + O(m⁴) which indicates that the residual distortion level is very small when m < 1.

Figure 5 shows a modification of the present invention. In order to improve the transmission efficiency of the transmitted signals, it is necessary to reduce the level of the carrier as compared with that of the SSB signals. However, this signal cannot be directly applied to the amplitude limiter, because the signal is not a full-carrier SSB signal. Prior to the amplitude limiter, the selective amplification circuit of the carrier component depicted in Figure 5 is required to regenerate a full-carrier SSB signal from the received signal. The circuit depicted in Figure 5 is inserted between the IF amplifier 11 and the amplitude limiter 12 shown in Figure 1. The circuit comprises an input terminal 51, a delay circuit 52, a bandpass filter 53, an IF amplifier 54, an adder 55 and an output terminal 56.

The signal at the input terminal 51, which is the output of the IF amplifier 11, is fed to the delay circuit 52 and the bandpass filter 53. The carrier component whose frequency is converted to the IF region is extracted by the bandpass filter 53. The output of the bandpass filter 53 is amplified in the IF amplifier 54 up to a sufficiently high level to satisfy the condition of a full-carrier SSB signal. The signal at the output of the IF amplifier 54 is combined with the output of the delay circuit 52 at the adder 55. Then, the signal at the output terminal 56 can be safely introduced to the amplitude limiter 12.

Figure 6 shows a modification of the present invention, where a modulating signal is pre-distorted in the transmitter portion of the system, so that the lineariser circuit depicted in Figures 2, 3 or 4 is not necessary in the receiver portion of the system. When the following relation is satisfied, that is, the signal demodulated by the frequency detector and the equaliser does not produce any distortion. The left hand side of equation (8) is the signal demodulated by the frequency detector and the equaliser, and the right hand side of equation (8) is the Hilbert transform of the original information signal sent by the transmitter. Equation (8) is easily rearranged aswhich is convenient for fabricating a pre-distortion circuit by using the existing devices. When g(t), which is pre-distorted in keeping with the relation of equation (9), is fed to the SSB modulator 3, the detected signal always corresponds to the original information signal even though it is Hilbert-transformed.

The pre-distortion circuit is inserted between the signal source 1 an d the SSB modulator 3. In Figure 6, the circuit comprises an input terminal 61, a Hilbert transformer 62, a tangent function generator 63, a DC (direct current) source 64, an adder 65, a multiplier 66, a Hilbert transformer 67 and an output terminal 68. The signal at the input terminal 61, which is derived from the signal source 1, is fed to the Hilbert transformer 62 and the adder 65. By the use of the DC source 64, the adder 65 produces a signal 1 + mg(t). The output of the Hilbert transformer 62 feeds both the tangent function generator 63 and the Hilbert transformer 67. The product of the output of the adder 65 and the tangent function generator 63 is produced by the multiplier 66. The signal at the output of the multiplier 66 is fed back to the input of the tangent function generator 63. The output of the Hilbert transformer 67 then comprises the pre-distorted signal. This output is fed to the output terminal 68.

The present invention may be combined with some of the conventional communication techniques, such as a syllabic compandor, a diversity reception system and/or a smearing filter, etc.

A syllabic compandor is available to suppress harmful noises such as thermal noise, man-made noise and/or click noise. In the transmitter portion, a compressor is introduced between the signal source 1 and the SSB modulator 3 shown in Figure 1. The compressor (not shown) compresses the signal level which is lower than a reference level. Then, the dynamic range between the highest and the lowest signal level is reduced. The compressed signal is fed to the SSB modulator 3 and is transmitted. In the receiver portion, an expandor (not shown) is introduced between the lineariser 15 and the AF amplifier 16 so that the received compressed signal is expanded. When noise is superimposed on the signal, it is reduced in the expansion process. The SNR (signal-to-noise ratio) is improved as compared with the case where the syllabic compandor is not introduced.

Figure 7 shows a modification of the invention. Diversity systems provide the most promising scheme for elimination of the effects of fading. In Figure 7, the reference numerals 10 to 17 represent the same integers as in Figure 1. The cicuit further comprises receiving antennas 9a and 9b, an antenna switch 71, a switch logic circuit 72 and a level detector 73. The instantaneous envelope of the signal received by the receiving antenna 9a, for example, is monitored by the level detector 73. If it falls below a predetermined threshold, the antenna switch 71 is activated by the switch logic circuit 72, so that it selects the second antenna, 9b. If the signal from the second antenna is above the threshold, switching ceases. If the signal in the second antenna is also subject to fading, we can revert to the first antenna. A signal in which amplitude fading is substantially eliminated is derived from the output of the antenna switch 71.

It should be appreciated that there are many diversity techniques available, for instance space diversity, frequency diversity, time diversity, etc. The diversity technique is also effective when introduced to the transmitter instead of the receiver.

A further modification of the present invention is the use of a smearing filter for improving the SNR of the detected signal. Smearing and desmearing filters (not shown) are placed before the modulator and after the demodulator, respectively. Both of these filters then operate on the information signal, and the filters are usually chosen to be complementary so that, in the ideal case, the net effect upon the information signal is merely a delay.

During severe fading, most of the amplitude information in conventional SSB signalling may be wiped out. In the present system, real zeros play an important role for conveying the information, just as in FM. Zeros of the signal possess great immunity to noise and interference. The above-mentioned pre-distortion circuit, a syllabic compandor, a diversity system, and/or a smearing filter are still useful for combatting noise encountered in a modible radio transmission path.

As described above in detail, the present invention possesses the following particular effects and is particularly applicable to use in mobile radio communication systems.

• (a) As an SSB modulation method is employed, the necessary radio bandwidth is comparable to the information bandwidth.
• (b) As an amplitude limiter is introduced into the receiver section, degradations caused by fading and/or man-made noise encountered in the mobile radio environments are removed.
• (c) As a conventional SSB transmitter and a PM receiver may be used merely by inserting a lineariser prior to the AF amplifier, there is no difficulty in manufacturing the transceiver of the present invention.