GB2207582A

GB2207582A - Phase error correction and carrier recovery

Info

Publication number: GB2207582A
Application number: GB08718062A
Authority: GB
Inventors: Stuart Michael Forsyth; Susan Mary Iversen
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1987-07-30
Filing date: 1987-07-30
Publication date: 1989-02-01
Anticipated expiration: 2007-07-30
Also published as: GB2207582B; GB8718062D0

Abstract

A carrier recovery scheme for a QPSK modulated signal uses a phase detector (33) which processes the in-phase and quadrature components (In, Qn) produced from an incoming signal Sk(t) in accordance with a discrete time algorithm such as to achieve an error signal e(t), for controlling a voltage controlled osciliator (23), which is independent of the incoming signal amplitude and linear over the full tracking period. The algorithm is in the form <IMAGE> The phase detector may be comprised by a PROM look-up table, and may be implemented in hardware or software. The error signal is D to A converted and used to control the phase of a local oscillator 23 of carrier frequency. The error signal is linear in phase over the full tracking range (Fig 3), and is not amplitude sensitive. <IMAGE>

Description

PHASE DETECTION AND CARRIER RECOVERY This invention relates to phase detection and to carrier recovery from a phase shift keying modulated signal, in particular but not exclusively a quadrature (four-level) phase shift keying (QPSK) modulated signal.

Phase modulation is particularly suitable for transmitting digital information. In phase-shift keying systems the phase of the carrier is shifted in order to transmit information. Where the phase of the carrier is shifted between Oo and 1800 the system is referred to as a bi-phase modulation system. Such a modulated signal is a suppressed carrier signal whereby the carrier frequency is suppressed and all of the energy appears in the modulation side bands. In order to demodulate such a bi-phase modulated suppressed-carrier signal it is necessary to provide a coherent demodulator. This requires the reintroduction (recovery) of a carrier which must be locked to the received signal in frequency and phase to permit coherent detection. For a biphase system a Costas phase-locked loop demodulator may be used.This involves a phase detection circuit for a local oscillator, sometimes referred to as an I-Q multiplier.

The local oscillator carrier phase must exactly track the phase of the signal for optimal detection. For QPSK modulated signals a basic Costas demodulator with conventional phase correction circuits is not suitable.

It is, therefore, an object of the present invention to provide a carrier recovery scheme for QPSK and higher order phase-shift keying modulation systems.

According to one aspect of the present invention there is provided a phase detection circuit in which two alternating current signals, which are derived from an incoming "" level (n being an integer) PSK modulated carrier signal and are in phase quadrature, are applied to a phase detector, the output from which is an error signal whose phase is such that it is transparent to the data related phase changes associated with the 2n -level PSK modulation, which error signal drives a local oscillator employed in the derivation of the two alternating current signals such that the local oscillator phase tracks the phase of the carrier exactly.

According to a further aspect of the present invention there is provided a method of recovering a carrier from a received QPSK modulated signal, comprising the steps of applying the received signal to first and second signal paths, each signal path including a mixer circuit followed by a low-pass filter and an amplifier stage, applying the output of each signal path to a common phase detector to generate an error signal, applying the error signal to a voltage-controlled oscillator whereby to control the phase of its output and applying the voltage-controlled oscillator dutput to said mixer circuits via a phase splitter whereby to achieve an in-phase component of the input signal in the signal path and a quadrature component of the input signal in the other signal path, the error signal being independent of the received signal amplitude, serving to maintain the quadrature relationship between the two said components and being transparent to the 900 data related phase changes associated with the QPSK modulation.

According to another aspect of the present invention there is provided a circuit for recovering a carrier from a received QPSK modulated signal, comprising first and second signal paths to which the received signal is applied, each signal path including a mixer circuit followed by a low-pass filter and an amplifier stage; a local oscillator running at the carrier frequency, a phase splitter coupled to the oscillator output and coupled to both said mixer circuits whereby one mixer circuit receives the local oscillator output with a quadrature phase shift with respect to the other mixer circuit; and a phase detector to which the amplifier stage outputs are applied, and whose an output comprises an error signal which drives the local oscillator, the phase detector error signal being independent of the received signal amplitude and being transparent to the 90 data related phase changes associated with the QPSK modulation.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates a conventional carrier recovery scheme for a bi-phase modulated system, Fig. 2 illustrates a carrier recovery scheme for a QPSK modulated system, according to the present invention, and Fig. 3 illustrates the tracking curve for the embodiment of Fig. 2.

A conventional bi-phase recovery system is illustrated in Fig. 1. A received RF signal on line 1 is applied to two signal paths 2 and 3. The output of a voltage controlled oscillator 4 running at the frequency of the received signal 1, whereby to make the IF zero frequency, is impressed on the received signal as applied to a respective mixer (phase detector) 5 or 6, through a 90 phase shift network 7. The mixer outputs are low pass filtered at 8 and 9, respectively, and amplified at 10 and 11, respectively. The resultant signals I and Q are required to be in phase quadrature. In order to ensure that they are always in phase quadrature, the phase of the output of the voltage-controlled oscillator 4 must be controlled appropriately, for which a phase locked loop is provided including a multiplier 12 (loop phase detector) and a phase filter 13 (loop filter).

When the loop phase detector 12 detects a difference from quadrature in the phase relation between I and Q it produces an error signal to adjust the phase of one or other of the voltage controlled oscillator outputs to restore quadrature.

For a carrier recovery system for QPSK the local oscillator phase must track the phase of the transmitting carrier in order to eliminate phase errors in the demodulated signals. This is achieved, as before, by controlling the phase of the voltage controlled oscillator with an error signal (voltage) derived from the received signal. This error signal must be transparent to the 900 data related phase changes associated with QPSK modulation.

Fig. 2 illustrates a carrier recovery system for QPSK modulation. A received radio frequency signal Sk(t) is filtered at 20 and applied to two signal paths 21 and 22. The output of a voltage controlled oscillator (VCO) 23 running at the frequency of the received signal is impressed on the received signal as applied to a respective mixer 24 or 25, through a phase split network 26, so that VCO outputs applied to the mixers are 900 out of phase. The resultant signals I and Q are low pass filtered at 27 or 28, respectively, amplified at 29 or 30 respectively, converted from analogue to digital from at 31 or 33 respectively and applied to a loop phase detector comprised by a PROM look-up table 33.

The loop phase detector 33 is an ARCTAN phase detector which processes the thus generated n-phase (I) and quadrature (Q) components of the received signal according to the following relationship, which was arrived at by combining knowledge of phase local loops, digital modulation and basic geometry in order to determine the error voltage ek(t),

In the absence of noise:: Ik = A cos #k(t) (2) 2 Qk = - A sin k(t) 2 #k(t) = (wo - w1)t + #o(t) - #1(t) + #(k+) 2 k = 0, 1, 2, 3 wO and wl are the angular frequencies associated with the input signal and the locally generated carrier, and #o(t) and #1(t) are their respective phases.

Substitution of (2) and (3) into equation (1) produces:

which may be written as:

The error voltage resulting from the use of the above algorithm (1) can thus be seen to have no amplitude dependence and is perfectly linear over #/4 radians (Fig. 3), which is the optimum tracking curve for carrier recovery from a QPSK modulated signal.

The phase detector 33 may obviously be implemented in hardware, however for implementation simplicity a PROM LUT is employed which stores the solutions of the algorithm (previously programmed into the PROM after being generated in software) for all possible values of 1k and Qk.

The output of the phase detector 33, that is the error voltage, is converted back to analogue form at 34 and applied to VCO 23 via a loop filter 35 as before. As indicated in Fig. 2 the signal applied to the mixers 24 and 25 are cos(w0t + 450) and cos(wot - 450) respectively, giving the instantaneous values IT and QT of A/2 cos(#k(t)+ 45 ) and A/2 cos (#k(t) - 45 ) respectively. The circuitry of Fig. 2 produces digital inphase and quadrature signals, (ik and fqg, for subsequent processing as required.

With the carrier recovery loop implemented as a.

Type 2 tracking loop, the system parameters for the phase detector 33 are KD = 2 volts/radian (KD is the phase detector gain).

##max = 45 #WL = #BN rad/sec (BN is the loop noise bandwidth and sswL is the frequency step acquisition range of the loop) 2 ##max = 3.6BN rad/sec (##/max is the 4 frequency ramp acquisition range).

These ranges are double those of conventional systems. To completely define the performance of a tracking loop requires analysis in the presence of noise. The noise performance of the ARCTAN phase detector is considered to be equivalent, to the first order, to that of a conventional Costas loop, so that the recovery system of the invention is equivalent to that of conventional systems, although a noise degradation generally will result at low SNR due to the inherent amplitude normalisation.

The advantages of the recovery system of the invention over a conventional Costas type or 4th power loop are that it is linear over the full tracking loop range, the error voltage is amplitude independent, i.e.

the recovery process is not dependent on the received signal amplitude, and implementation is simple.

Whereas the invention has been specifically described with respect to QPSK modulation, lower. or higher order modulation may also be treated in a similar manner, that is carrier recovery may be achieved from biphase, 8-phase or 16-phase etc. shift keyed modulated signal whereby less or more information is transmitted in a single interval than the 2 bits of QPSK. Thus for 8-phase and or 16-phase the data rate is increased for the same bandwidth. The invention is thus applicable to 2n - level PSK modulation where n is an integer i.e. n = 1 for biphase, n = 2 for QPSK, n = 3 for 8-phase, etc.

The algorithm (5) will vary depending on the number of phases levels for bi-phase ek(t) = ARCTAN [Q/I] or e(t) = g MOD (7r) 8-phase, e(t) =

for 16-phase e(t) =

The invention is applicable to any received RF PSK signal, including PSK optical signals.

Claims

CLAIMS:

1. A phase detection circuit in which two alternating current signals, which are derived from an incoming '" level (n being an integer) PSK modulated carrier signal and are in phase quadrature, are applied to a phase detector the output from which is an error signal whose phase is such that it is transparent to the data related phase charges associated with the 2 -level PSK modulation, which error signal drives a local oscillator employed in the derivation of the two alternating current signals such that the local oscillator phase tracks the phase of the carrier exactly.

2. A phase detection circuit in which two alternating current signals, which are derived from an incoming QPSK modulated carrier signal and are in phase quadrative, are applied to a phase detector the output f which is an error signal whose instantaneous phase angle is four times that of the incoming QPSK modulated carrier signal such that it is transparent to the 900 phase changes in the incoming signal, phase changes in the incoming signal of less than 450 causing corresponding change in the error signal, which error signal drives a local oscillator employed in the deviation of the two alternating current signals such that said changes are compensated for.

3. A circuit as claimed in claim 1 wherein n = 2 and said input signal is QPSK modulated, or as claimed in claim 2, the said two signals comprising an inphase (I) component and a quadrature (Q) component of the input signal, and wherein the phase detector processes the I and Q components to determine the error signal (e(t)) according to the expression

where A/2 is the amplitude of the said two signals.

4. A circuit as claimed in claim 2 wherein the phase detector is in the form of a PROM look up table.

5. A phase error correction circuit substantially as herein described with reference to and as illustrated in Figs. 2 and 3 of the accompanying drawings.

6. A method of recovering a carrier from a received QPSK modulated signal, comprising the steps of applying the received signal to first and second signal paths, each signal path including a mixer circuit followed by a low-pass filter and an amplifier stage, applying the output of each signal path to a common phase detector to generate an error signal, applying the error signal to a voltage-controlled oscillator whereby to control the phase of its output and applying the voltage-controlled oscillator output to said mixer circuits via a phase splitter whereby to achieve an in-phase component of the input signal in the signal path and a quadrature component of the input signal in the other signal path, the error signal being independent of the received signal amplitude, serving to maintain the quadrature relationship between the two said components and being transparent to the 900 data related phase changes associated with the QPSK modulation.

7. A method as claimed in claim 6 wherein the phase detector processes the in-phase (I) and quadrature (Q) components to determine the error signal e(ti according to the expression

where A/2 is the amplitude of said two signal components.

8. A method as claimed in claim 7, wherein the phase detector is in the form of a PROM look-up table.

9. A method of recovering a carrier signal from a received QPSK modulated signal substantially as herein described with reference to Figs. 2 and 3 of the accompanying drawings.

10. A circuit for recovering a carrier from a received QPSK modulated signal, comprising first and second signal paths to which the received signal is applied, each signal path including a mixer circuit followed by a low-pass filter and an amplifier stage; a local oscillator running at the carrier frequency, a phase splitter coupled to the oscillator output and coupled to both said mixer circuits whereby one mixer circuit receives the local oscillator output with a quadrature phase shift with respect to the other mixer circuit; and a phase detector to which the amplifier stage outputs are applied, and whose output comprises an error signal which drives the local oscillator, the phase detector error signal being independent of the received signal amplitude and being transparent to the 900 data related phase changes associated with the QPSK modulation.

11. A circuit as claimed in claim 10 wherein the amplifier stage outputs comprise an in-phase (I) component and a quadrature (Q) component of the received signal, and wherein the phase detector processes the I and Q components to determine the error signal (e(t)) according to the expression

where A/2 is the amplitude of said two components.

12. A circuit as claimed in claim 11 wherein the phase detector comprises a PROM look-up table.

13. A carrier signal recovery circuit for a QPSK modulated signal substantially as herein described with reference to Figs. 2 and 3 of the accompanying drawings.