WO2003098892A1

WO2003098892A1 - Modified psk constellation to facilitate carrier recovery

Info

Publication number: WO2003098892A1
Application number: PCT/GB2003/002248
Authority: WO
Inventors: Brian Herbert Beech
Original assignee: Tandberg Television Asa
Priority date: 2002-05-22
Filing date: 2003-05-22
Publication date: 2003-11-27
Also published as: GB0211767D0; EP1506655B1; DE60315287T2; GB2389019B; AU2003227984A1; DE60315287D1; EP1506655A1; ATE369000T1; GB2389019A

Abstract

Communication of a digital signal includes carrier generation and recovery for higher order modulation systems. The digital signal is phase shift key modulated with an asymmetric constellation pattern to produce a phase shift keyed signal and a carrier signal, which are processed to produce a transmission signal with no, or a weak, carrier signal component for transmission. At a receiver, a strong carrier signal is regenerated from the transmission signal for phase locking the receiver.

Description

MODIFIED PSK CONSTELLATION TO FACILITATE CARRIER RECOVERY

This invention relates to communicating a digital signal. In particular, the invention relates to carrier signal generation and recovery for higher order modulation systems.

It is a requirement for any modulation scheme that, as the received signal to noise ratio (SNR) is reduced, the carrier synchronisation should not fail before the forward error correction (FEC) decoding fails. In a phase shift keyed (PSK) modulation scheme, failure of carrier synchronisation or lock occurs as a result of a low SNR in combination with the presence of phase noise on the received signal. Loss of carrier synchronisation generally manifests itself as cycle slips which cause burst errors and these result in failure of the Reed-Solomon code and sometimes complete synchronisation failure of the system. Operation at low code rates, i.e. at high redundancy where the number of transmitted bits is much higher than the number of information bits, puts a requirement on the carrier recovery loop to operate with low values of signal to noise ratio (SNR). The use of new codes, for example turbo codes, imposes the requirement to operate at even lower values of SNR.

Many users are considering upgrading from quadrature phase shift keyed (QPSK) to 8-state phase shift keyed (8PSK) modulation and there is interest in the use of other higher order modulations, for example 16- state quadrature amplitude modulation (16QAM). Unfortunately the practical experience of 8PSK has shown that relatively low levels of phase noise can cause carrier recovery failure. Operation at low symbol rates, i.e. low rates at which symbols are transmitted, further increases the sensitivity to phase noise to the extent that operation on some satellite links may not be possible. For example operation of turbo coded 8PSK, rate 2/3 (i.e. three transmitted bits for every two information bits) at 5MSym/s requires phase noise levels better than can be easily achieved on typical satellite links and better than the Intelsat Earth Station Standard (IESS) 308. In order to guarantee reliable operation of such systems, a high level of performance is required of all elements in a transmission chain. The use of phase locked low noise block down- converters (LNBs) may be required to reduce phase noise. The disadvantage of this type of LNB is a higher cost compared with non- phase locked down-converters typically used for domestic satellite receivers.

Contrary to expectation, 16QAM, although sensitive to phase noise, is generally more tolerant of phase noise than 8PSK (or higher order PSK).

It is an object of the present invention at least to mitigate the aforesaid disadvantages in the prior art.

Therefore, it is an object of the invention is to provide a method of carrier recovery for PSK signals which allows operation with substantially higher levels of phase noise than is normally possible. Although the invention is illustrated in relation to 8PSK, the invention can be applied to any PSK modulation. The same principles could be applied to other modulations e.g. 16QAM, but with possible degradation of digital performance. The invention has applications for professional links particularly for operation of satellite transponders in multi-carrier mode, since in these cases the symbol rate may be low. This is because in single carrier mode a satellite transponder is normally filled with one carrier operating at a high symbol rate, whereas in multi-carrier mode the transponder is filled with multiple carriers and each one operates at a lower symbol rate. The symbol rate of each carrier is reduced as the number of carriers increases. The invention has particular application for consumer applications where it is desirable to have a high level of phase noise tolerance in order to allow the use of low cost technology for down-converter local oscillators. This may be particularly important for KA band satellite links where low levels of phase noise are difficult to achieve.

A further object of this invention is to provide a modification to a PSK signal for which the required S/N ratio for system failure is lower that that for unmodified PSK. Another object of this invention is to produce a carrier recovery circuit for which there are no phase ambiguities, that is having a phase locked loop with only one stable lock position. This is advantageous for subsequent signal processing, for example for FEC decoding, since there is no requirement to resolve phase ambiguities. This can lead to faster system lock up times which may be required in some applications.

Conventional n-level PSK (NPSK) carrier recovery algorithms have well- known false lock positions at multiples of 1/N * symbol rate. This invention provides a carrier recovery means which will not false lock allows faster carrier lock because the algorithms normally required to escape from false locks are not required.

According to a first aspect of the invention the invention there is provided a system for communicating a digital signal, the system comprising: a) phase shift key modulating means for modulating the digital signal with an asymmetric constellation pattern to produce a phase shift keyed signal and a carrier signal; b) signal processing means to process the phase shift keyed signal and carrier signal to produce a transmission signal with no, or a weak, carrier signal component; c) transmission means for transmitting the transmission signal to a receiver; d) receiving means at the receiver for receiving the phase shift keyed signal and regenerating a strong carrier signal from the transmission signal; and demodulation means for using the strong carrier signal for phase locking the receiver to demodulate the phase shift keyed signal.

Preferably, the asymmetric constellation pattern has at least one constellation point of reduced amplitude.

Advantageously, the at least one constellation point of reduced amplitude is zeroed.

Conveniently, the signal processing means is adapted for periodically changing the mapping of the asymmetric constellation to reduce the carrier signal component of the transmission signal. Advantageously, the signal processing means is adapted for changing the mapping of the asymmetric constellation for each symbol to be transmitted.

Preferably, the signal processing system is adapted for changing the mapping according to a pseudo-random binary sequence.

Preferably, changing of the mapping is achieved by phase rotation.

Advantageously, the signal processing means is adapted for the insertion of unmodified symbols among symbols modified by use of the asymmetric constellation pattern.

Conveniently, the signal processing means is adapted to allow variation of the ratio of modified symbols to unmodified symbols to optimize performance of the system.

Advantageously, the receiving means is synchronizable with the transmitting means by the transmission from the transmitting means to the receiving means of a predetermined sequence of modified and unmodified symbols.

Conveniently, the signal processing means comprises: a first mapping block connected to an input port for producing unmodified PSK symbols and a second mapping block connected to the input port for producing PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; selector means connected to the first and second mapping blocks for selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means, phase rotator means connected to the selector means for receiving the selected symbols from the selector to rotate the constellation of the selected symbol under the control of a pseudo-random bit sequence generator connected to the phase rotator and controlled by the synchronization means to output to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

Alternatively, the signal processing means comprises: a programmable mapping block connected to an input port for selectively producing unmodified PSK symbols or PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; under the control of parameter-controlled synchronizing means, phase rotator means connected to the programmable mapping block for receiving symbols from the programmable mapping block to rotate the constellation of the symbols under the control of a pseudo-random bit sequence generator connected to the phase rotator and controlled by the synchronization means to output to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

Preferably, a parameter controlling the parameter-controlled synchronizing means is a predetermined ratio of modified symbols to unmodified symbols.

Preferably, the receiving means comprises de-rotator means for receiving a complex signal and for implementing rotations which are multiples of π/4 for inputting to carrier recovery loop means.

Advantageously, the de-rotator means is under the control of pseudo random bit sequence generator means, controlled by synchronization detector means controlled by magnitude detector means for detecting the magnitude of received symbols to detect a predetermined sequence of modified and unmodified symbols.

Conveniently, the carrier recovery loop comprises a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a first selector under the control of the synchronization detector for selectively routing the signal to one of a first de-mapping block for de-mapping unmodified symbols and a second de-mapping block for de-mapping modified symbols and a second selector under control of the synchronization detector for selectively receiving output from the first and second de-mapping blocks and outputting a received symbol to an output port.

Alternatively, the carrier recovery loop comprises a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a programmable de-mapping block under the control of the synchronization detector for selectively de-mapping unmodified symbols or de-mapping modified symbols and outputting a received symbol to an output port.

Advantageously, the pattern noise canceller comprises a phase detector controlling a phase shifter, wherein the output from the phase detector is fed forward to the phase shifter to cancel pattern noise.

According to a second aspect of the invention there is provided a transmitter for generating a phase shift keyed (PSK) signal and carrier signal to transmit a transmission signal with no, or a weak, carrier signal component, the transmitter comprising: a first mapping block connected to an input port for producing unmodified PSK symbols and a second mapping block connected to the input port for producing PSK symbols modified by reducing the amplitude of at least one constellation state of a PSK constellation; selector means connected to the first and second mapping blocks for selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means, phase rotator means connected to the selector means for receiving the selected symbols from the selector to rotate the constellation of the selected symbol under the control of a pseudo-random bit sequence generator connected to the phase rotator and controlled by the synchronization means to output to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

Alternatively, the first mapping block, the second mapping block and the selector are replaced by a programmable mapping block for selectively producing unmodified PSK symbols or PSK symbols modified by zeroing of at least one constellation state of a PSK constellation under the control of parameter-controlled synchronizing means.

According to a third aspect of the invention, there is provided a receiver for carrier recovery from a transmission signal with no, or a weak, carrier signal component, the receiver comprising a de-rotator means for receiving a complex signal and for implementing rotations which are multiples of π/4 for inputting to a carrier recovery loop means, wherein the de-rotator means is under the control of pseudo random bit sequence generator means, controlled by a synchronization detector means controlled by magnitude detector means for detecting magnitudes of received symbols to detect a predetermined sequence of modified and unmodified symbols.

Advantageously, the carrier recovery loop comprises a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a first selector under the control of the synchronization detector for selectively routing the signal to one of a first de-mapping block for de-mapping unmodified symbols and a second de-mapping block for de-mapping modified symbols and a second selector under control of the synchronization detector for selectively receiving output from the first and second de-mapping blocks and outputting a received symbol to an output port.

Alternatively, the first selector, the first de-mapping block, the second de- mapping block and the second selector are replaced by a programmable de-mapping block for selectively de-mapping unmodified symbols or de- mapping modified signals under the control of the synchronization detector.

Conveniently, the pattern noise canceller comprises a phase detector controlling a phase shifter, wherein the output from the phase detector is fed forward to the phase shifter to cancel pattern noise.

According to a fourth aspect of the invention, there is provided a method for communicating a digital signal, the method comprising the steps of: a) phase shift key modulating the digital signal with an asymmetric constellation pattern to produce a phase shift keyed signal and a carrier signal; b) processing the phase shift keyed signal and carrier signal to produce a transmission signal with no, or a weak, carrier signal component; c) transmitting the transmission signal to a receiver; d) receiving at the receiver the phase shift keyed signal and regenerating a strong carrier signal from the transmission signal; and e) using the strong carrier signal for phase locking the receiver to demodulate the phase shift keyed signal.

Advantageously, the at least one constellation point of reduced amplitude has zero amplitude.

Conveniently, the step of processing the phase shift keyed signal and carrier signal comprises periodically changing mapping of the asymmetric constellation to reduce the carrier signal component of the transmission signal.

Advantageously, the step of processing the phase shift keyed signal and carrier signal comprises changing the mapping of the asymmetric constellation for each symbol transmitted.

Conveniently, the step of processing the phase shift keyed signal and carrier signal comprises changing the mapping according to a pseudorandom binary sequence.

Preferably, the step of processing the phase shift keyed signal and carrier signal comprises changing the mapping by phase rotation.

Advantageously, the step of processing the phase shift keyed signal and carrier signal comprises inserting for transmission unmodified symbols among symbols modified by use of the asymmetric constellation pattern.

Advantageously, the step of processing the phase shift keyed signal and carrier signal comprises varying a ratio of modified symbols to unmodified symbols to optimize performance of the method.

Conveniently, the step of receiving comprises the step of synchronising the receiving means with the transmitting means by the transmission from the transmitting means to the receiving means of a predetermined sequence of modified and unmodified symbols. Conveniently, the step of phase shift key modulating the digital signal with an asymmetric constellation pattern comprises: using a first mapping block connected to an input port for producing unmodified PSK symbols and using a second mapping block connected to the input port for producing PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; and the step of processing the phase shift keyed signal and carrier signal comprises using selector means connected to the first and second mapping blocks for selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means, using phase rotator means connected to the selector means for receiving the selected symbols from the selector to rotate the constellation of the selected symbol under the control of a pseudo-random bit sequence generator connected to the phase rotator means and controlled by the synchronization means and outputting to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

Alternatively, the step of phase shift key modulating the digital signal with an asymmetric constellation pattern comprises: using a programmable mapping block, under the control of parameter-controlled synchronizing means, connected to an input port for selectively producing unmodified PSK symbols or for producing PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; and the step of processing the phase shift keyed signal and carrier signal comprises selectively receiving unmodified or modified symbols at a phase rotator means connected to programmable mapping block, rotating the constellation of the selected symbol under the control of a pseudo-random bit sequence generator connected to the phase rotator means and controlled by the synchronization means and outputting to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

Preferably, the step of selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means comprises using a predetermined ratio of modified symbols to unmodified symbols as a parameter for controlling the parameter-controlled synchronizing means.

Advantageously, the step of receiving comprises using de-rotator means for receiving a complex signal and for implementing rotations which are multiples of π/4 for inputting to a carrier recovery loop means.

Conveniently, the step of using de-rotator means comprises using de- rotator means under the control of pseudo random bit sequence generator means, controlled by a synchronization detector means controlled by magnitude detector means for detecting the magnitude of received symbols to detect a predetermined sequence of modified and unmodified symbols.

Conveniently, the step of inputting to the carrier recovery loop means comprises inputting to a carrier loop recovery means comprising a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a first selector under the control of the synchronization detector means for selectively routing the signal to one of a first de-mapping block for de-mapping unmodified symbols and a second de-mapping block for de-mapping modified symbols and a second selector under control of the synchronization detector means for selectively receiving output from the first and second de-mapping blocks and outputting a received symbol to an output port.

Alternatively, the first de-mapping block, the second de-mapping block and the second selector may be replaced by a programmable de-mapping block for selectively de-mapping unmodified symbols or modified symbols under control of the synchronization detector means.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a constellation of allowed states for a known 8PSK signal as a plot of imaginary parts of the signal as ordinates against real parts as abscissa;

Figure 2a is a block diagram of a known carrier recovery loop for use with the constellation of allowed states shown in Figure 1;

Figure 2b is a plot of phase comparator output as ordinates against phase shift in degrees as abscissa for the phase comparator used in the known carrier recovery loop of Figure 2a;

Figure 3 shows a constellation of allowed states according to the invention as a plot of imaginary parts of a signal as ordinates against real parts as abscissa, in which point 1 has been remapped to point 9;

Figure 4 shows a block diagram of a transmitter according to the invention;

Figure 5 is a block diagram of a receiver according to the invention;

Figure 6 shows a constellation of allowed states at an output of the de- rotator of the receiver of Figure 5;

Figure 7 is a plot of bit error rate (BER) as ordinates against signal to noise ratio (SNR) in dB as abscissa for a conventional 8PSK signal and for 8PSK signals modified according to the invention;

Figure 8 is a plot of simulated results using the invention, showing no c slliitpCs;" a annrdl

Figure 9 is a plot of simulated results for conventional 8PSK carrier recovery, showing cycle slips.

In the figures, like reference numerals represent like parts.

The invention is described herein in relation to 8PSK signals, but it will be understood by one skilled in the art that the invention can be applied to other orders of PSK. A constellation, i.e. a plot of allowed phase shift values or states 10 in terms of real and imaginary parts of the signal, for a known 8PSK signal, is shown in Figure 1. A block diagram of a digital implementation of a carrier recovery loop according to well-known prior art, for use with the constellation of Figure 1, is shown in Figure 2a. Alternatively, analogue implementations may be used. It will be understood that Figure 1 indicates Cartesian representation of complex signals with I and Q imaginary and real vectors which are shown as inputs I_ln and Q_ln to the carrier recovery loop of Figure 2a. Other representations, such as polar coordinates, may alternatively be used. Referring to Figure 2a, a complex multiplier 26 has an imaginary vector input 261, a real vector input 262, an imaginary vector output 263 and a real vector output 264. The imaginary vector output 263, as well as being connected to an imaginary vector output port 271 of the carrier loop is also connected to an imaginary vector input 211 of a phase comparator 21 and the real vector output 264, as well as being connected to an real vector output port 272 of the carrier loop is also connected to an imaginary vector input 212 of the phase comparator 21. An output 213 of the phase comparator 21 is connected to an input of a loop amplifier 22, an output of which is connected to a numerically controlled oscillator (NCO) 23. An output of the numerically controlled oscillator is input to sine and cosine look-up tables 25, 24 which are connected to secondary inputs 265, 266 of the complex multiplier 26.

In use, the phase comparator 21 generates an output which is a measure of the phase of the incoming signal Iin Qi_n- An output of phase comparator

21 is fed into the loop amplifier 22, which typically generates an output signal comprising a sum of a proportional term and an integral term relating to well-known second order phase lock loop equations. This form of loop amplifier results in a second order feedback loop, but feedback loops with different orders may also be used. The output of loop amplifier

22 is fed to the numerically controlled oscillator 23, an output of which feeds the sine and cosine look-up tables 25 and 24. The NCO outputs a sawtooth waveform and the sine and cosine look-up tables convert the sawtooth wave into sine and cosine waves. This output of the look up tables feeds the complex multiplier 26 to complete the feedback loop.

The phase comparator 21 typically has a characteristic which is cyclic in 45-degrees. An example of such a characteristic is shown in Fig 2b. The loop can lock in one of eight phase positions. A cycle slip occurs when the loop temporarily unlocks and re-locks in a new phase position. A main factor influencing cycle slips is the phase comparator. For low values of SNR the phase comparator characteristic will change - the gain will reduce and the characteristic will change shape. The phase comparator characteristic changes for low SNR because the thermal noise causes the input signal vector randomly to fall in the wrong 45 degree phase segment. When the phase comparator output is filtered the effect is to generate a response which no longer conforms to the classic response shown in Figure 2b. Thus, when the thermal phase noise level becomes large in relation to the 45-degree range of the phase comparator, then anomalies occur. These anomalies will induce a cycle slip if a sufficiently high level of phase noise is present. This is a fundamental disadvantage with the simple approach of the prior art shown.

A modified constellation, having allowed states 2-9, for use with the invention, is shown in Figure 3. This constellation has two special features. Firstly, the average power of the signal is reduced to 7/8 of the power of an unmodified signal, i.e. by 0.58dB, because the constellation point normally in position 1 has been removed from the outside circle and has been re-mapped to the centre of the circle at position 9, that is the amplitude of the signal at position 9 is zero. Alternatively, the amplitude of constellation point 1 may be reduced to a non-zero value.

Secondly, due to the asymmetry of the constellation, there is a carrier component which is approximately 9.6dB lower than the average power of the signal. This is because the power of the carrier component is equal to the power of one point of the 8PSK constellation, so the power of the carrier component is 10* log (1/8) = -9.03 dB. Therefore the power of the carrier component relative to the power of the 9PSK signal is -9.03 -0.58 = -9.61dB. According to an aspect of the invention, a carrier recovery circuit is provided in a receiver which will lock to this carrier component, as described below.

On real satellite links it is not permissible for a signal to include a carrier component at this high level. This potentially would cause interference with other services and could cause difficulties in some receiver designs where the base-band signal is AC coupled. Different satellite operators set different requirements but typically the carrier component is required to be -30dB to -40 dB compared with the total signal strength and this range of values is to be understood by a weak carrier signal component herein. In order to overcome this difficulty a further modification of the signal is applied. The constellation mapping is changed on a symbol by symbol basis. Referring to Figure 3, nine possible combinations of constellation point constellations are possible by setting none or one of the points to the zero position 9:

a) 12345678

b) 2345678 9

c) 3456781 9

d) 4567812 9

e) 5678123 9

f) 6781234 9

g) 7812345 9

h) 8123456 9

i) 1234567 9

It will be noted that constellation point positions c) - i) can be generated by using mapping b) and applying a phase rotation to the signal. Constellation a) is the standard, unmodified 8PSK constellation.

In addition to random selection of the mappings b) to i), mapping a) is inserted at intervals. The ratio of the total number of constellations to the number of modified constellations b) to i) is referred to herein as M. For M=l, every symbol is of the modified type. For M=3 there are two standard symbols for every modified symbol. The parameter M allows a degree of flexibility to optimise system performance.

The embodiment of the invention described uses a phase rotation method. In order to prevent the generation of discrete frequency components in the transmitted signal, the rotation is changed according to a defined pseudo-random sequence at the transmitter. At a receiver, a corresponding pseudo-random sequence is used to de-rotate the signal to restore a constellation as shown in Figure 3. It should be noted that the signal which is transmitted over a transmission channel therefore has either no or only a very low level of carrier component which will not interfere with other services. However, a received signal after the de- rotation process has a strong carrier component which is used for carrier recovery.

The de-rotator in the receiver is synchronised to the rotator in the transmitter. Timing lock in the receiver can be maintained without carrier lock in a known manner. For this reason, the system is robust. If the carrier loop is not locked or suffers a cycle slip, then the de-rotating sequence is not corrupted.

In an embodiment of the invention, synchronisation is achieved by transmitting a known sequence of symbol types, i.e. a sequence of modified and unmodified symbols, as a synchronisation word. In the receiver, a magnitude detector monitors the magnitude of the received signal on a symbol by symbol basis. In one embodiment of the invention the output of the magnitude detector is processed by an averaging function in order to reduce the random influences of the channel data and of the transmission channel thermal noise. The output of the averaging function is compared against a threshold using a comparator. Considering that the average magnitude of a modified symbol is 7/8 times the magnitude of an unmodified symbol, then the output of the comparator can be made to be an indication of symbol type, i.e. whether modified or unmodified. This signal is compared against the known transmitted symbol sequence and when a match is found, the starting point of the pseudo-random binary sequence (PRBS) is reset.

Reliable information as to the type of each individual symbol is obtained from the state of the PRBS generator in combination with the knowledge of the transmitted symbol sequence. This information is used for de- mapping the signal.

Figs 4 and 5 illustrate a transmitter and receiver respectively for use in the invention.

Referring to Figure 4, an input port 40 is connected in parallel to inputs of a first mapping block 41 and a second mapping block 42 respectively. Outputs of the first and second mapping block 41, 42 are connected to respective inputs 431, 432 of a selector 43. An output of a synchroniser 44 is also connected to a synchronising port 433 of the selector. An output 434 of the selector 43 is connected to an input of a phase rotator 46 having an output to an I/Q output port 47. The output of the synchroniser 44 is also connected to an input of a pseudo-random binary sequence (PRBS) generator 45, an output of which is connected to a second input of the phase rotator 46.

In use, three bit data received at the input port 40 of the transmitter is passed to the mapping blocks 41,42 which correspond to mappings a and b, i.e. corresponding respectively to unmodified and modified constellations and symbols. The selector 43 selects which mapping is used, under the control of the synchronisation block 44. A parameter , which determines the ratio of unmodified symbols to modified symbols, is input to the synchronisation block 44. This synchronisation block 44 also controls synchronisation of the PRBS generator 45. An output of PRBS generator 45 has 8 states which control phase rotation by the phase rotator 46.

It will be understood that in the transmitter, the first and second mapping blocks may be replaced by a programmable mapping block, under the control of the synchronisation block 44, which selectively maps modified or unmodified symbols, thereby dispensing with the requirement for the selector 43.

Consider now the receiver block diagram Figure 5. An input port 50 is connected in parallel to inputs of a de-rotator 51, a magnitude detector 52 and a timing recovery block 53. An output of the magnitude detector 52 is connected to a first input of a synchronisation detector 54 and an output of the timing recovery block 53 is connected in parallel to a second input of the synchronisation detector 54 and a first input of a pseudo-random bit sequence generator (PRBS) 55. A first output of the synchronisation detector 54 is connected to a second input of the PRBS generator 55.

An output of the de-rotator 51 is connected to a first input of a complex multiplier 56. An output of the complex multiplier 56 is connected in parallel to inputs of a first phase detector 57, a phase shifter 58 and a second phase detector 59. An output of the second phase detector 59 is connected to an input of a loop amplifier 60 an output of which is connected to a numerically controlled oscillator (NCO) 61 having sine and cosine look-up tables. Output from the NCO is fed back to a second input of the complex multiplier.

An output of the first phase detector 57 is connected to a second input of the phase shifter 58. An output of the phase shifter 58 is connected to a first selector 62, outputs from which are connected to a first de-mapping block 63 and a second de-mapping block 64 respectively. Outputs of the first and second de-mapping blocks are connected to respective inputs of a second selector 65. An output of the second selector 65 is connected to an output port 66 of the receiver. A second output of the synchronisation detector 54 is connected in parallel to synchroniser input ports of the first and second selectors 62, 65 respectively.

In use, an input signal to the receiver is a complex signal which could be either in Cartesian or polar representation. This signal is first filtered by a receiver root-cosine Nyquist filter, not shown. The input signal is passed through the de-rotator block 51 which implements rotations which are multiples of π/4. The rotation is controlled from the PRBS generator 55. Synchronisation with the PRBS generator 55 in the transmitter is achieved by the magnitude detector 52 in combination with synchronisation detector 54.

The ratio M of unmodified to modified symbols received may be pre-set or may be determined by the receiver by measuring the proportion of modified to unmodified symbols received.

If the ratio of total symbols to modified symbols (M) is not infinite (i.e. if there are some modified symbols), then the constellation at the output of the de-rotator 51 appears as shown in Figure 6. The output of the de- rotator 51 feeds into the carrier recovery loop consisting of complex multiplier 56, phase detector 59, loop amplifier 60, and NCO with sine and cosine look-up tables 61. The phase detector 59 is cyclic in 360 degrees and operates on the carrier component of the signal.

In order to minimise generation of pattern noise by this loop a pattern noise canceller is included comprising the first phase detector 57 and the phase shifter 58. The first phase detector 57 is a conventional 8PSK phase detector cyclic in 45-degrees. Other types of phase detector may alternatively be used. The output of the first phase detector 57 is fed forward to the phase shifter 58 to cancel the pattern noise. Since the first phase detector gain will change with S/N ratio, the first phase detector has a programmable gain controlled from the received S/N ratio of the signal. The S/N ratio information is derived from a demodulator section of the receiver (not shown).

Output of the phase shifter 58 is fed to the first selector 62 which routes the signal to the de-mappers 63, 64. The outputs of the de-mappers are fed to the second selector 65, which provides the output to the decoder. For turbo code operation, the de-mappers 63, 64 will generate soft decision data.

It will be understood that in the receiver, the first and second de-mapping blocks 63,64 may be replaced by a programmable de-mapping block which selectively de-maps modified or unmodified symbols, under control of the synchronization detector 54 thereby dispensing with the requirement for first and second selectors 62,65.

Figure 7 shows curves of un-coded bit error rate (BER) plotted against signal to noise ratio (SNR) measured in relation to peak power. This relates to the case of single carrier transmission. Curves are shown for M=infinity (conventional 8PSK case) 71, M=l (all symbols are modified) 72 and M=3 73. For conventional 8PSK, the S/N failure points for turbo decoding are approximately:

Rate 2/3 7.5dB

Rate 5/6 lO.OdB

Rate 8/9 ll.OdB

Referring to Fig 7, this means that at rate 2/3, i.e. 7.5dB, for M=3 the invention incurs a performance penalty of 0.2dB, as indicated by double headed arrow 74 in Figure 7, which is being exchanged for the improved phase noise performance. For code rates 5/6, i.e. lOdB, and 8/9, i.e. lldB, there is no significant penalty since the plots for conventional carrier recovery and for M=3 virtually overlap at these values.

For satellite transmission in multi-carrier per transponder mode, M=l is recommended. In this case the system is average power limited and the SNR figures of Fig 7 for the M=l case should be reduced by 0.58dB to allow for the reduced power in the modified constellation. Taking this into account, there is no significant penalty for the rate 2/3 case. For rate 5/6 there is a performance improvement compared to conventional 8PSK of around 0.4dB. For rate 8/9 there is a performance improvement of around 0.55dB. So in these cases for multi-carrier transmission, not only is carrier recovery performance improved, but the system thermal noise failure point is also improved.

Figs 8, 9, which are plotted for every hundredth symbol, show simulated results for cycle slip using a carrier recovery loop according to the invention and using prior art respectively. The conditions of the simulation are as follows. 1) Symbol rate = 5 MSym/s.

2) SNR = 7.5dB.

3) Phase noise = -74dBc/Hz at lOKHz offset. Phase noise falling at the rate of 20dB per decade.

4) M=3

Fig 8 shows that over the period of the simulation (200 ms of real time), no cycle slips occurred using the invention. Fig 9 shows that using the prior art, large numbers of cycle slips occur. The cycle slips appear as a step change in phase (vertical axis). The smallest steps seen here correspond to a cycle slip of 45 degrees. Close examination shows that there are some phase slips which are multiples of the 45 degree slip. Statistically 45 degree slips occur most frequently. The larger the phase slip the less frequently it occurs. Thus the cycle slips are not always 45- degrees (0.78 radians) but are multiples of this. Additionally the prior art case of Figure 9 shows loss of lock events. Loss of lock events are indicated by bursts where the phase cycles through the full phase range. In Figure 9, these bursts can be seen to occur with varying durations. A simulation of ten seconds of real time using the invention shows no cycle slips or loss of lock events.

The results show that a communications system using the invention will accept phase noise levels in the region of 10 to 20dB higher than a prior art system before cycle slip occurs. Simulations have been done with phase noise falling off at 20 dB per decade. These simulations show that the cycle slip performance at a symbol rate of 5Msymbols/sec is better than the prior art operating at a symbol rate of 30 Msymbols/sec. The theoretical relation between symbol rate (Rs) and phase noise for this case is 20* log Rs. We have 20* log (30/6) =15.6dB. In practice phase noise will not always fall at 20dB per decade and so this is only an approximate assessment. Therefore the figure 10-20dB is an approximation to cover the range of practical cases. When cycle slip does occur, the slip is 360 degrees and the FEC decoder can immediately continue to decode correctly without re-synchronising. Furthermore the carrier lock provided by the invention does not have false lock positions. The system of the invention has only one stable lock position. Each time a signal is applied to the carrier recovery loop it will lock the signal in the same phase. For the prior art the loop will lock in one of 8 possible phases, and only one of these will result in correct decoding of the signal. As a separate issue, the invention will always lock to the centre frequency of the transmitted signal. In the prior art a system may lock at the centre frequency but also has false locks falling at multiples of symbol rate/8 from centre,

Claims

1. A system for communicating a digital signal, the system comprising:

a) phase shift key modulating means for modulating the digital signal with an asymmetric constellation pattern to produce a phase shift keyed signal and a carrier signal;

b) signal processing means to process the phase shift keyed signal and carrier signal to produce a transmission signal with no, or a weak, carrier signal component;

c) transmission means for transmitting the transmission signal to a receiver;

d) receiving means at the receiver for receiving the phase shift keyed signal and regenerating a strong carrier signal from the transmission signal; and

e) demodulation means for using the strong carrier signal for phase locking the receiver to demodulate the phase shift keyed signal.

2. A system as claimed in claim 1, wherein the asymmetric constellation pattern has at least one constellation point of reduced amplitude.

3. A system as claimed in claim 2, wherein the at least one constellation point of reduced amplitude is zeroed.

4. A system as claimed in claims 1 to 3, wherein the signal processing means is adapted for periodically changing mapping of the asymmetric constellation to reduce the carrier signal component of the transmission signal.

5. A system as claimed in claim 4, wherein the signal processing means is adapted for changing the mapping of the asymmetric constellation for each symbol transmitted.

6. A system as claimed in claims 4 or 5, wherein the signal processing system is adapted for changing the mapping according to a pseudorandom binary sequence.

7. A system as claimed in any of claims 4 to 6, wherein the signal processing means is adapted for changing of the mapping by phase rotation.

8. A system as claimed in any of the preceding claims, wherein the signal processing means is adapted for the insertion for transmission of unmodified symbols among symbols modified by use of the asymmetric constellation pattern.

9. A system as claimed in claim 8, wherein the signal processing means is adapted to allow variation of the ratio of modified symbols to unmodified symbols to optimize performance of the system.

10. A system as claimed in any of the preceding claims, wherein the receiving means is synchronizable with the transmitting means by the transmission from the transmitting means to the receiving means of a predetermined sequence of modified and unmodified symbols.

11. A system as claimed in any of the preceding claims, wherein the signal processing means comprises: a first mapping block connected to an input port for producing unmodified PSK symbols and a second mapping block connected to the input port for producing PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; selector means connected to the first and second mapping blocks for selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means, phase rotator means connected to the selector means for receiving selected symbols from the selector to rotate the constellation of the selected symbol under the control of a pseudo-random bit sequence generator connected to the phase rotator and controlled by the synchronization means to output to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

12. A system as claimed in any of claims 1 to 10, wherein, the signal processing means comprises: a programmable mapping block connected to an input port for selectively producing unmodified PSK symbols or PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; under the control of parameter-controlled synchronizing means, phase rotator means connected to the programmable mapping block for receiving symbols from the programmable mapping block to rotate the constellation of the symbols under the control of a pseudo-random bit sequence generator connected to the phase rotator and controlled by the synchronization means to output to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

13. A system as claimed in claims 11 or 12, wherein a parameter controlling the parameter-controlled synchronizing means is a predetermined ratio of modified symbols to unmodified symbols.

14. A system as claimed in any of the preceding claims, wherein the receiving means comprises de-rotator means for receiving a complex signal and for implementing rotations which are multiples of π/4 for inputting to carrier recovery loop means.

15. A system as claimed in claim 14, wherein the de-rotator means is under the control of pseudo random bit sequence generator means, controlled by synchronization detector means controlled by magnitude detector means for detecting the magnitude of received symbols to detect a predetermined sequence of modified and unmodified symbols.

16. A system as claimed in claims 14 or 15, wherein the carrier recovery loop means comprises a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a first selector under the control of the synchronization detector means for selectively routing the signal to one of a first de-mapping block for de-mapping unmodified symbols and a second de-mapping block for de-mapping modified symbols and a second selector under control of the synchronization detector means for selectively receiving output from the first and second de-mapping blocks and outputting a received symbol to an output port.

17. A system as claimed in claims 14 or 15, wherein, the carrier recovery loop comprises a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a programmable de- mapping block under the control of the synchronization detector for selectively de-mapping unmodified symbols or de-mapping modified symbols and outputting the de-mapped symbol to an output port.

18. A system as claimed in claims 16 or 17 wherein the pattern noise canceller comprises a phase detector controlling a phase shifter, and the output from the phase detector is fed forward to the phase shifter to cancel pattern noise.

19. A transmitter for generating a phase shift keyed (PSK) signal and carrier signal to transmit a transmission signal with no, or a weak, carrier signal component, the transmitter comprising : a first mapping block connected to an input port for producing unmodified PSK symbols and a second mapping block connected to the input port for producing PSK symbols modified by reducing the amplitude of at least one constellation state of the PSK constellation; selector means connected to the first and second mapping blocks for selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means, phase rotator means connected to the selector means for receiving the selected symbols from the selector to rotate the constellation of the selected symbol under the control of a pseudo-random bit sequence generator connected to the phase rotator and controlled by the synchronization means to output to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

20. A transmitter as claimed in claim 19, wherein the first mapping block, the second mapping block and the selector are replaced by a programmable mapping block for selectively producing unmodified PSK symbols or PSK symbols modified by zeroing of at least one constellation state of a PSK constellation under the control of the parameter-controlled synchronizing means.

21. A transmitter as claimed in claims 19 or 20, wherein a parameter controlling the parameter-controlled synchronizing means is a predetermined ratio of modified symbols to unmodified symbols.

22. A receiver for carrier recovery from a transmission signal with no, or a weak, carrier signal component, the receiver comprising a de- rotator means for receiving a complex signal and for implementing rotations which are multiples of π/4 for inputting to a carrier recovery loop means, wherein the de-rotator means is under the control of pseudo random bit sequence generator means, controlled by a synchronization detector means controlled by magnitude detector means for detecting magnitudes of received symbols to detect a predetermined sequence of modified and unmodified symbols.

23. A receiver as claimed in claims 22, wherein the carrier recovery loop comprises a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a first selector under the control of the synchronization detector means for selectively routing the signal to one of a first de-mapping block for de-mapping unmodified symbols and a second de-mapping block for de-mapping modified symbols and a second selector under control of the synchronization detector means for selectively receiving output from the first and second de-mapping blocks and outputting a received symbol to an output port.

24. A receiver as claimed in claim 23, wherein the first selector, the first de-mapping block, the second de-mapping block and the second selector are replaced by a programmable de-mapping block under control of the synchronization detector means, for selectively de-mapping unmodified symbols or de-mapping modified signals.

25. A receiver as claimed in claims 23 or 24 wherein the pattern noise canceller comprises a phase detector controlling a phase shifter, wherein the output from the phase detector is fed forward to the phase shifter to cancel pattern noise.

26. A method for communicating a digital signal, the method comprising the steps of:

a) phase shift key modulating the digital signal with an asymmetric constellation pattern to produce a phase shift keyed signal and a carrier signal;

b) processing the phase shift keyed signal and carrier signal to produce a transmission signal with no, or a weak, carrier signal component;

c) transmitting the transmission signal to a receiver;

d) receiving at the receiver the phase shift keyed signal and regenerating a strong carrier signal from the transmission signal; and

e) using the strong carrier signal for phase locking the receiver to demodulate the phase shift keyed signal.

27. A method as claimed in claim 26, wherein the asymmetric constellation pattern has at least one constellation point of reduced amplitude.

28. A method as claimed in claim 27, wherein the at least one constellation point of reduced amplitude has zero amplitude.

29. A method as claimed in claims 26 to 28, wherein the step of processing the phase shift keyed signal and carrier signal comprises periodically changing mapping of the asymmetric constellation to reduce the carrier signal component of the transmission signal.

30. A method as claimed in claim 29, wherein the step of processing the phase shift keyed signal and carrier signal comprises changing the mapping of the asymmetric constellation for each symbol transmitted.

31. A method as claimed in claims 29 or 30, wherein the step of processing the phase shift keyed signal and carrier signal comprises changing the mapping according to a pseudo-random binary sequence.

32. A method as claimed in any of claims 29 to 31, wherein the step of processing the phase shift keyed signal and carrier signal comprises changing the mapping by phase rotation.

33. A method as claimed in any of claims 26 to 32, wherein the step of processing the phase shift keyed signal and carrier signal comprises inserting for transmission unmodified symbols among the symbols modified by use of the asymmetric constellation pattern.

34. A method as claimed in claim 33, wherein the step of processing the phase shift keyed signal and carrier signal comprises selecting a ratio of modified symbols to unmodified symbols to optimize performance of the method.

35. A method as claimed in any of claims 26 to 34, wherein the step of receiving comprises the step of synchronising the receiving means with the transmitting means by the transmission from the transmitting means to the receiving means of a predetermined sequence of modified and unmodified symbols.

36. A method as claimed in any of claims 26 to 35, wherein the step of phase shift key modulating the digital signal with an asymmetric constellation pattern comprises: using a first mapping block connected to an input port for producing unmodified PSK symbols and using a second mapping block connected to the input port for producing PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; and the step of processing the phase shift keyed signal and carrier signal comprises using selector means connected to the first and second mapping blocks for selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means, using phase rotator means connected to the selector means for receiving the selected symbols from the selector to rotate the constellation of the selected symbol under the control of a pseudorandom bit sequence generator connected to the phase rotator and controlled by the synchronization means and outputting to an output port modified and unmodified symbols having pseudo- randomly rotated constellations.

37. A method as claimed in any of claims 26 to 35, wherein, the step of phase shift key modulating the digital signal with an asymmetric constellation pattern comprises: using a programmable mapping block, under the control of parameter-controlled synchronizing means, connected to an input port for selectively producing unmodified PSK symbols or for producing PSK symbols modified by the zeroing of at least one constellation state of the PSK constellation; and the step of processing the phase shift keyed signal and carrier signal comprises selectively receiving unmodified or modified symbols using phase rotator means connected to programmable mapping block for receiving the selected symbols from the mapping block to rotate the constellation of the selected symbol under the control of a pseudo-random bit sequence generator connected to the phase rotator and controlled by the synchronization means and outputting to an output port modified and unmodified symbols having pseudo-randomly rotated constellations.

38. A method as claimed in claim 36 or 37, wherein the step of selectively receiving unmodified or modified symbols under the control of parameter-controlled synchronizing means comprises using a predetermined ratio of modified symbols to unmodified symbols as a parameter for controlling the parameter-controlled synchronizing means.

39. A method as claimed in any of claims 26 to 38, wherein the step of receiving comprises using de-rotator means for receiving a complex signal and for implementing rotations which are multiples of π/4 for inputting to a carrier recovery loop means.

40. A method as claimed in claim 39, wherein the step of using de- rotator means comprises using de-rotator means under the control of pseudo random bit sequence generator means, controlled by a synchronization detector means controlled by magnitude detector means for detecting magnitudes of received symbols to detect a predetermined sequence of modified and unmodified symbols.

41. A method as claimed in claims 39 or 40, wherein the step of inputting to the carrier recovery loop means comprises inputting to a carrier loop recovery means comprising a complex multiplier with a phase locked feedback loop, a pattern noise canceller, a first selector under the control of the synchronization detector for selectively routing the signal to one of a first de-mapping block for de-mapping unmodified symbols and a second de-mapping block for de-mapping modified symbols and a second selector under control of the synchronization detector for selectively receiving output from the first and second de-mapping blocks and outputting a received symbol to an output port.

42. A method as claimed in claim 41, wherein the first de-mapping block, the second de-mapping block and the second selector are replaced by a programmable de-mapping block for selectively de- mapping unmodified or modified symbols under control of the synchronization detector means.

43. A method as claimed in claims 41 or 42 wherein the pattern noise canceller comprises a phase detector controlling a phase shifter, wherein the output from the phase detector is fed forward to the phase shifter to cancel pattern noise.