Polyphase receivers

A polyphase receiver comprises an RF front end (10 to 28) for receiving a wanted data signal modulated on a carrier signal and for producing quadrature related low IF signals, an image rejection filter formed by a polyphase filter (30) for filtering the quadrature related low IF signals, soft limiting means (36,38) for compressing the dynamic range of the filtered quadrature related IF signals and a signal demodulator (41) for recovering the data signal. The soft limiting means (36,38) has a characteristic which is substantially linear at signal levels 10 dB below a predetermined minimum wanted signal level, moves into compression for higher signal levels and hard limits at substantially 10 dB above the desired receiver sensitivity which avoids degrading the sensitivity of the receiver.

The present invention relates to a polyphase receiver and to a transceiver comprising a polyphase receiver as its receiving section. The present invention has particular, but not exclusive, application to integratable receivers/transceivers for use in portable telecommunications devices such as cellular and cordless telephones.

For convenience of reference in the present specification and claims, the word “receiver” is to be understood to include the receiving section of a transceiver.

Conventional radio receivers almost invariably use a superheterodyne architecture. In this architecture, the wanted signal, having previously been modulated onto a radio frequency (RF) carrier at the transmitter, is mixed with a local oscillator (LO) signal and thereby translated to a fixed intermediate frequency (IF) where unwanted interfering signals are removed by the use of highly selective filters.

Limiters have found widespread use in superheterodyne receivers designed to receive frequency modulated (FM) transmissions. They alleviate the need for automatic gain control (AGC) and enhance the receiver's immunity to amplitude modulated (AM) interference. Typically they operate at a point in the receiver where the IF is very high compared to the baseband modulation.

In the interests of making integrated receivers a zero-IF architecture has been the most successful so far but is not without problems in respect of recovering modulated signals containing wanted information at or near the carrier frequency and which are translated to DC and low frequencies.

“Polyphase” or low IF receiver architectures are becoming more widespread for receivers used in digital communication systems. These systems use an IF of the order of half the bit rate such that during a bit period there is only a single zero crossing in the quadrature related I and Q IF signals. The use of hard limiters in these circumstances has been found to impair severely the sensitivity of the receiver. Noise from the receiver front end will cause errors in the timing of the zero crossings and with a hard limiter, these timing errors translate into large phase errors, which in turn translate into bit errors.

An object of the present invention is to avoid large zero crossing timing errors in polyphase receivers.

According to the present invention there is provided a polyphase receiver in which quadrature related low IF signals are soft limited prior to being demodulated.

The present also provides a polyphase receiver comprising means for receiving a wanted data signal modulated on a carrier signal and for producing quadrature related low IF signals, soft limiting means for compressing the dynamic range of the quadrature related low IF signals and signal demodulation means for recovering the data signal.

The present invention further provides a polyphase receiver comprising means for receiving a wanted data signal modulated on a carrier signal and for producing quadrature related low IF signals, image rejection filtering means for filtering the quadrature related low IF signals, soft limiting means for compressing the dynamic range of the filtered quadrature related IF signals and signal demodulation means for recovering the data signal. The image rejection filtering means may comprise polyphase filtering means.

Compared to using hard limiters, the soft limiting means has the benefit that any noise in the I and Q components of the quadrature related low IF signals does not generate such large phase errors/bit errors. As a consequence a low IF receiver incorporating soft limiters achieves a similar sensitivity as that of a superheterodyne receiver and has the additional benefit that much of the receiver can be integrated.

In an embodiment of the present invention the soft limiting means has a characteristic which is linear for signal levels which are of the same order as that of the front end noise, is several dB into compression for signals at a specified sensitivity level of the receiver and applies hard limiting when the input signal signal is 10 dB above the receiver sensitivity.

In the drawings the same reference numerals have been used to indicate similar features.

The polyphase receiver shown inFIG. 1comprises an antenna10which is coupled via a band defining bandpass filter12to a low noise RF amplifier14. An output of the amplifier14is coupled to a signal splitter16from which signals are applied to signal inputs18a,20aof quadrature related mixers18,20. A local oscillator22implemented as a frequency synthesiser has an output frequency fLOwhich is applied to a phase shifter24providing an 0° phase signal to a local oscillator input18bof the mixer18and a 90° phase shifted signal to a local oscillator input20bof the mixer20. The local oscillator frequency fLOis chosen to provide a low IF signal having a frequency of the order half the bit rate. The quadrature related low IF I and Q signals are amplified in respective IF amplifiers26,28and their outputs are applied to an image rejection filter formed by a polyphase IF filter30. DC break capacitors32,34are shown connected to the respective outputs of the polyphase IF filter but in reality this is diagrammatic because DC breaks are distributed throughout the signal paths of the IF signals. Soft limiting amplifiers36,38are coupled to the I and Q signal paths from the polyphase IF filter30.

The soft limited output signals from the soft limiting IF amplifiers36,38are applied to a polyphase harmonic filter40which eliminates the harmonics of the wanted signal generated in the soft limiting amplifiers36,38. As these harmonics are displaced asymmetrically about zero frequency, it is preferred to use a polyphase filter.

A polyphase frequency discriminator41is coupled to the outputs of the harmonic filter40in order to demodulate the IF signals. The discriminator41comprises a bandpass filter42having I and Q inputs42a,42bcoupled to respective outputs of the harmonic filter40. Respective outputs42c,42dof the filter42are coupled to first inputs48,50of multipliers44,46.

Second inputs52,54of the multipliers are cross coupled to junctions55,53, respectively, on the signal paths to the inputs42a,42bof the filter42. A subtracting stage56has inputs coupled to outputs of the multipliers44,46, respectively, and an output coupled to a baseband filtering and bit slicer stage57. The stage57comprises a data filter58which removes high-frequency noise produced by the discriminator41as part of its intrinsic behaviour. The data filter58is a low pass filter whose cut-off frequency is in the region of half the bit rate to allow the passage of the highest fundamental frequency expected to be present in the data stream comprising an alternating sequence of logical 1 and logical 0. The data filter58is coupled to one input of a bit slicer circuit60which has another input for a threshold voltage supplied by a source62. The bit slicer circuit60supplies data bits to a decoder64.

Optionally an RSSI output66can be derived from the soft limiting amplifiers36,38.

For convenience of description the operation of the receiver shown inFIG. 1will be described with respect to the DECT (Digitally Enhanced Cordless Telecommunications) Standard.

In operation the RF signals received at the antenna10are applied to the band defining filter12after which they are amplified in the low noise amplifier14. The local oscillator frequency fLOis set at the lower edge of the wanted channel. Hence a wanted channel of width 1728 kHz becomes translated down to a low IF of 864 kHz. Simultaneously, all of the other potentially active channels in the DECT band are mixed down to frequencies on either side of the wanted channel. Conventional, real filtering could be used to reject most of these interfering signals but the adjacent channel positioned on the lower side of the wanted signal requires special treatment. As the adjacent channel is centred on an IF of −864 kHz, it is located precisely at the same frequency as the image of the wanted channel. In order to discriminate between these two signals the polyphase filter30is used to process the I and Q signals from the mixers18,20as a complex pair. The use of the polyphase filter30enables the desired rejection to be applied to the adjacent channel. The amplifiers26,28are provided to enable the I and Q signals to be in the appropriate dynamic range for entry into the polyphase filter. Sufficient gain must be applied to ensure that the noise generated by the polyphase filter30is insignificant when referred back to the front end of the receiver.

The distributed DC breaks represented by the capacitors32,34introduce a notch in the spectrum of the IF signal at DC which is wide enough to give the receiver adequate recovery time but not too wide to cause distortion of the modulation. In the case of DECT it has been found that 50 kHz is an optimum cut-off frequency.

The soft limiting amplifiers36,38compress the dynamic ranges of the I and Q signals obtained from the polyphase IF filter30in preparation for subsequent demodulation. In the case of a DECT receiver the amplifiers36,38have a compression characteristic of the form shown inFIG. 5. This characteristic is substantially linear at signal levels 10 dB below the minimum wanted signal level, in the same vicinity of the front-end noise at −107.6 dBm the characteristic progressively moves into compression as the signal level increases and at the sensitivity level of −96 dBm, the voltage swing of the limiters will have reached 95% of full scale. Hard limiting only occurs when the signal level is roughly 10 dB above the required receiver sensitivity. Consequently the limiting amplifiers36,38no longer degrade the sensitivity of the receiver.

The non-linearities in the limiting amplifiers36,38generate harmonics of the wanted signal which may be removed by filtering. As these harmonics are displaced symmetrically about zero frequency, then if it is decided to remove them by filtering it is desirable to continue with the complex signal processing and to use a polyphase device as the harmonic filter40

The polyphase discriminator41comprises a polyphase filter42which is centred on the IF of 864 kHz.

The signal bandwidth of the demodulated signal is filtered in the filter58to remove any high frequency noise produced by the discriminator41. As mentioned above, the cut-off frequency of the filter58is set in the region of the highest fundamental frequency expected to be present in the data stream which in this example is 576 kHz.

If the receiver shown inFIG. 1comprises the receiving section of a transceiver, a transmitter section68is also provided and is connected to the antenna10. Wherever possible the transmitter section makes use of components, such as the local oscillator22, provided in the receiving section.

Referring toFIGS. 2A,2B,FIGS. 3A,3B andFIGS. 4A,4B which illustrate respectively a) the receiver using no limiter at all (FIGS. 2A,2B), b) the receiver using a soft limiter (FIGS. 3A,3B), and c) the receiver using a hard limiter (FIGS. 4A,4B).

In the case where no limiter is used,FIGS. 2A,2B, and the signal is merely amplified in a linear amplifier, noise at the input approximately 10 dB below the wanted signal will cause occasional timing errors in the zero crossings of the I or Q component. In the case of the I component as shown inFIG. 2A, an early zero crossing at a time t1will result in a relatively small phase error, θe, in the complex signal vector. A small error like this would be unlikely to cause a bit error after demodulation since this would require a phase error of something approaching 90°. In the case of the soft limiter,FIGS. 3A,3B, the same noise at the input could also cause an erroneous zero crossing at time t1, but owing to the increased gain, the corresponding phase error θeis now increased as shown inFIG. 3A. Exactly how large the error becomes depends on the dynamic characteristic of the limiter. In practice the aim is to keep the increase small enough to ensure that a bit error is still unlikely for the given level of input signal. In the case of the hard limiter,FIGS. 4A,4B, the gain in the vicinity of a zero crossing is now extremely large. The input noise will generate full-scale transitions in the polarity of the I and Q signal components and therefore if the transitions take place at the wrong instant in time, the phase errors θeproduced will be a full 90°. In a low-IF receiver where the IF is approximately half the symbol rate (1152 kHz for DECT), there is only 1 zero crossing in each of the I and Q signal components per symbol which means the 90° phase error would be very likely to generate a bit error. This situation does not improve until the signal level rises to a point where the input noise becomes insignificant. Hence the hard limiter degrades the receiver sensitivity.

FIG. 6illustrates a BER simulation of a DECT receiver with soft limiting amplifiers having the compression characteristics shown inFIG. 5, the BER falls substantially at −92 dBm and achieves the required sensitivity figure 10−3, indicated by the horizontal broken line, at an input power of −96.6 dBm, the target input power being −96.0 dBm as indicated by the vertical emboldened line. Allowing for antenna losses of 3 dB the receiver has a sensitivity which passes the DECT specification by the substantial margin of 10 dB.

In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed.