GB2373689A - Demodulation apparatus for multi-carrier PSK signals which reduces inter-carier interference - Google Patents

Abstract

The disclosed apparatus is suitable for use with continuous phase signals which have a phase transition period 9 in which the phase varies non-linearly between the phase of a previous symbol and the phase of the present symbol 10. The frequency is greater within the transition period 9 than within the data period 10. Interference between the transition period 9 of a low frequency carrier and the data period 10 of a higher frequency carrier is avoided by synchronising the symbol positions of respective carriers and performing demodulation using only the data periods 10, preferably using a FFT. The transition periods 9 are ignored and can therefore be considered guard intervals. The demodulation apparatus is also suitable for use with a combined amplitude/phase modulation scheme such as QAM.

Description

Bandwidth Efficient Demodulator This invention relates to a bandwidth efficient demodulator which does not compromise power efficiency. Its application is likely to benefit mobile communications where several geographically dispersed transmitters are communicating with the same receiver.

In radio transmission digital information is modulated onto a carrier signal at the transmitter and demodulated back into digital information at the receiver. This process is known as modulation and several different types are in use, typically modifying the carrier amplitude, frequency or phase to represent digital information. The modulation process spreads the carrier signal over a finite bandwidth, which is related to the modulation technique and the data rate of the information.

The power efficiency of a modulation technique determines the relationship between the received signal to noise ratio and the data rate that can be supported for a given probability of bit error. The bandwidth efficiency of a modulation technique determines the bandwidth that is used to transmit a specific data rate.

Both power and bandwidth are valuable resources limiting the capacity of a communications system. Unfortunately modulation techniques which are bandwidth efficient are usually less power efficient requiring more signal power to operate.

The object of this invention is to provide a demodulator which combines both power and bandwidth efficiency.

This invention enables the demodulation of carriers which are closely spaced without sustaining high levels of inter carrier interference or compromising on power efficiency.

To achieve this carrier phase/amplitude changes are synchronised in time for all carriers. The demodulator processes the signals between these transitions and ignores the transition period avoiding inter carrier interference associated with the transitions.

Synchronisation of individual carrier transitions is achieved by slaving them to a central clock which is broadcast to users of the system. This is achieved in a straightforward manner using conventional signalling and synchronisation techniques when setting up the circuit.

To ensure that the instantaneous carrier bandwidth between transitions is kept to a minimum the filtering of the individual carrier signals at the transmitter is relaxed so that the phase/amplitude transition may take place rapidly. Such an approach is counter intuitive as one would expect interference to increase as filtering is relaxed. Whilst this increases the bandwidth occupied by the individual carriers averaged over time, it reduces the occupied bandwidth during the period that is being processed by the demodulator.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 shows in schematic form the main components of the demodulator.

Figure 2 illustrates the amplitude of signal which is being processed by the demodulator against time with the individual samples highlighted. Figure 1 shows a schematic of an example of such a demodulator, which is based on a system using differential Quadrature Phase Shift Keying (QPSK). In this system information is represented by comparing the phase of the carrier signal during one symbol period with its phase during the previous period. Four phases are used, enabling each symbol to represent two bits of digital information.

The incoming signal 1 is converted to a suitable frequency for sampling through a conventional frequency converter 2, and filtered 3 to reject unwanted noise and signals.

The resultant analogue signal is then sampled digitally in an Analogue to Digital Converter (ADC) 4 to generate a stream of numbers. In this example design a sampling frequency of 1,400 kilo samples per second is used.

The Fast Fourier Transform (FFT) section 5 then processes each block of thirty five samples 8 by performing a thirty two point Discrete Fourier Transform (DFT) on the last thirty two samples 10, ignoring the first three samples 9. This process resolves the incoming signals into thirty two frequency components or bins. Each bin has an associated complex number representing the frequency equivalent of the input signal that was processed. The amplitude of the complex numbers represents the amplitude of the signal in each bin and the phase of the number represents the phase of the signal.

In this example design amplitude is not used however systems using combined amplitude/phase modulation could use the same technique.

The phase comparator 6 stores the complex frequency bin values and compares them with the values from the previous DFT for the corresponding bin. This comparison determines the value of any phase transition that has taken place between the current and previous symbol after allowing a fixed conversion value for each frequency bin corresponding to the phase change that would be experienced with a non modulated carrier at the frequency corresponding to the bin. Carrier frequency errors are compensated for by monitoring carrier phase variations over a long period of time enabling the demodulator to handle frequencies which are not exactly aligned with the centre of the frequency bins.

The carrier phase differences are then converted into digital information in a straightforward manner and pass out of the demodulator as a binary bit stream 7.

The transmitting devices are time synchronised such that carrier phase transitions occur during the first sampling period (which is ignored by the DFT). The filtering in the transmitting devices and communications path is wide enough to ensure that the carrier phase will have stabilised at its new value by the time the fourth sample in each block is taken. This will ensure that no phase transitions occur during the period that the demodulator measures the phase of the carriers, and thus interference between signals in different frequency bins will not occur. This scheme is illustrated in Figure 2.

By eliminating phase transitions during the DFT period, each carrier component will be processed as though it represented a snapshot of an un-modulated carrier, and will therefore generate no adjacent channel interference disrupting the carriers in the adjacent frequency bins. The only errors introduced will be associated with carriers which are not centred in the frequency bins. This will introduce a small amount of interference to adjacent carriers which may be neglected for frequency errors of less than five percent of the spacing between carrier bins.

The example demodulator processes carrier signals in frequency bins 4 though 8 to reduce the effect of fringing which occurs at bins close to bins 0,16 and 32. A bank of several such demodulators enables a continuous band of many tens or hundreds of equally spaced carriers to be processed.

A data rate of 73142 bits per second is achieved for a carrier spacing of 40 kHz with a power efficiency loss of 0.4 dB from the theoretical performance associated with differential QPSK. Three dB improved performance may be achieved for moderate additional complexity using coherent PSK techniques to establish a reference phase for each carrier.

Claims (4)

1. CLAIMS 1. A demodulator which blanks out signals during the phase transition period in order to avoid inter carrier interference.
2. 2. A demodulator as claimed in Claim 1 which performs phase demodulation of signals by applying the Discrete Fourier Transform to incoming signals.
3. 3. A demodulator as claimed in Claim 1 which reduces the required spacing between carrier signals by increasing the instantaneous bandwidth occupied by the signals during the phase transition period by widening the transmitter filter bandwidth.
4. 4. A demodulator as claimed in Claim 1 which tracks carrier frequency errors by comparing phase errors between sequential discrete Fourier transforms of the incoming signal.