Timing and carrier recovery in TDMA without preamble sequence

Abstract: A method of detecting the clock rate and recovering the carrier in a TDMA signal comprises the steps of: storing for a predetermined amount of time quadrature baseband components (I,Q) as sampled data (at 54); estimating the clock rate phase error of said baseband samples (at 55); correcting the clock phase of said baseband samples from the estimated clock phase error using interpolation over said sampled quadrature components (at 56); estimating the carrier phase error of said stored baseband time corrected quadrature samples (at 58); correcting the carrier phase of said stored baseband time corrected samples according to said estimated phase error (at 59) ; and forwarding said corrected clock and carrier signals to decision circuit means (60) for providing demodulated data. This method permits to minimize the frame overhead allocated to clock and carrier training sequences while maintaining a reliable data signal for error free transmission.

Description: This invention relates to mobile radio systems and more particularly to the time division multiple access (TDMA) class of systems. In the TDMA class of mobile radio systems, several users share one carrier. Each user is allocated a separate time slot on that carrier, and each transmits and receives short bursts of data packets separated by a guard time. A typical TDMA burst starts with a preamble. The preamble consists of bit timing and carrier recovery information, equalizer initialization data FEC (forward error correction), decoder synchronization and other overhead if needed. This is typically followed by a start flag, a control field, a data field, a CRC (cyclic redundancy check) and an end flag. The number of users on one carrier is limited to the number of slots allocated for that particular carrier. The problem associated with the use of clock and carrier training sequences (preambles or midambles) is that they increase the bit frame overhead and they reduce transmission efficiency. There is a requirement for minimizing the bit overhead of the frame while maintaining a reliable data signal for error free transmission. EP-A-0 222 593 discloses a method for detecting the clock rate and recovering the carrier, which is applicable among others to a QAM signal. In this method, the input analog signal is converted into a digital signal with a free running clock rate, and the digital data are supplied to a timing recovery circuit for estimating the clock rate phase error. The clock rate phase error is averaged over a certain number of digital data words. Simultaneously the digital data are supplied to a digital interpolation filter which comprises a shift register and multipliers associated with each stage of the shift register for convolving the data clocked through the shift register with a predetermined function so as to obtain a Nyquist filter characteristic. The coefficients defining this function are modified in accordance with the estimated clock rate phase error so that the clock rate phase error is corrected. The clock phase corrected data are then supplied to a carrier phase estimating circuit and to a phase correction circuit. In the phase estimating circuit, the carrier phase error is estimated by averaging over a plurality of data words and is then fed forward to the phase correction circuit for performing a phase correction. If this known method were applied to a TDMA signal, the averaging processes in the timing recovery circuit and the phase estimating circuit would still require clock and carrier training preambles in each TDMA burst. It is an object of the present invention to minimize the frame overhead allocated to clock and carrier training sequences (preambles or midambles). Another object of the present invention is to minimize signal processing load by maintaining signal integrity and reliability. According to the invention, this object is achieved by the method specified in claim 1 and by the circuit specified in claim 9. Optional features of the invention are indicated in the dependent claims. Description of the Drawings Particular embodiments of the invention will be understood in conjunction with the accompanying drawings, in which: Figure la is an illustration of a typical TDMA frame; Figure lb is an illustration of a TDMA frame as can be used in the present invention; Figure 2 is a block diagram of a typical TDMA mobile radio system; Figure 3a is a block diagram of a first stage of a Baseband Processing Circuit according to an embodiment of the present invention; Figure 3b is a block diagram of the second section of the Baseband Processing Circuit of Figure 3a; Figure 4 is a block diagram of the Clock Phase Estimator of Figure 3b; and Figure 5 is a block diagram of the Carrier Phase Estimator of Figure 3b. Description of the Preferred Embodiment Referring now to Figure la, we have shown generally at reference numeral 10, a typical TDMA frame representation. The TDMA carrier is divided into a series of contiguous frames 11 and slots 12. As is shown, each frame is comprised of n number of slots. Each user on the carrier is assigned one or more slots. The transmission commences at the start of the slot and is completed before the end of that slot. The size and number will of course depend on the application. A typical TDMA burst is separated from an adjacent burst by a guard time 13 and starts with a preamble 14. The preamble consists of bit timing and carrier recovery information, equalizer initialization data, FEC (forward error correction), decoder synchronization and other overhead if needed. This is followed by a start flag 15, an address field 16, a control field 17, a user data field 18, a CRC (cyclic redundancy check) 19 and an end flag 20. If we now refer to Figure 1b, we have shown a TDMA frame proposed for use with the present invention. In this representation, the overhead allocated to the clock and carrier training preambles has been deleted. Only the guard time 21 separates each burst. Each burst would similarly have a start flag 22 for synchronization purposes and could consist of a Barker code, an address field 23, a control field 24, a user data field 25, a CRC 26 and an end flag 27. A signal having this representation can be recovered and demodulated as will be described below. Figure 2 is a block diagram of a typical receiver used in mobile radio systems which would include a front end section and a baseband processing circuit. This front end is shown generally at reference numeral 30 and the baseband processing circuit at reference numeral 31. The front end 30 is basically comprised of a receiving and transmitting antenna 32 connected to a duplexer or transmit/receive switch 33. The received signal first enters a band pass filter 34 to provide some extra transmitter image rejection in the receiver. The filtered signal is then fed through a typical low noise amplifier 35 before being down-converted to an IF by mixer 36. At the IF processing stage 37, the signal is filtered to remove some of the out-of-band unwanted signals and amplified before being mixed down to baseband by quad mixer 38. At this point, two quadrature baseband components are recovered, namely the I-phase signal and the Q-phase signal processed at circuit 31. At the transmitter section, the I and Q signals are first filtered to remove the out of band frequencies (not shown) before being up-converted by modulator 39. The mixer 36 and modulator 39 are frequency locked to the same reference frequency by synthesizers 40 and 41. These synthesizers can consist of a dielectric resonator VCO as the RF signal source controlled by a synthesizer IC in a phase locked loop. The Q channel is mixed with the 90° phase RF carrier, while the I channel is mixed with in-phase RF carrier. The RF mixer outputs are combined and the resultant waveform is fed to the power amplifier 42. A PIN diode attenuator (not shown) can be used to control the transmit power level into the power amplifier 42. The power amplifier output is fed to duplexer or transmit/receive switch 33 which is used to combine transmitter and receiver signals at the antenna terminal. If we now refer to Figure 3a, we have shown the first stage of the baseband processing circuit shown in Figure 2. The two quadrature signals, namely I and Q, that have been recovered from quad mixer 38 of Figure 2, can then be amplified before being low pass filtered by filters 43 and 44 respectively, to remove high frequency that can cause aliasing distortion. The respective signals are then digitally sampled by A/D converters 45 and 46. The baseband digital signals 47 and 48 are then digitally filtered by channel filters 49 and 50 respectively to achieve the required adjacent channel performance and to remove out-of-band energy and intersymbol interference. A constant signal level is maintained by means of automatic gain control circuits 51 and 52. An adaptive slope equalizer 53 may be used to obtain a relatively constant frequency/power response across the channel bandwidth for enhanced performance. In Figure 3b, we have shown the second stage of the baseband processing circuit of Figure 2. The second stage provides the timing and carrier recovery of the frame. As indicated, no overhead is allocated to clock and carrier training preambles. Instead, the baseband quadrature signal components are stored as sampled data in delay buffers while the clock and carrier phase are estimated. There are advantages if the entire slot is stored. A receiver assigned n slots per frame has an interval equal to the frame duration divided by n to process each slot. This can allow the signal processing circuit to operate with a lower clock rate which may be advantageous. In addition, the processing may be non-causal. Averaging can be done forwards and backwards. This stage is comprised of a first delay buffer 54 for storing the digitized samples received, a clock phase estimator 55 providing an estimate of the clock phase error to a data interpolation circuit 56. Delay buffer 57 provides additional time to process the time recovered samples at the carrier phase/gain estimator 58 which will provide a carrier phase/gain error estimate to correction circuit 59 the output of which is connected to a decision circuit 60. In Figure 4, we have shown a block diagram of an implementation of the clock phase error estimator of Figure 3b. In this embodiment, the clock is recovered using a Costas loop applied to the filtered baseband signals on one or both axes. This clock has a harmonic relationship to the baud rate fs/n where fs is the baud rate and n is greater than or equal to 1. Circuits 62 and 63 are used to process the sampled I and Q baseband signals by the reference clock signal 64 via divide-by-two circuits 65 and 66. The real and imaginary components x and y respectively for each axis are filtered by means of low pass filters 67, 68 and 69, 70 respectively. The results are then squared by means of squaring circuits 71 and 72 for removing the 180 degree phase ambiguity. The resultant real and imaginary signals from each axes are then summed at 73 and 74 for improved accuracy and then averaged at 75 over the burst and further processed at rectangular-to-polar conversion circuit 76 to give the estimated clock phase error. One embodiment of circuit 76 is a table look-up circuit implemented in a ROM in which the input is address information and the output is a set of coefficients for a finite impulse filter. These coefficients are used in circuit 56, the data interpolation circuit in an FIR (finite impulse response) filter to recover a sequence of I and Q samples with the correct clock phase. The start frame reference signal is used here to initiate the clock recovery process. The estimated clock phase error is used in conjunction with the clock interpolation circuit 56 shown in Figure 3b to interpolate the correctly timed data from the stored samples in delay buffer 54. The results are T/n-spaced sampled data for the I and Q axes where n is greater than or equal to 1. These samples are sent to a decision circuit 60, the output of which is the demodulated data. The demodulated data is also input to the frame detection circuit 61 which provides a start frame reference signal for the entire process. The decision circuit could be a simple threshold detector on the I and Q axes or an adaptive equalizer (decision feedback, Viterbi, etc.). It will be understood by those knowledgeable in this art that the input and output rates of the clock recovery circuit may or may not be the same. Figure 5 is a block diagram of an implementation of the carrier phase and magnitude estimator of Figure 3b. The carrier measurement includes raising the I and Q baseband samples to the mth-power for m-PSK modulation at circuit 77 to account for four quadrant carrier phase ambiguity. The signals can then be converted at circuit 78 to provide a polar coordinate before being averaged at circuit 79, providing a carrier phase estimate 80 and a magnitude estimate 81. These are applied to circuit 59 of Figure 3b to restore symbols back to their proper locations. It will be understood to those knowledgeable in the art that a magnitude correction would not be required if an AGC circuit was used in this instance.

Claims: A method of detecting the clock rate and recovering the carrier in a TDMA signal in absence of clock and carrier training preambles, comprising the steps of: a) storing for a predetermined amount of time digitally sampled quadrature baseband components (I,Q) of said TDMA signal; b) estimating the clock rate phase error of said quadrature baseband components; c) correcting the clock phase of said baseband components from the estimated clock phase error using interpolation over said digitally sampled quadrature baseband components; d) estimating the carrier phase error of said clock phase corrected quadrature baseband components; e) correcting the carrier phase of said clock phase corrected quadrature baseband components according to said estimated phase error; and f) forwarding said corrected clock and carrier signals to decision circuit means (60) for providing demodulated data and to frame detection circuit means (61) for providing a start frame reference signal. A method as defined in claim 1 wherein said clock phase error is estimated by applying a Costas loop to said baseband components for recovering the correct clock information; comprising: filtering the real and imaginary components of each baseband component; squaring said real and imaginary components to remove phase ambiguity; summing the resultant real and imaginary components from each base-band component; and averaging to provide said clock phase error estimate. A method as defined in claim 2 wherein said clock phase estimate is used to interpolate over said stored samples to generate the clock-phase corrected quadrature components. A method as defined in any of the claims 1 to 3, wherein, said carrier phase is estimated by applying a four-power complex multiplication to said clock phase corrected quadrature baseband components; and the resultant signal is averaged over a clock burst to provide said carrier phase estimate. A method as defined in claim 4 wherein said carrier phase is recovered by using said carrier phase estimate to restore the samples to the true I and Q position. A method as defined in any of the claims 1 to 5, further comprising the steps of: g) estimating the magnitude error of said clock phase corrected quadrature baseband components; and h) correcting the carrier magnitude of said clock phase corrected quadrature baseband components according to said estimated magnitude error. A method as defined in claim 6 wherein said magnitude error is estimated by applying a fourth-power complex multiplication to said clock phase corrected quadrature baseband components; and the resultant signal is averaged over a clock burst to provide said magnitude estimate. A method as defined in claim 7 wherein said carrier magnitude is recovered by using said magnitude estimate to restore the samples to the true I and Q position. A circuit for detecting the clock rate and recovering the carrier in a TDMA signal in absence of clock and carrier training preambles, according to the method of any of the claims 1 to 8, comprising a first delay buffer (54) for storing for a pre-determined amount of time digitally sampled quadrature baseband components (I,Q) of said TDMA signal, a first estimating circuit (55) for receiving said quadrature baseband components for estimating the clock rate phase error of said quadrature baseband components; data interpolation means (56) for correcting the clock phase of said quadrature baseband components from the estimated clock phase error using interpolation over said digitally sampled quadrature baseband components; a second delay buffer (57) for storing the clock-phase-corrected quadrature baseband components obtained from said data interpolation means; a second estimating circuit (58) for estimating the carrier phase error of said stored clock-phase-corrected quadrature baseband components; a correction circuit (59) responsive to the output of said second estimating circuit for correcting the carrier phase of said clock phase corrected quadrature baseband components stored in said second delay buffer (57) according to said estimated phase error; decision circuit means (60) for converting the signals received from said correction circuit (59) into demodulated data; and a frame detection circuit (61) connected to said decision circuit means for providing a start frame reference signal. A circuit as defined in claim 9, wherein said second estimating circuit (58) is adapted to further estimate the magnitude error of said clock phase corrected quadrature baseband components and said correction circuit (59) is adapted to further correct the magnitude of said clock phase corrected quadrature baseband components according to said estimated magnitude error.

Inventors: MCNICOL JOHN D, MCNICOL, JOHN D.
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1989
Main classification: H04L27
IPC codes: H04L7, H04L27, H04J3