A method of synchronizing a radio receiver with an incoming radio signal

Technical field

The present invention relates to a method for Synchronizing a radio receiver with an incoming Radio signal transmitted from a signal blocks in the form of Radio station is sent, each block a having predetermined number of time slots. Especially the invention relates to the synchronizing incoming Radio signals in demodulating a baseband signal in the receiver of a radio station, with the aid of a solid Synchronization word into each time slot.

State of the art

In the case of a digital cellular radio system, which operates in accordance with the TDMA principle, be Radio broadcasts in blocks of a radio station broadcast, for example, a base station, each block a predetermined number of time slots having. In this regard, it is necessary to Radio receiver (mobile device) with the radio station in to synchronize with respect to the time slot for the Radio receiver is determined. This synchronization must fast and independent of the time of the dispersion Radio medium as a result of multipath propagation and of the Fading occur. Furthermore, the synchronization must be carried out independently of the frequency errors that may occur during the radio broadcast.

It is known that the time dispersion of the radio medium by coherent correlating the received and demodulated to compensate radio signal; see. For example, Swedish Patent Application 89 02844-3. As shown in this Patent application describes the received and demodulated radio signal correlated coherent with the Intention to stop an equalizer or adjust, the so operates to compensates the echoes or equalized, which are obtained in the multipath propagation. As in the aforementioned patent application and is also described in WO 88/05 981, each received time slot to a synchronization word, which to activate and adjust the equalizer is used. The synchronization word is used, to allow the equalizer to perform its function to correct time relative to the time slots in the execute received radio signal. The equalizer is normally in the demodulator for the baseband signal provided that, for example, by phase shift modulated (QPSK) may be. If in the demodulator not Equalizer is provided so requires the demodulator still synchronizing signals to its function to run.

Describing the invention

The received by coherent correlation radio signals achieved advantage is that the correlation practically regardless of the dispersion properties of the Radio mechanism can be used, so even when Occurrence of significant scattering of reflected and received radio signals. The coherent correlation of incoming and demodulated radio signal gives a Estimate for the impulse response of the radio channel with respect on the time slot in which the Synchronization word finds. The synchronization word is sometimes called the "training sequence", respectively. In this case, however, assumed that a correction for each frequency error has been made during the Propagation of the radio signal from the transmitter to the receiver might have occurred, as a result of Doppler shift during the radio broadcast to a mobile receiver or due to a deviation of Receiver synthesizer frequency of the transmission frequency. If a high frequency error occurs, so a working Methods not employing coherent correlation. It is therefore necessary, the frequency error in the determine radio receiver and this error compensate, before the correlation can be performed.

The present invention relates to another method the correlation of the received radio signal, namely a called differential correlation.

In this method the properties in which are Demodulating the radio signal used to a differential signal between a determined Symbolic value and a next previous or next form following symbolic value, partly in the received radio signal and partly in the known Synchronization word that in the radio receiver is stored. After the correlation is a signal obtained, the absolute value or absolute size a Time position indicates the same time position the appropriate synchronization word.

The inventive method is characterized by the in the characterizing part of claim 1. stated features in.

The inventive method can also be used to are obtained from the correlation function of the estimating frequency error, said method by the features is in the in the following Claim 3 are given.

Brief Description of Drawings

The invention is described in more detail with described with reference to the accompanying drawings. It shows

<b>Fig.</b> 1 is a schematic illustration of a radio station and a radio receiver;

<b>Fig.</b> 2 shows a radio channel having eight time slots, and a schematic representation of the contents a time slot, if in between the <b>Fig.</b> 1 shown radio stations is sent;

<b>Fig.</b> 3 a multipath propagation;

<b>Fig.</b> 4, the time dispersion; and

<b>Fig.</b> 5 is a block diagram showing in more detail the receiving units of a radio station explained in which the invention Method is used.

The best embodiment of the invention

<b>Fig.</b> 1 generally illustrates two radio stations <b>1</b> and <b>2</b>, the used for sending and receiving radio signals so that the radio station <b>1</b> Radio signals and notes of the radio station <b>2</b> be transmitted and vice versa. The inventive method can be used both in the station <b>1</b> as well as in the station <b>2</b> when receiving radio signals be used. In particular, the radio station<b>1</b> a its base station, and the radio station <b>2</b> can be one of be more mobile stations.

The between the station <b>1</b> and the station <b>2</b> exchanged Radio signals are radio frequency signals are baseband-modulated, and in accordance with the TDMA principle are divided temporally. Sending and Receiving via the radio channels is divided with a given duplex spacing in blocks and time slots correspondingly <b>Fig.</b> 2, so that a block a predetermined including duration and, for example, eight time slots contains, each having a duration of about one millisecond who, if the block is eight milliseconds. One of the radio channels, or a small number of channels, is or are generally for controlling and transmitting Information to the mobile stations from the base stations in coordinated manner used. At the center of Data message D is a partial synchronization S₀, S₀ containing a synchronization pattern (i), wherein i denotes the number of symbols. Of the S o can sync part also at the beginning of be arranged data message D. The partial synchronization has a bit pattern in both the transmitter and Also in the receiver is known, and which for a predetermined time slot in a periodically recurring is the same blocks, so that therefore a time slot CH2 the same bit pattern in its synchronization word in the having next following blocks. The Synchronization pattern s₀ (i) can do likewise are used, the demodulator in the receiver part to synchronize a radio station, a burst fashion in the Time slots CH0-CH7, in addition to its use in estimating the time dispersion.

In the case of the embodiment according to <b>Fig.</b> 3, the Radio station <b>1</b> a base station B, and the radio station <b>2</b> is a mobile station M. The base station B sends Radio waves to the mobile station M in the TDMA a given channel, as in <b>Fig.</b> is shown. 2 yourself in a direction propagating radio waves through stationary or moving obstacles reflected, whereas other waves propagate freely to the mobile station M to which they are received. In the<b>Fig.</b> 3 explained Multipath propagation resulting in a fading which can take different forms. If the Time differences between received waves in a Time interval are concentrated, which was significantly shorter as the bit time Tbit (see <b>Fig.</b> 4), step in so-called flat fading on. If the time differences are larger, so occur two or more separate waves, each of which is more or less independent fading shows. This fading is displayed when receiving a varying amplitude and phase. The receiver equalizer forcing a coherent demodulator in the receiver to this phase change to follow. The phase positions be can be clearly identified by sending a known sequence s₀ (i) in the above-mentioned Partial synchronization S o. If the channel is not too fast varies, so varies with low bit rate, it is not necessary that the demodulator the Information in relation to the phase position of the received Signal during the time of determination of the data message updated, but it is at a high bit rate required, the propagation parameters at the beginning of each adjust time slot, and sometimes even even the duration of the time slot.

<b>Fig.</b> 4 illustrates the manner in which one of Transmitter sent in the base station B by the pulse Receiver M of the mobile device as a result of the above said fading is received. The resulting Impulse response consists of two impulses I and II, of which I was by a time delay t₁, according to the Propagation time and the pulse attenuated II and further was a time delay t₂, due to reflection to X in the illustration of <b>Fig.</b> . 3 <b>Fig.</b> 4 represents a imaginary case in that only for explaining the principle is intended. In reality, enters a so-called Inter-symbol interference in the receiver, so that the Pulses I and II are interwoven. Furthermore, it was assumed that the impulse response of only two pulses, consists. In reality, an interference pattern obtained, which consists of a number of reflected pulses consists. <b>Fig.</b> However, Figure 4 illustrates the so-called Time dispersion, ie that a transmitted pulse rise for a number of time-shifted pulses are (in <b>Fig.</b> 4, only two pulses, namely the pulses I and II), as a result multipath propagation. The bit time T_bit is in this Connection significantly. Thus the passage of the free Time dispersion is considered, the bit time T should_bit have such a long time that the significant momentum II in the T_bit-Intervallfeld Falls, so that therefore t₂- t₁ small compared T_bit is. The time dispersion can give a bit error reason above as a result of mentioned inter-symbol interference. The influence of these Time dispersion can be low by using reduce symbol speeds, ie such that T_symbol relatively large (speed is less than 25 kbaud / s), or by using an equalizer.

If the above-mentioned time dispersion is such that reflected significant impetus II within a very limited interval t₂-t₁ occur (for example, 10 microseconds), so it is not necessary to provide a to use coherent correlation, which according to the foregoing embodiments due to a frequency error the received radio signal occurs. According to the the present invention is instead in this case a so-called differential correlation used, wherein wherein a simultaneous estimation of frequency error is obtained. Since the frequency error can be estimated can, it is possible, the error in the RF / IF demodulator compensate and then, if necessary, in a coherent toggle correlation.

<b>Fig.</b> 5 illustrates in more detail the receiver part the in <b>Fig.</b> 1 shown Radio Station <b>2</b>In which the The method according to the invention is used.

The antenna of a receiver <b>21</b> received radio signal is to an amplifier unit <b>22</b> and from there to a RF / IF demodulator <b>23</b> transfer. By demodulator<b>23</b> becomes a baseband signal is obtained, which an analog signal is and the transmitted data flow (data symbols) represents the predetermined according to a Modulation program is modulated, for example, QPSK phase shift modulated (Quadrature Phase Shift Keying).

Sampling and analog-digital conversion, refer to the unit <b>24</b> instead, and there is obtained a digital signal y (n), which is phase-shifted, modulated baseband signal represents, and that the subsequent demodulator <b>25</b> is supplied. Thus, the demodulator correct can perform demodulation, is a Synchronization signal required, or it is required, the demodulator with the phase-shifted, to synchronize the modulated baseband signal outputted from is sent to the transmitter. Therefore, the signal y (n) one supplied correlator differential correlation between the received signal y (n) and the Synchronization word with a known Synchronization pattern s₀ (i) performs, which correspondingly <b>Fig.</b> 1 of the transmitter portion of the radio station <b>1</b> Gets transferred. The known synchronization pattern s₀ (i) is in the synchronization word generator Receiver stored.

A corresponding signal s₀ (i) is known from the receive synchronization pattern, and in the correlator <b>26</b> is a differential signal formed

Δ_s(I) = S o (i + 1) s₀ _* (I)

wherein S o (i + 1) is the (i + 1) th symbol in the Synchronization patterns S₀ (i), and s₀_*(I) complex conjugate of s₀ (i). Assuming that

s₀ (i) = A_i · e^j^Ri

so it follows that s₀_*(I) = A_i · e^-j^Ri, where A₁ the amplitude and R_i the phase of the i-th symbol is. If

then results

which means that Δ_s(I + 1) the change in angle represents that during the modulation of s₀ (i) has occurred, and that s₀ (i + 1) · A_i, R_i and A_{i + 1}. R_{i + 1} the signal points represent, and that ΔO_{i + 1} the angular change (positive, negative or zero) represents that in the phase shifter modulation has occurred (eg, QPSK).

Similarly, in the correlator <b>26</b> from the received signal y (n) a differential size

Δ_y(I) = y (i) y_* (Iu)

formed, where y_*(Iu) the complex conjugate of y (IU) is, and u is a sampling factor equal to the Simplification can be assumed to be the first

The differential correlation in the correlator <b>26</b> by forming

formed when the sampling factor u = 1, where L is the Number of symbols in the synchronization part S o is, and k = 0 is when the entire synchronization part in the Correlation is used. Higher accuracy is achieved if the received and demodulated signal in the A / D converter <b>24</b> is sampled when the Up-Samplingf is actuator u selected such that it &lt;1.

Therefore, the differential correlation involves the formation of Product of differential size Δ_s(I) from the known sync patterns S₀ (i) for each Signal point (each symbol), and the formation of the complex Conjugate of the differential size Dy (n + i), where n the time position of the samples of y (n) means the i and Time position for a given symbol in the Synchronization pattern D₀ (i).

The correlation function formed r (n) for each sample in the signal y (n) has an absolute maximum value amount | R (n) | max on for a given value of n, which the correct position for the sync pulse indicates, demodulator the <b>25</b> to activate, so

| R (n) |_Max R = (n₀)

wherein n₀ the time position of the synchronization pulse designated. Furthermore, r of the function (s) is a Estimation of the frequency error obtained, by Education arg [r (n)]. Specifically results below Formula for the frequency error Δ_f

wherein n₀ is the selected sampling time, and T_s the symbol time.

The correlator <b>26</b> consists of a signal processor known type, which is so programmed to the above stated calculations. The calculation unit <b>27</b> performs the calculation of | R (n) | max = r (n₀), and the calculation of arg [R (n₀)] of r (n) in a known manner.

demodulator <b>25</b> where in <b>Fig.</b> 5 receiver shown has no equalizer, and the only purpose of the obtained synchronization signal | r (n₀) | is the Finding the correct signal point in the by Phase shift modulated signal y (n). The value arg [r (n₀)] is supplied to the RF / IF modulator, so that the modulator frequency error .DELTA.f according to the can compensate for the above-mentioned relationship. It However, in a further demodulator in the receiver according to <b>Fig.</b> 5 (not shown) may be provided, as in the described and explained in Swedish patent application is, which was indicated in the introduction. It is hereby possible, the demodulator <b>25</b> in the absence of an equalizer use, and a differential correlation carried out in order to compensate for the frequency error. Of the Demodulator is then disconnected or turned off, and then is actuated, a demodulator, an equalizer with the is provided, in which a coherent correlation known manner is performed.