HK1072669A - A method for synchronization of received signals - Google Patents

Description

Method for synchronizing received signals

Technical Field

The present invention relates generally to digital transmission systems. And more particularly to a method for improving receiver synchronization in a digital transmission system using orthogonal frequency division multiplexing.

Background

The increasing popularity of digital multimedia applications (e.g., short message service, internet, web television, etc.) has created an ever-increasing demand for digital broadband communication systems. Accordingly, various transmission techniques are utilized to enable more and more user data to be transmitted over a limited frequency band, thereby increasing the throughput and capacity of the system. However, these wideband transmission techniques are very sensitive to transmission impairments such as noise, adjacent channel interference, intersymbol interference, multipath effects and other impairments.

Orthogonal Frequency Division Multiplexing (OFDM) has recently become more popular (particularly in the united states) and has been used to help address these negative effects from multi-user digital broadband transmission. OFDM, which is selected as the transmission method of the european radio (DAB-digital audio broadcasting) and TV (DVB-T-digital video broadcasting) standards, is a multi-carrier transmission technique that divides the available spectrum into a number of carriers, each modulated by a low-rate data stream. Similar to Frequency Division Multiple Access (FDMA), OFDM enables multi-user access by subdividing the available bandwidth into multiple narrowband channels that are assigned to users.

However, OFDM uses the spectrum more efficiently by separating the channels more densely (and in fact overlapping). Dense separation of user channels is achieved by preventing interference between closely spaced carriers by making all carriers orthogonal to each other. This orthogonal relationship is established by means of each carrier having an integer period over the symbol period (using IFFT-inverse fast fourier transform). As shown in fig. 1, due to this periodicity, the spectrum of each carrier (a, b, c, d) is zero at the center frequency of every other carrier in the system, so that there is no interference between carriers and the carriers are theoretically as densely spaced as possible. At the receiving end, each carrier (or subcarrier) can be estimated at a specific frequency (or time period) and all other carrier signals should be zero (eliminating adjacent carrier interference). In addition, as shown in fig. 2, to help eliminate multipath effects, a guard period (guard period)205 is added between the transmitted symbols (information conveyed on the carrier), which is the most common combination of cyclic expansion of the symbols and a zero amplitude signal (no signal period).

One key factor in maintaining the orthogonal relationship between the carriers is the synchronization (operating on the same modulation frequency and time scale) between the transmitter and the receiver. This no-signal (zero) period of the transmitted OFDM signal is typically detected by comparing the power (energy) of the received OFDM signal with a number of predetermined threshold levels, thereby maintaining (restoring) synchronization at the receiving end. Fig. 3 shows a preprocessed signal 805 and a processed (filtered) signal 810 that includes zero period detection 815.

Typically, in practical applications of this technique, the detected maximum and minimum signal levels are set to thresholds and then negative and positive adjustments are made, respectively, to find the exact point (for timing reference, the rise period) at which the (part of the) transmitted data frame followed by the guard period 815 begins. For example, referring to fig. 3, a first maximum level (threshold) 10 of the filtered signal 810 may be found at time 380, and a first minimum level (threshold) 5 of the filtered signal 810 may be found at time 255. Thereafter, the maximum level threshold may be repeatedly decreased (e.g., initially decreased by 50%) and the minimum level threshold may be repeatedly increased (e.g., initially increased by 25%) to find the exact starting point for transmitting the data frame, thereby restoring time synchronization. Further description of using successive calculations of power (energy) thresholds to detect the end of a zero period is provided in U.S. patent No.6,246,735, the disclosure of which is incorporated herein by reference. However, this threshold adjustment technique requires complex calibration of multiple thresholds and requires computation of the power (energy) level to make it sensitive to noise and sensitive to the shape of the no-signal period.

Therefore, in view of the shortcomings of the current synchronization methods, there is a need to provide a synchronization method that can efficiently recover a timing reference (e.g., no signal period) from a received OFDM signal in the presence of noise, and that does not depend on the shape of the no signal period, nor does it require complex calibration of multiple energy level thresholds.

Summary of The Invention

It is an object of the present invention to provide a method for efficiently restoring synchronization at a receiving end of a digital transmission system. A digital signal comprising a transmitted data portion and a guard period is received at a receiving end. The signal envelope of the received data signal is determined and filtered to find the center of the guard period, which provides a time reference for the received digital signal. Embodiments of the invention described herein may be used for optimized operation of a digital transmission system by efficiently recovering synchronization from a received digital signal during noisy conditions independent of the shape of the signal and without the need for complex threshold calculations.

Brief Description of Drawings

Fig. 1 is an exemplary OFDM spectrum diagram.

Fig. 2 is an exemplary OFDM time domain signal diagram.

Fig. 3 is a diagram of an OFDM time domain signal before and after filtering.

Fig. 4 is a block diagram of an exemplary digital transmission system according to an embodiment of the present invention.

Fig. 5 is a flow chart illustrating a process for synchronization recovery according to an embodiment of the present invention.

Fig. 6 is a diagram of a digital time domain signal before and after filtering according to an embodiment of the invention.

Fig. 7 is a block diagram of a receiver according to an embodiment of the present invention.

Detailed Description

Fig. 4 shows an exemplary digital transmission system 400 using Orthogonal Frequency Division Multiplexing (OFDM). Transmission system 400 includes a transmitter 405, a receiver 410, and a radio channel 415. During operation, an OFDM transmission signal is generated by inputting user data into modulator 420 to modulate the input data at a predetermined carrier frequency using a modulation scheme (e.g., OPSK, QAM, etc.) corresponding to predetermined amplitude and phase requirements of the carrier. The frequency signal (carrying the data) is then converted back to a time domain signal using an inverse fast fourier transform 425, the inverse fast fourier transform 425 also ensuring orthogonality of each carrier. The time domain signal is then sent via a D/a (digital to analog) converter 430 to an RF carrier for transmission to the receiver 410 via a radio channel 415. The transmitter 405 may also include a frequency converter to change the frequency of the RF carrier used for transmission.

Transmitter 405 may also include a serial-to-parallel converter (not shown) disposed before modulator 420 that receives input user data (in the form of a serial data stream) and converts/formats the input user data into a predetermined word size for transmission (e.g., 2 bits/word for OPSK) and into a parallel format. The user data is then transmitted in parallel by assigning each data word to a carrier (channel) in the transmission. Also, a parallel-to-serial converter (not shown) may be inserted after the IFFT 425 before actual transmission, so as to convert a plurality of data streams into one signal waveform for transmission. Further, the transmitter 405 may further include a guard interval inserting unit (not shown) disposed after the parallel-to-serial converter, which inserts a guard period (no-signal period) at the start of each symbol transmitted by the carrier. In addition, the receiver 410 may include a guard period elimination unit and a serial-to-parallel converter (both not shown) to perform the reverse process.

Typically, the radio channel 415 may add a negative impact to the transmitted signal including noise, multipath effects, and signal fading (power clipping). The transmitted signal is received at an analog front end 407, which analog front end 407 may include or be subsequently coupled to a down converter to convert the received RF carrier signal to a lower frequency. a/D (analog-to-digital) converter 445 then converts the received analog RF carrier signal back to digital form. Next, a fast fourier transform 440 is used to transform the digital signal back to the frequency domain and a demodulator 435 is used to extract the incoming user data signal.

Fig. 5 depicts a flowchart 500 showing the process of synchronization recovery according to an embodiment of the invention. These steps are performed in the receiving end (e.g., receiver) of the digital transmission system. At step 505, a transmitted digital signal (from a transmitter) is received. The digital signal is a time domain signal carrying transmission data (e.g., user data) and a guard (zero) period (or frequency band) during which no signal is transmitted before being corrupted by the channel. The transmission data and the guard period may be carried in a time slot of a Time Division Multiplexing (TDMA) transmission system. The guard periods may constitute gaps between transmitted data periods of the digital signal.

At step 510, a signal envelope of a received digital signal is determined based on sampling transmitted data and a guard period over a plurality of time slots, the transmitted data and the guard period both being transmitted within one period of the digital signal. The receiver may use a predetermined time slot duration (period) from the transmitter as an initial timing reference to help generate the signal envelope. The power (energy) of the sampled signal period may be calculated and averaged over a number of time slots. For example, the received digital signal may be re-sampled (e.g., 1200 samples) over a period of the digital signal (e.g., 375 microseconds), where the digital period includes a guard band (period-e.g., 50 microseconds), and each sample amplitude is stored in a buffer. The sampling process may be repeated a number of times (e.g., 10-12) wherein the resulting sampled amplitudes are averaged in the process to produce a signal envelope.

At step 515, the signal envelope may be filtered out. The filter may be a matched filter that matches the guard period (gap). For example, a rectangular filter (the impulse response is rectangular) may be used to generate the filtered signal, the rectangular filter being approximately matched to the filter for substantially rectangular gaps and having a length (time period) approximately (substantially) equal to the length of the guard period of the digital signal. The filtered signal can be used as a basis for resolving the synchronization timing reference of the receiver.

At step 520, a minimum level (e.g., power level) of the filtered signal envelope is determined, which indicates the center of the zero (gap) period. This minimum level provides a time reference for the received digital signal. Thereafter, the timing of the receiver in the digital transmission system can be adjusted to correspond to the timing of the transmitter in order to find the start of transmitting a data frame.

Fig. 6 shows a time-domain digital signal diagram 600 before or after filtering according to an embodiment of the invention. As shown in fig. 6, the pre-filtered digital signal 605 includes a guard period 610 and a transmit data portion 615 comprising a plurality of time slots. After the digital signal 605 has been processed as in fig. 5, the filter outputs a filtered digital signal envelope 620. The signal envelope 620 includes a gap (zero) portion having a minimum (power) level 625. The minimum power level 625 indicates the center of the zero period and the gap edge (the beginning of the data portion transmitted) may be determined by offsetting by half the gap width. Thus, after finding the gap center, the start of the transmitted data frame can be determined (looking for the slot synchronization signal) by shifting the gap center by half the gap width to the start of the transmitted data period. The gap width of the guard period may be predetermined from the transmitter. For example, the minimum level of the guard period (gap center) may be found at time 255 of fig. 6. Adding the time offset 45 moves the reception to the beginning of the transmitted data frame (portion), just as at time 300 of receiving the digital signal 605 shown in fig. 6. After the gap center is determined to provide a time reference for the received digital signal, the receiver can achieve time slot (time) synchronization. Also, the rectangular filter may be a moving average filter that spans about the width of the gap. The moving average filter takes into account the continuously calculated average as the sample values are taken from one time slot to the next without waiting for the full sample of the subsequent time slot to be taken. For example, if the first slot takes 100 samples to calculate an average, the first sample of the subsequent slot may be added (while subtracting the first sample of the previous slot, the earliest sample taken) to continue averaging 100 samples without waiting for the next 100 samples to be taken. The time span of the filter is about the same as the time interval (period) of the guard band.

In addition, filtering the received signal 605 (performed according to the process of fig. 5) may cause a timing delay (time offset to the received signal 605) of the signal 620. This timing delay may be corrected immediately by the receiver before further processing, or may be factored in when calculating the time offset to move from the slot center 625 to the beginning of the transmitted data frame.

Fig. 7 shows a block diagram of a receiver 700 that may perform the method of fig. 5 in accordance with an embodiment of the invention. The analog front end 702 and the a/D converter 705 perform step 505 by receiving an analog RF carrier (from a transmitter) and converting the RF signal to digital form. Also, the analog front end may include or be followed by a down-converter to convert the received RF carrier signal to a lower frequency prior to digital conversion by A/D705. After the digital conversion, the frequency tracking/time synchronization unit 710 receives the converted signal from the converter 705, determines a signal envelope of the received digital signal according to a transmission data period and a guard (zero) period sampled over a plurality of time slots, and thus performs step 510. A filter 715 is included in unit 710 that may perform step 515 by filtering out the signal envelope using a rectangular filter (the impulse response is rectangular) spanning a time of substantially zero cycle length. Unit 710 then performs step 520 by determining a minimum level of the filtered signal envelope indicating the center of the zero period and provides a time reference for the received digital signal. The receiver 700 then achieves timing and frequency synchronization recovery for the transmitted data frames. The frequency tracking performed by unit 710 may be implemented in software, or a feedback control network may be used to adjust the frequency of the receiver oscillator to match the transmit frequency of the received signal from the transmitter. As shown in fig. 7, the time synchronization performed by unit 710 may be implemented using a programmable Digital Signal Processor (DSP)713, a Field Programmable Gate Array (FPGA), a microcontroller, or other type of programmable control device to perform step 510 and 520 shown in fig. 5 to provide a time reference (e.g., detection of a guard period) for the received digital signal.

FFT 720 and demodulator 725 then follow unit 710, transform the time domain digital signal to the frequency domain, and use demodulator 435 to extract the incoming user data signal originally transmitted (carried) by the digital signal. A transmitter may be used to transmit a digital signal via an RF carrier to receiver 700 to produce an end-to-end digital transmission system in which receiver 700 performs the steps described with respect to fig. 5 to restore synchronization.

Embodiments of the present invention may be used in a variety of applications. The process steps and digital transmission system described herein may be applied in DVB/DAB applications, wireless/cellular applications, and also allow for the recovery of synchronization for weaker signals (below a predetermined threshold for regular traffic). In particular, for wireless/cellular applications, the embodiments of the invention shown in fig. 5-7 may be used by the receiver of a remote wireless unit. The remote wireless unit may include, but is not limited to: handheld or other wireless telephones, laptops, Personal Digital Assistants (PDAs), pagers, and other remote wireless units that can be used by a user to communicate within a wireless communication system.

Although the invention has been described primarily using specific embodiments, it will be understood by those skilled in the art that modifications and variations can be made to the invention without departing from the spirit and scope of the invention. Also, the methods disclosed herein are not to be limited by what has been particularly shown and described, except as indicated by the appended claims.

Claims (20)

1. A method for obtaining synchronization in a digital transmission system, comprising the steps of:

receiving a digital signal carrying transmission data and a guard period, during which no signal is transmitted,

a signal envelope of the received digital signal is determined,

the signal envelope is filtered using a matched filter having a span corresponding to the guard period,

a guard period is determined from the filtered signal envelope to provide a time reference for the received digital signal.

2. The method of claim 1, wherein the step of determining the signal envelope comprises sampling the transmitted data and the guard period.

3. The method of claim 1, wherein the filtering step comprises filtering the signal envelope using a filter having a length approximately equal to a predetermined length of a guard period of the received digital signal.

4. The method of claim 1, wherein the step of determining a guard period comprises determining a minimum level of the filtered signal envelope, thereby indicating a center of the guard period.

5. The method of claim 1 wherein the digital transmission system uses orthogonal frequency division multiplexing for transmission.

6. The method of claim 5 wherein the digital transmission system is a wireless communication system and said method is performed by a remote wireless unit.

7. The method of claim 1, wherein the digital signal is received at a signal level below a predetermined threshold for regular traffic in the digital transmission system.

8. The method of claim 1, wherein the digital transmission system is one of a digital audio broadcasting system or a digital video broadcasting system.

9. An apparatus for obtaining synchronization in a digital transmission system, comprising

A controller programmable to perform the steps of:

determining a signal envelope of a received digital signal, the digital signal carrying transmission data and a guard period, during which no signal is transmitted,

the signal envelope is filtered using a matched filter having a span corresponding to the guard period,

a guard period is determined from the filtered signal envelope to provide a time reference for the received digital signal.

10. The apparatus of claim 9, wherein said step of determining a signal envelope includes sampling transmit data and a guard period.

11. The apparatus of claim 9, wherein the filtering step comprises filtering the signal envelope using a filter having a length approximately equal to a predetermined length of a guard period of the received digital signal.

12. The apparatus of claim 9, wherein the step of determining a guard period comprises determining a minimum level of the filtered signal envelope, thereby indicating a center of the guard period.

13. The apparatus of claim 9 wherein the digital transmission system uses orthogonal frequency division multiplexing for transmission.

14. The apparatus of claim 13 wherein the digital transmission system is a wireless communication system and said steps are performed by a remote wireless unit.

15. A digital transmission system comprises

A transmitter for transmitting a digital signal carrying user data using an RF carrier;

a receiver, comprising:

a controller programmable to perform the steps of:

determining a signal envelope of a received digital signal, the digital signal carrying transmission data and a guard period during which no signal is transmitted;

determining a signal envelope of the received digital signal;

the signal envelope is filtered using a matched filter matched to the guard period,

a guard period is determined from the filtered signal envelope to provide a time reference for the received digital signal.

16. The system of claim 15, wherein said step of determining a signal envelope includes sampling the transmitted data and the guard period.

17. The system of claim 15, wherein the filtering step comprises filtering the signal envelope using a filter having a length approximately equal to a predetermined length of a guard period of the received digital signal.

18. The system of claim 15, wherein the step of determining a guard period comprises determining a minimum level of the filtered signal envelope to indicate a center of the guard period.

19. The system of claim 15 wherein the digital transmission system uses orthogonal frequency division multiplexing for transmission.

20. The system of claim 19 wherein the digital transmission system is a wireless communication system and said steps are performed by a remote wireless unit.