Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1999-08-23
2003-06-24
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S259000, C375S295000, C375S316000, C370S503000
Reexamination Certificate
active
06584164
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming a training sequence in communication in a communication system where information is transmitted at one or several carrier frequencies, the training sequence being used at least as a synchronization signal. The invention relates further to a transmitter for transmitting information in one or several data frames at one or several carrier frequencies, the transmitter comprising means for forming at least one training sequence and attaching the same to the data frame to be transmitted, and the training sequence being intended for synchronizing the receiver. Furthermore, the invention relates to a receiver comprising means for synchronizing the receiver to a received signal which at the transmitting stage is provided with at least one training sequence, as well as a communication system comprising a transmitter for transmitting information into a communication channel in one or several data frames, and means for forming at least one training sequence and attaching the same to a data frame to be transmitted, a receiver for receiving transmitted information from the communication channel, and means for synchronizing the receiver.
2. Description of the Related Art Including Information Disclosed Under 37 CRF 1.97 and 1.98
Transmission of signals from a transmitter to a receiver via the radio channel is subject to a number of interference factors. For example, weather changes cause so-called fading, wherein at the receiving end, the strength of the signal varies and may prevent correct reception of information. Other interference factors include channel noise and ignition interference caused by electric devices. Particularly in urban areas, one significant interference factor affecting the reliability of data transmission is multipath propagation which is caused by the fact that the signal enters the receiver via several different routes, and the path traveled by these different signals can differ in length. As a result, the different signals enter the receiver in different phases, thereby disturbing correct transmission of information. The situation is even more complicated when the transmitter or the receiver is moving, because the situation changes all the time, wherein synchronization of the receiver is difficult.
One method for reducing the effect of these said interference factors is to transmit information in parallel. Thus, the bits of information to be transmitted are divided into groups, and each group is demodulated at subcarriers of different wavelengths. At the receiving end, these signals of different carrier frequencies are received and modulated, and subsequently connected to a bit string corresponding to the original information that was transmitted. In such a method, it is possible to reduce the bit rate of each group and still obtain the same final bit rate. For example, in a situation where eight subcarriers are used, the bit rate of each group can be reduced to an eighth part. Thus, particularly interference of short duration affect the transmission of information substantially less than in a situation where information is transmitted in series mode, i.e. modulated by one carrier. One method used for such serial transmission of information is the so-called OFDM modulation technique, i.e. orthogonal frequency division multiplexing. Below in this description, this technique will be referred to by the abbreviation OFDM.
The information to be transmitted can be any information as such, including audio signals, video signals, data files, text messages, etc. However, information in analog form is converted into binary form with an analog/digital converter before modulation. The binary information is divided into blocks of preferably fixed size. Each of these blocks is coded and transmitted as one data frame. Usually, such a block is further divided into smaller groups, each group consisting of typically two to five bits. Each of these groups is modulated at one subcarrier frequency. In the modulation, the amplitude and phase of the subcarrier frequency is set to a value determined on the basis of the value of the bits in the group. Thus, in a situation where the group consists of m bits, the modulation results in 2
m
different alternatives for the phase and the amplitude. In this description, these alternatives are called the constellation. For example, in quadrature amplitude modulation (QAM), the size of the modulating group is given in the form 2
m
-QAM, for example 4-QAM, 16-QAM or 64-QAM. It is obvious that also other modulation techniques can be used in connection with this invention.
In the receiver, each subcarrier frequency is demodulated to find out which symbol was transmitted in each group. This is conducted by examining from the received signal at each subcarrier frequency the phase and amplitude of the signal, to decide which signal point is closest to the received signal point. This signal point determines which symbol was probably transmitted. Subsequently, the original block can be reconstructed by combining the groups received at different sub-carrier frequencies and demodulated.
Irrespective of whether the information was coded according to either or both the phase and the amplitude of the signal transmitted, it is necessary to synchronize the receiver with the received signal. Typically, each block must be synchronized independently, because in many systems the receiver does not know the exact moment of transmission. Therefore, each block to be transmitted must include synchronization data, on the basis of which the receiver tries to synchronize itself with the received signal and to find out the information transmitted. To eliminate phase and frequency errors, the transmitter modulating frequencies should be kept exactly constant, and correspondingly, the demodulation frequencies in the receiver should be at the correct frequency and the frequency should be as constant as possible, which requires the use of high-quality components as well as relatively complicated circuits. Known synchronization methods are often based on the use of training components which are typically identical. In synchronization, it is known to use an optimal matched filter (MF) whose coefficients are matched with the training parts of the received signal. The coefficients are usually fixed and they must be known to the receiver. The design of such matched filters requires complicated computing, and furthermore, each OFDM symbol must be equipped at the transmission stage with a guard time to eliminate the effect of multipath propagation. In the case of a matched filter, multipath propagation is not taken into account in the filter coefficients. If the guard time is not added to the OFDM symbol, the output of the matched filter includes extra peaks formed by incoming signals in the receiver at different delay times. Another known solution is the use of a simpler synchronizer, where subsequent similar training parts are correlated with each other, wherein multipath propagation of the channel affects both of the symbols to be correlated in the same way. The receiver does not need to know the training parts.
In a simpler receiver synchronizer, a correlation of the training parts is determined, resulting in a step function in the case that the training parts are identical. However, it is very difficult to deduce the correct timing and frequency, particularly with varying power levels of the signal to be received. Timing and frequency errors are caused by different unideal conditions, the oscillator, the communication channel, etc. Bit errors occur if timing and frequency errors are not corrected in the receiver. In the case of a timing error, Fourier conversion is made at a wrong location. The frequency error leads to rotation of signal points (constellation points). The transmitter and the receiver must be synchronized with each other, to el
Corrielus Jean
Nokia Corporation
Perman & Green LLP
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