Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-07-08
2003-05-13
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Reexamination Certificate
active
06563856
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to communication systems, and more particularly to an apparatus for achieving frame synchronization in a digital receiver.
2. Related Technology
In radio transmission, information is conveyed by uniformly spaced pulses and the function of any receiver is to isolate these pulses as accurately as possible. However, due to the transmission channel, the received signal has undergone alterations during transmission, and a complete estimation of certain reference parameters is necessary prior to data detection. These unknown parameters can cover such factors as the optimum sampling location, the start of a data packet (for burst mode transmission) or of a frame marker for continuous transmission, or the phase offset introduced in the channel or induced by instabilities between the transmitter and receiver oscillators. The extraction of the phase or frequency of the incoming carrier is known as phase/frequency estimation. Alternatively, non-coherent demodulation such as differential demodulation can be applied where the phase difference between one data symbol and the next is assumed constant.
In traditional analog receivers, synchronization of the phase and frequency is typically performed in the intermediate frequency (IF) stage of the receiver. However, the IF analog components are costly and prone to undesirable variations over time. Flexibility in the design of the receiver synchronization unit has increased in recent times with the advent of increasingly powerful silicon chips, which are considerably cheaper and more stable. This has led to a reduction in the amount of signal processing being performed at IF. In the current state of the technology, IF sections are reduced to an asynchronous sampling device for analog to digital conversion and a free-running oscillator for down conversion to baseband. The term “baseband” refers to when the carrier frequency has been completely removed from the received signal and the signal is centered at DC (0 Hz). In typical digital receivers, the asynchronous sampling device operates at a rate of two or more samples per symbol. The term “symbol” is used in this context to refer to transmitted signals that are phase modulated with discrete phase and or amplitude relationships. Each assigned phase and or amplitude relationship is a symbol that is subject to detection at the receiver.
In communication systems, information is transmitted either continuously or in bursts. In both cases, the data from the information source at the transmitter is sub-divided into units known as frames. The purpose of data frames in continuous transmission is to provide a marker to track the received data at the end-user destination as well as to organize the data stream into uniformly sized groups of bits. Even more importantly, frame synchronization is essential in any system utilizing block error control coding (H. Meyr, M. Moeneclaey and S. Fechtel, “Digital Communication Receivers: Synchronization, Channel Estimation and Signal Processing”, John Wiley Publishers, 1998, pp. 542-545) wherein codewords are identified with respect to the frame synchronization reference. Moreover, frame synchronization is very important in continuous transmission when frames can be lost due to adverse channel conditions if the receiver cannot track and remove the condition quickly enough. When the receiver settles again, there should be some mechanism to indicate to the receiver when the detected data is meaningful. This function is performed by the periodic insertion of frame markers and midambles in packet transmission to indicate the start of valid data and to assist in updating the parameter acquisition and tracking mechanism. In burst mode transmission, the data bursts are received starting at a random location within a predefined time slot. The purpose of frame synchronization in this case, as well as before for continuous data, is to estimate the location of the start of the data as well as assisting in the estimation of the unknown parameters for the receiver detection.
The most common technique used in frame synchronization is the insertion of fixed data patterns at the transmitter, known as frame markers, at the start of the data frame to assist synchronization. The purpose of frame synchronization is to isolate the position of the start of the arbitrary data stream, which follows these frame markers, as illustrated in
FIG. 1. A
compromise is necessary between the length of the frame marker to ensure minimal loss of synchronization and the length of the associated information bits in the frame to achieve an efficient data throughput. Data throughput refers to the amount of information bits sent in a frame with respect to the total number of bits sent in the frame.
To achieve frame synchronization at the receiver using the frame marker method, the receiver searches the entire data stream for a sequence matching the known frame marker inserted at the transmitter. From a signal theory perspective, the receiver performs a cross correlation of the frame marker with the received signal. If the receiver is not in synchronization with the framing pattern, the accumulated correlation will be low. When the receiver comes into frame synchronization, however, the correlation should be nearly perfect, blemished only by an occasional detection error. Synchronization is achieved by implementing a filter with the values of the coefficients at the filter taps matched to the frame marker sequence inserted at the transmitter. Depending on the sampling rate N, the filter taps are spaced N delays apart to isolate the correct sample at which the frame marker sequence ends (the value of N is the same). Matching the coefficients at the transmitter and receiver ensures that the correlation energy is maximized at the filter output when the two sequences coincide. The frame marker sequence in the literature is also referred to as a unique word or synchronization sequence; hereafter the frame marker sequence is referred to as the unique word due to its special correlation properties.
The unique word sequence is chosen for its near-optimum correlation properties, a Dirac (or impulse) auto correlation characteristic is ideal for frame synchronization. However, in practice, the auto-correlation characteristic of a unique word sequence contains a strong peak where the two sequences coincide as well as sidelobes at fixed intervals on both sides of the main peak as illustrated in FIG.
2
.
FIG. 2
shows the situation where the input signal is sampled at one sample per symbol, which implies there is only one sample on the main lobe of the correlation. A good unique word has the property that the absolute value of its correlation sidelobes is small with respect to the absolute value of the main correlation lobe. A correlation sidelobe is the value of the correlation of the unique word with a time-shifted version of itself.
The next stage in any receiver is the detection of the correlation peak. In the case of complex modulation schemes where data is transmitted on both an In-phase (I) branch and a Quadrature (Q) branch, the unique word is simultaneously transmitted on both branches. For complex modulation schemes where the unique word is repeated on both the I and Q branches, a complex matched filter is unnecessary. Instead two real matched filters outputs for both the in-phase and quadrature components are combined to yield the equivalent complex matched filter output. This avoids unnecessary complexity in the receiver implementation. Therefore, to obtain the overall autocorrelation function, the magnitude or magnitude squared of the I and Q correlation outputs is taken. The magnitude of the correlation eliminates the effect of any phase offset present on the received signal at the input to the correlators. This technique gives reliable results for the case where the phase offset is of the order of 10
−3
of the inverse of the midamble (or unique word if no midamble is used) duration. The next step is to pass the absolute v
Hatim Baya
Lakkis Ismail
O'Scolai Cathal
O'Shea Deirdre
Safavi Saeid
Burd Kevin M
Pham Chi
Wireless Facilities, Inc.
LandOfFree
Frame synchronization and detection technique for a digital... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Frame synchronization and detection technique for a digital..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Frame synchronization and detection technique for a digital... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3011686