Pulse or digital communications – Pulse code modulation – Differential
Reexamination Certificate
1999-01-12
2003-04-08
Pham, Chi (Department: 2631)
Pulse or digital communications
Pulse code modulation
Differential
C375S220000, C375S224000, C375S286000, C375S320000, C375S326000, C375S330000, C375S353000, C375S363000
Reexamination Certificate
active
06546055
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for determining a carrier frequency offset encountered in wireless systems and using this information for synchronization purposes, and in particular to determining a large carrier frequency offset using symbols having a cyclic prefix.
BACKGROUND OF THE INVENTION
Wireless communications systems using radio-frequency (RF) signals for transmitting data are rapidly gaining popularity. These include both continuous data transmission systems, such as digital broadcast TV, as well as systems transmitting data at random times in bursts, e.g., wireless local area networks (WLANs)
In a typical RF data transmission system baseband data is transmitted by a transmitter which processes the baseband data and modulates it on a transmit carrier frequency f
ct
to generate an RF signal. The RF signal is usually composed of groups of symbols called data frames.
FIG. 1
illustrates a data frame
10
in the time domain. Frame
10
is composed of a number of consecutive data symbols
12
A through
12
M. Symbol
12
N is shown in more detail to reveal its useful portion spanning a symbol interval T
s
and its guard portion containing, e.g., a cyclic prefix, and spanning a guard interval T
g
. Guard interval T
g
precedes symbol interval T
s
. Therefore, each symbol
12
has a total duration of T
g
+T
s
seconds.
A receiver receives data frame
10
, demodulates symbols
12
and processes them to retrieve the transmitted baseband data. In order to properly perform this function the receiver has to achieve proper symbol timing and frequency synchronization with the transmitter. There are several aspects of synchronization that require careful attention for proper reception of data frame
10
.
First, the receiver must determine the exact timing of the beginning of each symbol
12
within frame
10
. If correct timing is not known, the receiver will not be able to reliably remove the cyclic prefixes and correctly isolate individual symbols
12
before performing further processing.
Second, the receiver has to also perform a generally more difficult task of determining and correcting for carrier frequency offset &Dgr;f
c
. Ideally, the receive carrier frequency f
cr
should exactly match the transmit carrier frequency f
ct
. If this condition is not met the mis-match contributes to a non-zero carrier offset &Dgr;f
c
. Depending on the type of symbols transmitted, inability to correct for carrier offset may prevent the receiver from recognizing symbols
12
. Orthogonal frequency division multiplexing (OFDM), although a robust technique for efficiently, transmitting data over a channel, is very susceptible to a non-zero carrier offset &Dgr;f
c
. The technique uses a plurality of sub-carrier frequencies within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). In case symbols
12
are generated by OFDM, the consequence of a carrier offset will be a loss of orthogonality between the OFDM sub-carriers and inter-carrier interference (ICI). This, in turn, will result in a high bit error rate (BER) in the recovered baseband data.
The third synchronization issue is of concern in OFDM communications. Specifically, the transmitter's sample rate has to be synchronized to the receiver's sample rate to eliminate sampling rate offset. Any mis-match between these two sampling rates results in an increased BER.
The transmission of data through a channel via an OFDM signal provides several advantages over more conventional transmission techniques. These advantages include:
a) Tolerance to multipath delay spread. This tolerance is due to the relatively long symbol interval T
s
compared to the typical time duration of the channel impulse response. These long symbol intervals prevent inter-symbol interference (ISI).
b) Tolerance to frequency selective fading. By including redundancy in the OFDM signal, data encoded onto fading sub-carriers can be reconstructed from the data recovered from the other sub-carriers.
c) Efficient spectrum usage. Since OFDM sub-carriers are placed in very close proximity to one another without the need to leave unused frequency space between them, OFDM can efficiently fill a channel.
d) Simplified sub-channel equalization. OFDM shifts channel equalization from the time domain (as in single carrier transmission systems) to the frequency domain where a bank of simple one-tap equalizers can individually adjust for the phase and amplitude distortion of each sub-channel.
e) Good interference properties. It is possible to modify the OFDM spectrum to account for the distribution of power of an interfering signal. Also, it is possible to reduce out-of-band interference by avoiding the use of OFDM sub-carriers near the channel bandwidth edges.
Although OFDM exhibits these advantages, prior art implementations of OFDM also exhibit several difficulties and practical limitations. The most important difficulty with implementing OFDM transmission systems involves timing and frequency synchronization between the transmitter and the receiver, as discussed above.
Prior art solutions to obtaining proper timing and synchronization in RF transmission systems depend, among other factors, on the transmission technique, i.e., type of symbol keying. In simple systems appropriate phase lock loops (PLLs), or zero-crossing circuits can be used in the receiver for determining the transmit carrier frequency f
ct
. In addition, or independently of these solutions, data frame
10
may include training symbols which are recognized by the receiver and used to achieve timing and synchronization.
Specifically, in the case of OFDM signals, several solutions have been proposed. In U.S. Pat. No. 5,444,697, Leung et al. suggest a technique for achieving timing synchronization of a receiver to an OFDM signal on a frame-by-frame basis. The method, however, requires that a plurality of the OFDM sub-carriers be reserved exclusively for data synchronization, thus reducing the number of sub-carriers used for encoding and transmitting data. Furthermore, Leung does not suggest a technique for correcting the carrier frequency offset or sampling rate offset. Finally, Leung's technique requires a loop-back to determine the phase and amplitude of each sub-channel, thereby rendering the technique unsuitable for broadcast applications such as digital TV.
In U.S. Pat. No. 5,345,440, Gledhill et al. present a method for improved demodulation of OFDM signals in which the sub-carriers are modulated with values from a quadrature phase shift keying (QPSK) constellation. However, the disclosure does not teach a reliable way to estimate the symbol timing. Instead, assuming approximate timing is already known, it suggests taking a fast Fourier transform (FFT) of the OFDM signal samples and measuring the spread of the resulting data points to suggest the degree of timing synchronization. This technique, however, requires a very long time to synchronize to the OFDM signal since there is an FFT element in the timing synchronization loop. Also, their method for correcting for carrier frequency offset assumes that timing synchronization is already known. Furthermore, the achievable carrier offset acquisition range is limited to half a sub-channel bandwidth. This very limited range for carrier offset correction is insufficient for applications such as digital television where carrier frequency offsets are :likely to be as much as several tens of sub-carrier bandwidths. Finally, the disclosure does not teach a method for correcting for sampling rate offset.
In U.S. Pat. No. 5,313,169, Fouche et al. suggest a method for estimating and correcting for the carrier frequency offset and sampling rate offset of a receiver receiving an OFDM signal.
Cox Donald C.
Schmidl Timothy M.
Kumar Pankaj
Lumen Intellectual Property Services Inc.
Pham Chi
The Board of Trustees of the Leland Stanford Junior University
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