Multiplex communications – Generalized orthogonal or special mathematical techniques – Fourier transform
Reexamination Certificate
2000-02-22
2003-11-18
Ton, Dang (Department: 2666)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Fourier transform
C370S208000, C370S281000, C370S290000, C370S292000, C370S295000, C370S310000, C375S232000, C375S260000, C375S350000, C375S355000, C375S362000
Reexamination Certificate
active
06650617
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processing orthogonal frequency division multiplexed (OFDM) signals.
BACKGROUND OF THE INVENTION
A wireless LAN (WLAN) is a flexible data communications system implemented as an extension to, or as an alternative for, a wired LAN within a building or campus. Using electromagnetic waves, WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs. Some industries that have benefited from the productivity gains of using portable terminals (e.g., notebook computers) to transmit and receive real-time information are the digital home networking, health-care, retail, manufacturing, and warehousing industries.
Manufacturers of WLANs have a range of transmission technologies to choose from when designing a WLAN. Some exemplary technologies are multicarrier systems, spread spectrum systems, narrowband systems, and infrared systems. Although each system has its own benefits and detriments, one particular type of multicarrier transmission system, orthogonal frequency division multiplexing (OFDM), has proven to be exceptionally useful for WLAN communications.
OFDM is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit data. These sub-carriers are arranged for optimal bandwidth efficiency compared to conventional frequency division multiplexing (FDM) which can waste portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier.
The transmission of data through a channel via OFDM signals also provides several other advantages over more conventional transmission techniques. Some of these advantages are a tolerance to multipath delay spread and frequency selective fading, efficient spectrum usage simplified sub-channel equalization, and good interference properties.
Referring now to
FIG. 1
, an OFDM signal
10
is transmitted as blocks of user data
12
separated by guard intervals known as cyclic prefixes
14
. A cyclic prefix
14
is a copy of a portion of an adjacent block of user data
12
and is used to reduce Inter-Symbol Interference (ISI) caused by multipath fading. More particularly, only cyclic prefixes
14
, as opposed to user data
12
, are effected by ISI, as is known by those skilled in the art. Thus the removal of cyclic prefixes
14
by an OFDM receiver removes the effects of ISI from the received OFDM signal.
At the OFDM receiver a received OFDM signal
10
is digitized or sampled to convert the OFDM signal from an analog to a digital signal. Afterwards, the OFDM receiver applies Fast Fourier Transform (FFT) windows to the OFDM signal to remove the cyclic prefixes from a received OFDM signal. Ideally, an OFDM window
16
only passes user data
12
to an FFT unit
18
and discards cyclic prefixes
14
. However, if there is a sampling frequency offset between the OFDM transmitter and the OFDM receiver, FFT window
16
may drift beyond the boundaries of user data
12
. If this drift occurs, as shown in
FIG. 2
, a portion or sample
20
of cyclic prefix
14
may be passed to FFT unit
18
and a portion or sample
22
of user data
12
may be lost. As a result, the window drifting effect may result in the presence of ISI in a received OFDM signal. Furthermore, an offset of FFT window
16
will result in a phase rotation in the output of FFT unit
18
. The rotation occurs because a time shift in the time domain results in a phase rotation in the frequency domain. The phase rotation may generate errors in the user data recovered by the OFDM receiver.
One way to correct for the drifting effect is to lock the frequency of the receiver's sampler or ADC to the transmitter sampling frequency using a phase-locked loop. Turning to
FIG. 3
, an exemplary phase-locked loop configuration
24
includes an ADC
26
that samples a received OFDM signal. An FFT window unit
28
receives the OFDM samples, removes cyclic prefixes, and passes user data to a FFT unit
30
, as discussed above. A pilot extractor
32
extracts pilots imbedded in the. user data and passes the pilots to a phase difference calculator
32
. A pilot is a reference signal (having a known phase) that is embedded in an OFDM symbol on a predetermined subcarrier. Phase difference calculator
32
calculates the phase difference between the pilots within the OFDM symbols and passes the calculated difference to a sampling offset detector
36
. Sampling offset detector
36
detects a sampling offset between the transmitter and receiver using the calculated difference and outputs the sampling offset to a digital phase-locked loop
38
. Digital phase-locked loop
38
controls the sampling clocks of ADC
26
and ensures consistent FFT window positioning throughout the reception of the transmission once digital phase-locked loop
38
has locked.
Although PLL configuration
24
ensures consistent FFT window positioning once digital phase-locked loop
38
has locked, PLL configuration
24
has several drawbacks. One drawback is that PLL configuration
24
may not correctly position the FFT window due to noise and channel effects. The incorrect positioning (i.e., window offset) may cause a phase rotation in the output of FFT unit
30
that, in turn, may cause errors in the user data recovered by the OFDM receiver. Another drawback is that digital phase-locked loop
38
of PLL configuration
24
is costly to implement.
If the local sampling clock of the OFDM receiver has a small offset with respect to the transmitter sampling frequency it may be advantageous (e.g., to reduce costs) to remove the digital phase-locked loop and utilize a free-running local clock. However, by utilizing a free-running clock without a phase-locked loop, a small sampling offset, over time, can accumulate and shift the FFT window beyond the user data boundaries. As noted above, the FFT window shift may introduce errors, such as ISI, into the user data portion of a received OFDM symbol. The present invention is directed to the correction of this problem.
SUMMARY OF THE INVENTION
An Orthogonal Frequency Division Multiplexing (OFDM) receiver that extracts pilots from a fast Fourier transformed and equalized OFDM signal, and processes the extracted pilots to derive an FFT window adjustment factor and an associated equalizer tap adjustment value. The OFDM receiver simultaneously controls the position of an FFT window and the phase of equalizer taps using the FFT adjustment factor and equalizer tap adjustment value.
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J. A. C. Bingham, “Multicarrier Modulation For Data Transmission: An Idea Whose Time Has Come”, May 1990—IEEE Communications Magazine.
Andréas Czylwik,Degradation of Multicarrier and Single Carrier Transmission with Frequency Domain Equalization due to Pilot-Aided Channel Estimation and Frequency Synchronization, IEEE Telecommunications Conference, vol. 1, No. 3, Nov.
Belotserkovsky Maxim B.
Litwin, Jr. Louis Robert
Duffy Vincent E
Hom Shick
Kurdyla Ronald H
Thomson Licensing S.A.
Ton Dang
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