Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
2000-01-20
2004-10-19
Pham, Chi (Department: 2663)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C375S346000
Reexamination Certificate
active
06807145
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of transmission systems. More particularly, the present invention relates to wireless transmission systems.
BACKGROUND OF THE INVENTION
Orthogonal frequency division multiplexing (OFDM) systems are employed or proposed in many commercial wireless data transmission systems, for example, cellular telephones, digital audio broadcasting (DAB), and high-definition television (HDTV). These applications require the transmission of data at very high rates ranging from a few hundred kb/s to a few Mb/s.
OFDM systems are well known in the art of signal transmission. U.S. Pat. No. 5,610,908 to Shelswell et al., entitled “Digital Signal Transmission System Using Frequency Division Multiplex,” discloses an OFDM system for signal transmission, and is incorporated herein by reference.
In OFDM systems, a high rate serial data stream is converted to several parallel low rate data streams so that the symbol duration of each of the data symbols transmitted in each of the parallel streams is very large in comparison to the expected channel delay spread. The channel delay spread arises from multipath scattering. Multipath scattering occurs when a transmitter transmits a data pulse and because of the data pulse reflecting off of natural and man-made objects, the data pulse becomes many pulses which arrive at the receiver at different times. The difference in time between the first and last of the multipath data pulses is the channel delay spread. Each one of the parallel data streams are transmitted on a sub-carrier frequency such that the parallel data streams are orthogonal to each other. Since the sub-carrier frequencies of the parallel streams are less than the frequency of the serial data stream, the effects of delay spread are greatly reduced. Hence, unlike in single carrier transmission systems, equalization is not a difficult task in OFDM systems.
In the frequency domain, the bandwidth of each of the parallel data streams in the OFDM system is very small in comparison to the coherence bandwidth of the channel. The coherence bandwidth is the frequency range over which two frequency components will have a strong potential for amplitude correlation and is inversely related to the delay spread. Strong amplitude correlation will result in portions of the channel being “flat” (i.e., having a constant amplitude and linear phase response). Portions where signals cancel each other out are referred to as flat fading regions. Large flat fading regions result in poor signal reception at the receiver. Since the total transmission bandwidth is typically several times larger than the channel coherence bandwidth, some of the parallel data streams within the total transmission bandwidth will be subjected to flat fading regions. The flat fading is referred to as Rayleigh fading. Rayleigh fading arises from the transmitted signal being reflected off of different objects and arriving at a receiver at different times. These multipath signals create standing waves at the receiver and result in poor signal reception. The bit error rate (BER) of a signal tends to increase as a function of Rayleigh fading. Flat fading regions due to multipath signals creating standing waves will also arise in multiple transmitter systems where each transmitter is transmitting the same signal.
FIG. 1A
illustrates a prior art orthogonal frequency division multiplexing (OFDM) system transmitter
10
for use in a single frequency network (SFN) of transmitters. A SFN comprises more than one transmitters transmitting the same OFDM signal in order to increase broadcast coverage. In transmitter
10
, data bits
12
are initially received at the transmitter
10
. The data bits
12
are then encoded using a convolutional coder
14
which encodes the data bits
12
into a continuous bit stream (1s and 0s) in a known manner using an error protection code. After encoding, the continuous bit stream is phase shift keyed using a phase shift keying PSK modulator
16
to obtain a stream of data symbols.
The data symbols are then frequency interleaved using frequency interleaver
18
to randomize the data. Interleaving is a data communication technique used in conjunction with error protection codes to reduce errors. In the interleaving process, coded data symbols are reordered before transmission in such a manner than any two successive coded data symbols are separated in the transmitting sequence. Upon reception, the interleaved coded data symbols can be reordered in their original sequence, thus effectively spreading or randomizing the errors in time to enable more complete correction by a random error protection code.
The symbols are then differentially encoded using differential coder
20
. Differential encoding assists in making the data signal insensitive to phase distortion. The differentially encoded complex symbols are then fed into an inverse fast Fourier transformer
22
. The inverse fast Fourier transformer
22
shifts the data symbols from the frequency domain to the time domain for data transmission. The signal generated by the inverse fast Fourier transformer
22
is an orthogonal signal in the time domain suited for data transmission. The time domain signal is then transmitted by antenna
24
.
FIG. 1B
illustrates a prior art orthogonal frequency division multiplexing (OFDM) system receiver
30
for receiving signals from a single frequency network (SFN) of transmitters. In the receiver
30
, an OFDM signal is received at a receiver circuit
34
through antenna
32
. The OFDM signal is then transformed from the time domain to the frequency domain by fast Fourier transform (FFT)
36
. The transformed signal is then differentially decoded and de-interleaved by differential decoder
38
and de-interleaver
40
, respectively. Next the signal is passed through a PSK de-modulator
42
to convert the complex symbols back into a bit stream. The bit stream is then decoded by convolutional decoder
44
to obtain the original data bits
12
from transmitter
10
.
To improve signal reception, many OFDM systems, such as digital audio broadcasting (DAB) systems, use a single frequency network (SFN) of transmitters broadcasting identical signals in the same frequency band with the transmitters synchronized in time. The signals from these different transmitters appear as multi-path energy at the receiver and may improve signal reception because of frequency diversity.
Frequency diversity schemes are used to develop information from several signals transmitted over independently fading paths. The independently fading paths are combined to reduce the effects of flat fading regions. This scheme minimizes the effects of flat fading regions since flat fading regions seldom occur simultaneously during the same time interval on two or more paths.
The transmit power and the distance between the transmitters in a SFN are carefully designed so that the signal paths from the different transmitters arrive within a specified guard interval of an OFDM frame so that intersymbol interference from adjacent OFDM frames can be avoided. In a SFN, depending upon the path delays, the combination of signal energies from two or more transmit stations may lead to destructive addition of some frequencies of the signal at the receiver. In channels that have very small delay spreads this destructive addition of the signals will give rise to very poor performance gains.
In a multipath environment, if the signals from the transmit stations have very small delay spreads, frequency diversity is unavailable. To illustrate the effects of small delay spreads consider the following example for a digital audio broadcasting DAB system in a single frequency OFDM network. Consider two stations transmitting a 4 MHz wide signal centered at a frequency of 2.4 GHz. If the receiver has only line-of-sight reception from both transmit stations and is located equi-distant from the two transmit stations, at this location, as long as the path delay spread is not large (i.e., <50 m, corresponding to a delay difference of 0.167 &mg
Sayeed Zulfiquar
Weerackody Vijitha
Ferris Derrick W
Lucent Technologies - Inc.
Pham Chi
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