Pulse or digital communications – Receivers
Reexamination Certificate
1997-10-22
2002-03-19
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Receivers
C375S332000, C370S206000
Reexamination Certificate
active
06359938
ABSTRACT:
COMPUTER PROGRAM LISTING APPENDIX
This application was orginally filed with Code Listings
1-35
which are now included in a computer program listing appendix. This appendix includes a total 203 frames of microfiche which are placed on a compact disc.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to receivers of electromagnetic signals employing multicarrier modulation. More particularly this invention relates to a digital receiver which is implemented on a single VLSI chip for receiving transmissions employing orthogonal frequency division multiplexing, and which is suitable for the reception of digital video broadcasts.
2. Description of the Related Art
Coded orthogonal frequency division multiplexing (“COFDM”) has been proposed for digital audio and digital video broadcasting, both of which require efficient use of limited bandwidth, and a method of transmission which is reliable in the face of several effects. For example the impulse response of a typical channel can be modeled as the sum of a plurality of Dirac pulses having different delays. Each pulse is subject to a multiplication factor, in which the amplitude generally follows a Rayleigh law. Such a pulse train can extend over several microseconds, making unencoded transmission at high bit rates unreliable. In addition to random noise, impulse noise, and fading, other major difficulties in digital terrestrial transmissions at high data rates include multipath propagation, and adjacent channel interference, where the nearby frequencies have highly correlated signal variations. COFDM is particularly suitable for these applications. In practical COFDM arrangements, relatively small amounts of data are modulated onto each of a large number of carriers that are closely spaced in frequency. The duration of a data symbol is increased in the same ratio as the number of carriers or subchannels, so that inter-symbol interference is markedly reduced.
Multiplexing according to COFDM is illustrated in
FIGS. 1 and 2
, wherein the spectrum of a single COFDM carrier or subchannel is indicated by line
2
. A set of carrier frequencies is indicated by the superimposed waveforms in
FIG. 2
, where orthogonality conditions are satisfied. In general two real-valued functions are orthogonal if
∫
a
b
ψ
p
(
t
)
ψ
q
*
(
t
)
ⅆ
t
=
K
(
1
)
where K is a constant, and K=0 if p≠q; K≠0 if p=q. Practical encoding and decoding of signals according to COFDM relies heavily on the fast Fourier transform (“FFT”), as can be appreciated from the following equations.
The signal of a carrier c is given by
s
c
(t)=A
c
(t)e
j[&ohgr;
c
t+&phgr;
c
(t)]
(2)
where A
c
is the data at time t, &ohgr;
c
is the frequency of the carrier, and &phgr;
c
is the phase. N carriers in the COFDM signal is given by
s
s
(
t
)
=
(
1
/
N
)
∑
n
=
0
N
A
n
(
t
)
ⅇ
j
[
ω
n
t
+
φ
n
(
t
)
]
(
3
)
&ohgr;
n
=&ohgr;
0
+n&Dgr;&ohgr;
(4)
Sampling over one symbol period, then
&phgr;
c
(t)
&phgr;
n
(5)
A
c
(t)
A
n
(6)
With a sampling frequency of 1/T, the resulting signal is represented by
s
s
(
t
)
=
(
1
/
N
)
∑
n
=
0
N
A
n
(
t
)
ⅇ
j
[
(
ω
n
+
n
Δ
ω
)
kT
+
φ
n
]
(
7
)
Sampling over the period of one data symbol
T
=NT, with &ohgr;
0
=0,
s
s
(
kT
)
=
(
1
/
N
)
∑
n
=
0
N
-
1
A
n
ⅇ
j
φ
n
ⅇ
j
(
n
Δ
ω
)
kT
(
8
)
which compares with the general form of the inverse discrete Fourier transform:
g
(
kT
)
=
(
1
/
N
)
∑
n
=
0
N
-
1
G
(
n
/
(
kT
)
)
ⅇ
jπ
n
(
k
/
N
)
(
9
)
In the above equations A
n
e
j&phgr;
n
is the input signal in the sampled frequency domain, and S
s
(kT) is the time domain representation. It is known that increasing the size of the FFT provides longer symbol durations and improves ruggedness of the system as regards echoes which exceed the length of the guard interval. However computational complexity increases according to Nlog
2
N, and is a practical limitation.
In the presence of intersymbol interference caused by the transmission channel, orthogonality between the signals is not maintained. One approach to this problem has been to deliberately sacrifice some of the emitted energy by preceding each symbol in the time domain by an interval which exceeds the memory of the channel, and any multipath delay. The “guard interval” so chosen is large enough to absorb any intersymbol interference, and is established by preceding each symbol by a replication of a portion of itself. The replication is typically a cyclic extension of the terminal portion of the symbol. Referring to
FIG. 3
, a data symbol
4
has an active interval
6
which contains all the data transmitted in the symbol. The terminal portion
8
of the active interval
6
is repeated at the beginning of the symbol as the guard interval
10
. The COFDM signal is represented by the solid line
12
. It is possible to cyclically repeat the initial portion of the active interval
6
at the end of the symbol.
Transmission of COFDM data can be accomplished according to the known general scheme shown in
FIG. 4. A
serial data stream
14
is converted to a series of
15
parallel streams
16
in a serial-to-parallel converter
18
. Each of the parallel streams
16
is grouped into x bits each to form a complex number, where x determines the signal constellation of its associated parallel stream. After outer coding and interleaving in block
20
pilot carriers are inserted via a signal mapper
22
for use in synchronization and channel estimation in the receiver. The pilot carriers are typically of two types. Continual pilot carriers are transmitted in the same location in each symbol, with the same phase and amplitude. In the receiver, these are utilized for phase noise cancellation, automatic frequency control, and time/sampling synchronization. Scattered pilot carriers are distributed throughout the symbol, and their location typically changes from symbol to symbol. They are primarily useful in channel estimation. Next the complex numbers are modulated at baseband by the inverse fast Fourier transform (“IFFT”) in block
24
. A guard interval is then inserted at block
26
. The discrete symbols are then converted to analog, typically low-pass filtered, and then upconverted to radiofrequency in block
28
. The signal is then transmitted through a channel
30
and received in a receiver
32
. As is well known in the art, the receiver applies an inverse of the transmission process to obtain the transmitted information. In particular an FFT is applied to demodulate the signal.
A modern application of COFDM has been proposed in the European Telecommunications Standard ETS 300 744 (March 1997), which specifies the framing structure, channel coding, and modulation for digital terrestrial television. The specification was designed to accommodate digital terrestrial television within the existing spectrum allocation for analog transmissions, yet provide adequate protection against high levels of co-channel interference and adjacent channel interference. A flexible guard interval is specified, so that the system can support diverse network configurations, while maintaining high spectral efficiency, and sufficient protection against co-channel interference and adjacent channel interference from existing PAL/SECAM services. The noted European Telecommunications Standard defines two modes of operation. A “2K mode”, suitable for single transmitter operation and for small single frequency networks with limited transmitter distances. An “8K mode” can be used for either single transmitter operation or for large single frequency networks. Various levels of quadrature amplitude modulation (“QAM”) are supported, as are
Alam Dawood
Collins Matthew
Davies David H.
Foxcroft Thomas
Keevill Peter A
Bayard Emmanuel
Discovision Associates
Masaki Keiji
Pham Chi H.
Stokey Richard
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