Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-05-17
2001-11-06
Trost, William (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S003020, C455S010000, C714S746000, C714S758000, C375S225000
Reexamination Certificate
active
06314289
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to concepts for digital broadcasting and, in particular, concepts for digital broadcasting suited for fading channels for wireless communication.
BACKGROUND OF THE INVENTION
Satellite-based broadcasting systems provide an adequate communication link only in rural areas, in which only a small number of e.g. bridges exist. Additionally, rural areas usually do not have skyscrapers. Skyscrapers as well as bridges or, generally, densly built-up areas are obstacles to satellite-based communication systems, since carrier frequencies used for such communication links involve that a channel between a sender, e.g., a satellite, and a receiver, i. e. a mobile or stationary receiver, is characterised by the line of visual contact (line of sight) between the sender and the receiver. If a skyscraper comes into the line of visual contact, i.e., the transmission channel between the satellite and the receiver, which may be positioned in a car, the received signal power will decrease substantially.
Generally, it can be stated that in wireless systems (radio systems), changes in the physical environment cause the channel to fade. These changes include both relative movement between transmitter and receiver and moving scatters/reflectors in the surrounding space. In theoretical studies of wireless systems, the real channels are usually modelled so that they result in trackable analysis. The two major classes of fading characteristics are known as Rayleigh and Rician. A Rayleigh-fading environment assumes no line of sight and no fixed reflectors/scatters. The expected value of the fading is zero. If there is a line of sight, this can be modelled by Rician-fading, which has the same characteristics as the Rayleigh-fading, except for a non-zero expected radio.
Modern digital broadcasting systems know several means for reducing the impact of a channel fading. These concepts comprise channel coding on the one hand and several kinds of diversity on the other hand. The European standard for digital audio broadcasting (DAB), set out in Radio Broadcasting Systems; Digital Audio Broadcasting (DAB) To Mobile, Portable and Fixed Receivers, ETS 300 401, ETS I—European Telecommunications Standards Institute, Valbonne, France, February 1995, uses differential quadrature phase-shift keying (DQPSK) as modulation technique. The channel encoding process is based on punctured convolutional coding, which allows both equal and unequal error protection. As a mother code, a convolutional code having a code rate of 1/4, a constraint length 7, and octal polynominals is used. The puncturing procedure allows the effective code rate to vary between 8/9 and 1/4. Channel coding by means of punctured convolutional codes is described in “Punctured Convolutional Codes of Rate (n−1)
and Simplified Maximum Likelihood Decoding”, J. Bibb Cain et al., IEEE Transactions on Information Theory, Vol. IT-25, No. 1, January 1979.
Punctured convolutional codes can be used in connection with many modulation techniques, such as OFDM, BPSK, QAM, etc.
Different channel encoding techniques are outlined in “Channel Coding with Multilevel/Phase Signals”, Gottfried Ungerboeck, IEEE Transactions on Information Theory, Vol. IT 28, No. 1, pages 55 to 66, January 1982.
Bitstreams encoded by means of a convolutional encoder can be decoded by a decoder, in which the well-known Viterbi algorithm is implemented. This algorithm is capable of using the channel state information (see P. Hoeher “TCM on Frequency-Selective Length-Mobile Fading Channels”, Proc. Tirrenia International Workshop Digital Communication, Tirrenia, Italy, September 1991). The Viterbi algorithm can be modified to provide reliability estimates together with the decoded sequence. This enables soft decoding. By applying a soft-output Viterbi algorithm, an improvement of about 2 dB is obtained in comparison to systems that implement “hard” decision.
DESCRIPTION OF PRIOR ART
With reference to
FIG. 6
, a simplified overview of a transmitter receiver system described in the European DAB Standard is illustrated. The transmitter receiver system generally comprises a transmitter section
60
and a receiver section
70
. The transmitter section
60
, in the simplest case, comprises a bitstream source
62
, a channel encoder
64
and a transmitter
66
. The receiver section
70
, in the simplest case, comprises a receiver
72
and a channel decoder
74
.
FIG. 7
illustrates a transmitting receiving setup providing for time diversity as well as space diversity. The transmitter section
60
′ comprises the bitstream source
62
and the encoder
64
that have already been described with respect to FIG.
6
. In addition, the receiver section
60
′ comprises a first transmitter
66
a
and a second transmitter
66
b
. Both transmitters
66
a
and
66
b
are fed by the same signal output by the encoder
64
that is duplicated by a duplicator
67
.
To obtain time diversity, a delay element
68
is coupled between the duplicator
67
and the second transmitter
66
b.
In the case of satellite communication, the transmitters
66
a
and
66
b
are realised by two satellites that reside on different orbital positions spaced apart from each other.
The first channel is defined by the line of sight between the first transmitter and the receiver, for example, a car, whereas the second channel is defined by the line of sight between the second transmitter
66
b
and the car that comprises the receiving section
70
′. In the scenario, in which the car travels on a street to the right and to the left of which are high buildings, the possibility is increased that the car will receive the transmitted signal from at least one satellite.
When the case is considered, in which the car is driving through a tunnel or under a bridge, the lines of sight to both transmitters
66
a
and
66
b
are interrupted. The time diversity method implemented by this system shown in
FIG. 7
, however, ensures that the receiver will not be affected by the interrupted channel, since the transmission signal is delayed by the delay stage
68
. Optimally, no transmission interruption will result, when the delay time is equal to or greater than the travelling time of the car through the tunnel or under the bridge. Thus, the receiving section will, once again, receive the transmission signal sent by the transmitter
66
a
, when it was under the bridge, via a channel
2
. Naturally, the receiving section
70
′ comprises another delay stage
78
. As it is shown in
FIG. 7
, the delay stage
78
of the receiving section has to be in the channel that has not been delayed in the transmitter section. Thus, the signals at the output of the receivers
72
a
and
72
b
are identical, when the delay values of the delay stages
78
and
68
are equal.
A decision stage
79
, which is symbolised as a switch in
FIG. 7
, determines which channel provides the signal with the better signal to noise ratio. When it is determined that channel
1
provides the stronger signal, the decision stage
79
is operative to conduct the signal received by the receiver
72
a
into the channel decoder
74
When it is determined in block
79
that the signal transmitted over the other channel (channel
2
) is the stronger one, the decision stage
79
is operative to conduct the signal received by the receiver
72
b
to the channel decoder
74
.
To summarise, the system illustrated in
FIG. 7
comprises the following essential features:
the signal output by the encoder
64
is duplicated by the duplicator
67
;
exactly the same signals, whether delayed or not, are transmitted via both channels;
the signals transmitted over both channels are derived from the bitstream output by the bitstream source
62
in exactly the same way by means of the encoding process carried out in the redundancy adding encoder
64
(repetition code);
the decision stage
79
compares the signal to noise ratio of both channels and selects the channel in which the signal having the better signal to noise ratio is transmitte
Breiling Marco
Eberlein Ernst
Gerhauser Heinz
Stoessel Jan
Dougherty & Clements LLP
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung
Tran Congvan
Trost William
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