Pulse or digital communications – Transceivers – Transmission interface between two stations or terminals
Reexamination Certificate
1998-06-19
2003-06-24
Bayard, Emmanuel (Department: 2631)
Pulse or digital communications
Transceivers
Transmission interface between two stations or terminals
Reexamination Certificate
active
06584147
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a receiver at the head-end or side in a communications system or network. More specifically, the invention relates to a receiver for signals in the upstream direction which is the direction from user to head-end or a centralizing unit that is linked to a number of users, the number being equal to or larger than one.
BACKGROUND OF THE INVENTION
The technology area of communications systems is subject to a vast engineering effort in order to allow for an always increasing number of applications.
Currently, the public access network for television (CATV) is being prepared for bi-directional communication. The goal of upgrading the CATV network is to provide bi-directional communication of digital data at speeds well beyond that of traditional data communication over telephone lines. This way, CATV data communication allows new types of applications such as video-on-demand and fast Internet access. It also provides an alternative to existing telephone services. In that case, the analog voice data is digitized and transmitted as a collection of data packets. For CATV communications, one defines the downstream direction, going from the broadcasting head-end side to the user side, and the upstream direction, going from the user side to the broad-casting head-end side. These definitions of upstream and downstream direction in the sequel are adopted also for any communications system or communications network. The upstream direction is defined as going from a user side or a subscriber residence to a central office or head-end or a centralizing unit that is linked to a number of users, the number being larger than one.
The communication process between the end-user and head-end is typically organized as a number of hierarchical subprocesses, each running at a different level of abstraction. This hierarchy is needed to express the access of the communications network between communicating parties in an efficient way. This invention is concerned with the communication subprocess on the lowest hierarchical layer, i.e., the physical layer.
The goal of communications on the physical layer is:
to provide a reliable means of data communication by applying methods of data modulation and demodulation, and
to transfer this reliable communication method towards the higher layers. This is done by means of an interface and a protocol.
The topology of the physical layer a typical public access network for television is shown in FIG.
1
. Both the head-end side and user side have a transmitter and receiver to make bi-directional communication possible. The network has a tree-like topology, and consists of both active elements (bi-directional amplifiers) and passive elements (cable, splitters and taps). For each head-end transmitter/receiver pair, many user-end transmitter/receiver pairs may exist. Typically, 400 users can be served through 1 head-end. The up- and downstream communications path run over a single electrical path and are differentiated through frequency multiplexing, as shown in FIG.
2
.
U.S. Pat. Nos. 3,962,637 and 5,127,051 describe improved modems for high frequency data transmission. In particular, U.S. Pat. No. 5,127,051 discloses a modem system that can adapt to fast channel variations by rapidly deriving accurate channel estimate without excessive storage of data overhead. Accordingly, the data are transmitted in a frame carrying at least two identical data sequences. This approach, however, has as a disadvantage a major overhead for static type of channel and therefore has a slow performance for these channel types. The publication “CATV Return Path Characterization for Reliable Communications” by C. A. Eldering, IEEE Communications Magazine, August 1995 addresses the problem of reliable solutions for bi-directional communication. In said publication, an emphasis is given to the key problem of the understanding of the communication channel characteristics in the upstream direction. One of the problems in upstream communication is as follows. As we move from the user to the head-end, the physical transmission medium (the cable or the channel) is shared by an increasing amount of users. Therefore, users will share the medium in the upstream direction by means of an appropriate multi-access protocol. This invention is concerned with the time-division multi-access protocol which uses burst-mode signals. In this protocol, each user gets in turn connection to the head-end during a fixed time-slot. The start and end of the time-slot is decided at the head end by allocation algorithms running in the higher hierarchical communication layers. A discussion of these allocation algorithms is out of the scope of this patent application since it is concerned with the transport of data on the physical layer only.
The key problems to solve in order to establish a reliable upstream communication between the user and the head-end are the following:
1. As the signals emitted at the user side propagate through the upstream channel, they are attenuated and delayed. This attenuation and delay is different for each user, since each user is connected at a different position in the tree network as seen from the head-end. Therefore the head-end must estimate these modulation distortions on a per-burst basis. It must also do this as fast as possible, since during estimation time, no useful data can be transmitted.
2. Besides attenuation and delay, the signals also suffer from group delay distortion. Group delay distortion is caused by the non-linear phase characteristics of the (mainly active) components located in the up-stream channel. The effect of group delay distortion is that the time-domain shape of the distorted signal is changed. The distortion is a linear effect, which means that it can be removed by passing the received signal through a proper filter before detecting it. The required shape of this filter is dependent on the amount and type of group delay distortion, and is again different for each user. Therefore, the head-end receiver must estimate the coefficients of this filter on a per burst basis. Failure to do so causes an effect at the head-end receiver side called inter-symbol-interference (ISI). ISI degrades the quality of the data detection process, and therefore should be avoided.
The process of estimating attenuation, delay and group delay distortion is jointly called channel estimation.
3. The upstream communications CATV path is also susceptible of noise influences. These can be caused by electrical appliances or spurious emissions of radio-band users (mobile communication, amateur, CB, . . . ), and other, unknown sources. Since the actual time-domain shape of noise is unknown, it cannot be removed at the receiver. It will therefore also degrade data detection performance of the receiver. The transmission can however be protected against noise influences by applying a proper encoding of the data. The encoding increases the redundancy of the transmitted data pattern. At the receiver side, the removal of this redundancy can then be used to identify locations of errors in the received data pattern. Eventually, the redundancy can even be used to correct the errored values.
4. All estimation processes active in the head-end receiver must proceed as fast as possible. During the estimation the actual delay, attenuation and group delay distortion is unknown and no data can be detected successfully. The signal transmitted by one user is of a bursty nature. Therefore, the shorter the estimation time, the more time will be left in the signal burst that can be used for the transmission of actual data.
In the remainder of this document, we will first summarize the key properties of the invention, which is a digital receiver for these burst mode signals. Next, we will give a detailed description of the receiver and its operation.
SUMMARY OF THE INVENTION
The present invention relates to a receiver at the head-end or a centralizing unit side in a communications system or network. The receiver is adapted for receiving signals in the upstream
De Ceulaer Bart Jozef Maria
De Meyer Karel Stefaan Martha Maria
Engels Marc
Moonen Marc Suzanne Paul
Schaumont Patrick
Bayard Emmanuel
IMEC
Knobbe Martens Olson & Bear LLP
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