Multiplex communications – Channel assignment techniques – Combining or distributing information via code word channels...
Reexamination Certificate
1996-07-19
2002-03-12
Ngo, Ricky (Department: 2664)
Multiplex communications
Channel assignment techniques
Combining or distributing information via code word channels...
C370S442000, C370S478000, C370S479000, C370S480000, C370S503000
Reexamination Certificate
active
06356555
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to the field of bidirectional communication of digital data over coaxial cable or other transmission media. More particularly, the invention pertains to the field of provision of multiple channels of digital data including interactive TV services, digital telephony, video teleconferencing, video on demand, internet access at 10 megabit/second or media data rates etc., all provided to home or business establishments over cable TV coax or combinations of coaxial cable, fiber optic links, microwave or satellite links or other wireless systems using synchronous CDMA system technology. These new services delivered over the CATV system will hereafter be called supplemental services.
BACKGROUND OF THE INVENTION
In order to provide bidirectional digital data communication over a cable TV coaxial network to multiple subscribers with multiple digital services including data, digitized video and digitized audio, all made available over a single cable TV hybrid fiber coax (hereafter HFC) network (hereafter called CATV systems), several problems have to be solved. First, video applications such as video teleconferencing and video on demand movies require that a flexible amount of bandwidth be awarded to support the digitized video transmissions and that the amount of bandwidth awarded be guaranteed, i.e., the awarded bandwidth will be made available continuously without interruption until the video service is completed. These requirements force the use in the CATV environment of higher level protocols that have been developed or which are in the process of development for local area networks such as ATM or ISDN that are designed for delivery of digitized video, digitized audio and digital data over point to point LAN connections. Thus, the a major problem exists in adapting these point to point LAN protocols to the point-to-multipoint CATV environment.
Second, there is the problem of bandwidth availability and sharing the available bandwidth among a plurality of users who are physically distributed and all of whom have changing bandwidth needs over time. In CATV systems, typically only 6 mHz of bandwidth is available for communications of digital data downstream from the head end to the subscribers and another 6 mHz elsewhere on the spectrum of the CATV media is made available for upstream communications. These 6 mHz bands must be shared by all the users to carry data for all the services provided. Since video is a high bandwidth demand application which requires continuous availability, the available bandwidth must be fully utilized with maximum efficiency so as to provide bandwidth capacity to as many users as possible.
Third, there is the problem of noise and interference on the CATV system which could degrade the supplemental services to the point of unacceptability if techniques are not employed to lessen the affect of the noise on reception or otherwise deal with the problem. Cable TV media are high noise environments because of numerous reasons which are well known to those skilled in the art.
A fourth major problem, but related to the second problem, is synchronization of data transmission. Synchronization must be maintained between all remote units and the central unit for maximum efficiency use of the available bandwidth. If synchronization is not maintained, the number of users which can simultaneously share the available bandwidth is reduced.
In addition, there is the problem of eliminating the intersymbol interference and partial cross correlation between codes of code division multiple access (herafter CDMA) technology. CDMA technology is used in the genus of the invention to support multiple user with simultaneous pseudo point-to-point connections to the head end. CDMA is known in the prior art and has the advantage of not requiring the bandwidth allocation of frequency-division multiplexing nor the time synchronization of time-division multiplexing. However, it has the problems mention above which must be solved for a system acceptable to a user to exist. In addition, to maximize the number of users supported, frame synchronization is used in the invention and one of the problems solved by the inventors is how to achieve this frame synchronization in a physically distributed system of transmitters.
Of course all nonbaseband CDMA systems require two additional forms of synchronization as is well known in the art: first, there must be synchronization in phase and frequency between the transmitting carrier and the local oscillator in the receiver which feeds the demodulator. Also, as is known in the art all CDMA systems require clock recovery synchronization so that the pseudonoise code (hereafter PN code) sequence fed into the despreading circuitry is not only identical to the PN code fed into the spreading circuitry of the transmitter, but also exactly in phase therewith. Further, clock recovery is necessary in CDMA systems so as to know symbol boundaries so as to be able to correctly sample the baseband data stream exiting the despreading circuitry. How to achieve these forms of synchronization in a CATV CDMA system are another set of problems that must be solved.
Further, in CDMA systems with multiple transmitters which are physically distributed each using different spreading codes, it is possible for signals from different transmitters to arrive at the central unit at different power levels. This causes nonzero cross-correlations between the different codes, and gives rise to a problem known as the “near-far” problem. To prevent high error rates from strong signals overwhelming weaker ones, the near-far problem must be solved.
An example of one attempt to transmit digital data bidirectionally over a CATV system is the technology disclosed in U.S. Pat. Nos. 4,912,721 and 5,235,619 assigned to Scientific Atlanta. In these systems, direct sequence spread spectrum technology is used to overcome the noise problem in CATV systems, but no support of multiple users by code division multiple access is sought. The Scientific Atlanta patents are single user direct-sequence spread spectrum systems, and there is no teaching of how to achieve frame synchronization which is necessary to maximize the number of users in a CDMA multiuser system. In the Scientific Atlanta patents, each user sends billing and request data to the head end and the head end sends data related to supplemental services other than the cable TV programming to the remote units at the sites of users during a timeslot assigned to that user. The transmission of data is accomplished on a one user per timeslot basis using a pseudonoise spreading code. No teaching of the use of orthogonal codes is present in U.S. Pat. No. 4,912,721. Therefore, only one user can use the available bandwidth devoted to the supplemental services at any particular time. Accordingly, for all users to share the available bandwidth, a time division multiplexing scheme must be used and the direct-sequence, spread spectrum technology is used only to overcome the noise and other impairments on the CATV channel.
CDMA cellular telephone systems are known. One example is U.S. Pat. No. 5,416,797 assigned to Qualcomm Inc. of San Diego, Calif. In this system multiple users of a cellular telephone systems are supported by a direct-sequence spread spectrum system which minimizes interference between users by providing orthogonal PN codes for spreading and controlling transmit timing such that the code frames from different users are aligned in time with each other at the receivers. Each cell site transmits a pilot carrier signal which is unique to that cell and which is used by the mobile units to obtain initial system synchronization and to provide time, frequency and phase tracking information. Each cell spreads the pilot signal using the same code, but cell uses a unique phase offset thereby allowing mobiles to achieve synchronization by searching all phase possibilities for the single code. The strongest pilot signal is tracked, and then the mobile unit acquires a synchronization channel which transmits sy
Azenkot Yehuda
Rakib Selim Shlomo
Craig Ronald
Fish Ron
Fish, A Law Corp.
Ngo Ricky
Terayon Communications Systems, Inc.
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