Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-05-06
2001-06-05
Pham, Chi (Department: 2631)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S503000, C370S342000, C375S219000
Reexamination Certificate
active
06243369
ABSTRACT:
FIELD OF USE
This application incorporates by reference the teachings of PCT Publication WO97/08861, published Mar. 6, 1997 for details of SCDMA transmitters and receivers, and incorporates by reference the teachings of PCT publication WO97/34421, published Sep. 18, 1997 for apparatus and methods of using SCDMA on hybrid fiber coax plants to use ATM cells for data transfer.
The invention is useful in the field of digital communications over any media, but, in particular, over hybrid fiber coax (HFC).
An emerging technology for cable TV systems is the provision of interactive, bidirectional digital data communications over the same HFC media as is used to provide TV signals to subscribers. Terayon Communication Systems first started shipping cable modems in July of 1997 that had the capability to send digital data using synchronous code division multiple access (SCDMA) multiplexing of digital data from multiple sources bidirectionally over HFC. In that system (described fully in the PCT publications incorporated herein, hereafter sometimes simply referred to as the PCT publications), downstream and upstream transmissions had gaps between frames. Frame synchronization in the upstream between the same frame number transmitted by different remote units (RUs) at differing distances from a central unit (CU) was important for low ISI noise. This was achieved by transmitting barker codes upstream at variable delays from each RU until a delay was found for each RU which resulted in its barker code arriving in the center of the gap between CU frames. This transmit frame timing delay determined for each modem in a trial and error process called ranging is then used for transmission of subsequent upstream frames by that RU. All RUs align to the same frame.
Downstream synchronization was achieved by transmission of barker codes from the CU to the RUs during every downstream gap. The RUs detected the CU barker codes and used that information to determine the CU downstream frame boundaries. The downstream barker codes also were encoded to include the downstream chip clock so that all the RUs could synchronize to the CU master chip clock. Known pilot channel data transmitted during timeslot 0 in the downstream (the SCDMA multiplexing is done over a TDM input stream) from the CU was used to do carrier recovery and monitor frame synchronization and to transmit kiloframe information. Clock recovery in the RUs was from the downstream barker codes using early-late gating techniques. In subsequent evolutions, the pilot channel data was only used to send frame sequence data and kiloframe marker information, and no carrier recovery was performed using the pilot channel data. Carrier recovery was done from the recovered downstream clock since at the CU the downstream carrier was generated to be phase coherent with the downstream clock. The arrival of the barker code in the downstream frame gaps also served as a reference for each RU from which to measure a transmit frame timing delay to achieve upstream frame synchronization.
An emerging standard for use in digital multi-service delivery through TV distribution systems is MCNS. In this standard, MAC layer data frames are broken down into MPEG packets which are 64-QAM or 256-QAM modulated and sent downstream in a continuous stream after FEC encoding. The FEC encoding involves four layers of processing: the MPEG packets are broken up and encoded into Reed-Solomom blocks with block boundaries bearing no relationship to MPEG packet boundaries; an interleaver mixes up the resulting 7 bit symbols so symbols formerly contiguous in time are no longer contiguous; a randomizer that takes the output of the interleaver and scrambles the symbols in pseudorandom order; and a trellis encoder adds some redundant bits. There are no gaps in the downstream data in which the CU can send a barker code which carries the master chip clock and which signals frame boundaries. There are no downstream frame boundaries related to the MPEG packet frames, but there are FEC frames delineated by a 42 bit FEC sync trailer appended to the end of 60 R-S blocks for 64-QAM, each R-S block containing 128 7 bit symbols. There is a 28-bit unique sync pattern in the first 4 symbols of the trailer. The remaining 14 bits are utilized for interleaver control. The trailer is inserted by the R-S encoder and detected by the R-S decoder to locate FEC frame boundaries. There is no synchronization coupling between the FEC and transport layers where MPEG packets are processed.
SCDMA upstreams require that all RUs be synchronized in frequency and have their frame boundaries aligned in time at the CU. To do this, the upstream must be synchronized to the downstream and mechanisms are needed to account for the fact that the propagation delay in transmissions from each RU to the CU encounter different propagation delays. The difficulty in resynchronizing an SCDMA upstream to a downstream with a different clock rate is that there are a wide range of different standards and different clock rates. Further, digital resampling can lead to two different downstream clock rates even within the same downstream.
If an SCDMA upstream is to be used with a downstream with an arbitrary clock rate such as an MCNS downstream or a IEEE 802.14 standard downstream, there arises a need for:
(1) a way to maintain a rational relationship between the downstream and upstream clock rates through the use of PLLs or digital resamplers;
(2) circuits for generating timestamp messages to establish a CU reference replacing the barker codes from which the round trip delay can be measured for fast ranging;
(3) a circuit for reducing the jitter of the timestamp messages to improve the accuracy of delay estimates for fast ranging;
(4) a circuit for detecting upstream clock slips.
SUMMARY OF THE INVENTION
There is described herein two examples of systems for transmitting digital data bidirectionally over shared media such as cable TV plants using upstream Synchronous CDMA multiplexing. The distinguishing characteristics of a genus to which these two species belong is the generation in the RUs of an upstream carrier and upstream chip clock both of which are synchronized in phase with a CU master clock. They are synchronized in phase by virtue of being synchronized in phase with a downstream symbol clock which is recovered from the downstream data transmitted by the CU, the downstream symbol clock being both synchronized in phase with the CU master clock and having a different frequency than the upstream chip clock. The advantage of the aspect of the invention that synchronizes the upstream clock to the CU master clock or at least makes it phase coherent therewith and which generates a phase coherent upstream carrier from the recovered downstream clock is that it eliminates the need for upstream clock and carrier recovery circuitry in the CU since the upstream clock and carrier are phase coherent with locally generated clock and carrier signals in the CU. In other words, since the upstream clock and carrier are phase synchronous with the CU master clock, the upstream data can be demodulated and demultiplexed using upstream clock and carrier signals generated from the CU master clock after suitable phase and amplitude adjustments for each RU derived from known preamble data transmitted by the RU using the upstream clock and carrier derived from the recovered downstream clock. Although a PLL is used in the CU, it is used to generate an upstream local clock signal from a master clock signal which has its frequency set so as to generate a downstream clock signal or at some multiple of the downstream symbol clock frequency. In other words, a distinguishing characteristic of the first genus of inventive species is that they employ a master clock to generate a clock signal which can be used directly or indirectly for whatever clock frequency is used for one direction of transmission and a PLL or digital resampler which generates a phase synchronous clock signal which can be used for the other direction of transmission. That is, if the CU master cloc
Artman Doug
Grimwood Michael
Knittel Jim
Lind Paul Alan
Rakib Selim Shlomo
Emmanuel Bayard
Falk & Fish
Fish Ronald C.
Pham Chi
Terayon Communication Systems, Inc.
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