Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-01-28
2001-01-30
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Transceivers
Modems
C375S242000, C370S523000
Reexamination Certificate
active
06181737
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data communications equipment. More particularly, the present invention relates to translation tables and constellation designs for a PCM modem and methods of generating the same.
2. State of the Art
With the ever-increasing importance of telecommunications for the transfer of data as well as voice, there has been a strong effort to increase data transfer rates over the telephone wires. In 1994, the ITU-T adopted the V.34 Recommendation (International Telecommunication Union, Telecommunication Standardization Sector Recommendation V.34, Geneva, Switzerland 1994). The V.34 standard and subsequent amendments define modem operating speeds of 28.8 kbps up to 33.6 kbps, and the vast majority of modems being sold today adhere to the V.34 Recommendation. However, with the explosion in the use of the Internet, even at the V.34 transfer rates, downloading of large files available on the Internet can take long periods of time. Thus, even as the V.34 standard was being adopted, there was a thrust to provide additional standards recommendations which would increase data transfer rates even further.
Recognizing that further increases in data rates is theoretically limited where the telecommunication network is an analog system (see C. E. Shannon, “A Mathematical Theory of Communication,”
Bell System Technical Journal
, 27:379-423, 623-656 (1948)), there have been various proposals to take advantage of the fact that much of the telecommunication network is now digital. For example, U.S. Pat. No. 5,394,437 to Ayanoglu et al., U.S. Pat. No. 5,406,583 to Dagdeviren, and U.S. Pat. No. 5,528,625 to Ayanoglu et al. (all assigned to AT&T/Lucent and all of which are hereby incorporated by reference herein in their entireties) all discuss techniques which utilize the recognition that the network is mostly digital in order to increase data transmission rates to 56 kbps and higher. Similarly, Kalet et al., “The Capacity of PAM Voiceband Channels,”
IEEE International Conference on Communications '
93, pages 507-511 Geneva, Switzerland (1993) discusses such a system where the transmitting end selects precise analog levels and timing such that the analog to digital conversion which occurs in the central office may be achieved with no quantization error. PCT application number PCT/US95/15924 (Publication WO 96/18261) to Townshend which is hereby incorporated by reference herein in its entirety) discusses similar techniques. All of the disclosures assume the use of PAM (pulse amplitude modulation) digital encoding technology rather than the QAM (quadrature amplitude modulation) currently used in the V.34 Recommendation. The primary difference between the AT&T technology and the Townshend reference is that the AT&T technology suggests exploiting the digital aspect of the telephone network in both “upstream” and “downstream” directions, while Townshend appears to be concerned with the downstream direction only. Thus, systems such as the “x2” technology of US Robotics which are ostensibly based on Townshend envision the use of the V.34 Recommendation technology for upstream communications.
Recently, a new Recommendation for standard was adopted by the ITU-T for the purposes of standardizing a PCM-type modem. The new standard, known as “V.90”, which is hereby incorporated by reference herein in its entirety, relates primarily to the transmitter of a PCM-type modem. In Section 8.4.1, the V.90 Standard requires the provision of a probing signal; also known in the art as digital impairment learning or “DIL”. The purpose of the DIL is to give the receiver of the receiving (analog) modem the opportunity to measure network impairments. The measurements and determinations made by the receiving modem are used by the receiving modem in formulating an appropriate constellation for the transfer of data. The constellation formulated by the receiving modem is transmitted back to the transmitting modem as a DIL descriptor set forth in Section 8.3.1 of the V.90 standard.
While much attention has been paid in the prior art to the transmitter in the V.90 modem, it will be appreciated that ability to design an appropriate transmission constellation plays a critical role in producing a high quality modem. In particular, according to V.90, the transmitter transmits 8-bit binary numbers (octets) which correspond to 128 positive and 128 negative &mgr;-law or A-law levels. These octets go through the digital network and are finally transformed into analog levels in a digital-to-analog (D/A) converter in the central office. To maximize data rates in the presence of network impairments, an optimal signal constellation must be utilized. Thus, it is necessary to relate (correspond) the transmitted octets to the levels received at the D/A output. This relation or correspondence is accomplished by reference to a translation table. Determination of the translation table is not a trivial task because the digital channel has uncertain parameters and the PCM signal is subjected to both digital and analog distortions including digital attenuation (PAD), robbed bits, etc. However, preparation of an appropriate translation table is critical to the high-quality functioning of the data communications. In addition, the translation table is necessary for generating an appropriate constellation design.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide methods for generating translation tables in the receivers of PCM modems, and apparatus which utilize the methods.
It is another object of the invention to provide methods for generating translation tables in the receivers of PCM modems where account is made for digital and analog impairments.
If is a further object of the invention to provide methods for optimizing construction of constellation design based on translation table determinations.
It is an additional object of the invention to provide algorithms useful for generating translation tables in V.90-type modem receivers.
In accord with the objects of the invention, in generating translation tables, the receiver of the invention utilizes information regarding each of the slots of the received signal. In particular, for each slot, the receiver must first determine: a) whether any of the slots are subjected to some type of robbed-bit-signaling, b) an estimation of the scaling factor, c) an estimation of the PAD applicable to the session, and d) the code (A-law or &mgr;-law) utilized. More particularly, the receiver receives a DIL probing signal and separates the signal slot by slot into tables of signal levels which are ordered (e.g., by value), and stored. From the tables of signal levels, tables of distances between the adjacent levels are calculated and stored. According to a first preferred aspect of the invention, based on the number of zero distances (i.e., distance between adjacent levels=zero) calculated, a determination is made for each slot as to whether or not that slot has been subjected to robbed bits.
According to a second preferred aspect of the invention, the tables of distances are used in finding candidate scaling factors for each of the slots. More particularly, N distances representing the sequence of the distances between N+1 probing signal levels are preferably utilized by comparing them to upper and lower bounds which are functions of the first three non-zero components of the sequence. The number of members of the set of distance differences falling within the range defined by the upper and lower bounds is determined, along with a first average which is the average of those distances. The N distances are then compared to upper and lower bounds which are a function of the previously determined first average to provide a second set of distances and a second average. In the preferred embodiment, this process is repeated to obtain third, fourth and fifth sets and third, fourth, and fifth averages. The sum of the first through fifth numbers is calculated for each of the three non-zero components,
Drucker Vitaly
Goldstein Yuri
Okunev Yuri
Wang Oin
Gallagher Thomas A
Gordon David P.
Jacobson David S.
PC Tel Inc.
Pham Chi H.
LandOfFree
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