Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1999-02-26
2003-09-23
Chin, Stephen (Department: 2634)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C714S758000
Reexamination Certificate
active
06625219
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the field of data communications and processing. Specifically, the present invention relates to a method and apparatus for encoding/framing a data stream of multitone modulated signals to improve impulse burst immunity.
BACKGROUND OF THE INVENTION
Digital data communications systems are commonly used to transmit and/or receive data between remote transmitting and receiving locations. A central facet of any data communications system is the reliability and integrity of the data which is being communicated. Ideally, the data which is being transmitted from the transmitting location should be identical to the data which is being received at the receiving location. Practically however, the data to the data which is being received at the receiving location. Practically however, the data which is received at the receiving location has oftentimes been corrupted with respect to the original data that was transmitted from the transmitting location. Such data communication errors may be attributed in part to one or more of the transmission equipment, the transmission medium or the receiving equipment. With respect to the transmission medium, these types of data errors are usually attributed to the less than ideal conditions associated with the particular transmission medium. An example of such a communication medium or channel is the hybrid fiber coaxial cable television network, HFC CATV.
In certain channels, such as the HFC channel, errors may be caused by noise or other interference. One type of noise is ingress or narrowband interference which typically occurs at a fixed frequency and lasts for a long time. Another type of noise is impulse or burst interference which typically occurs at unexpected times, lasts for a short period of time (e.g., several microseconds), and corrupts all tones or bands.
Multitone modulation is a signal transmission scheme which uses a number of narrow-band carriers positioned at different frequencies, all transmitting simultaneously in parallel. Each narrow band carries a fraction of the total information being transmitted. The discrete bands or sub-channels are independently modulated, and each have a carrier frequency at the center frequency of the particular band.
One type of multitone transmission scheme is discrete multitone, often referred to as DMT. In DMT, a 1.1 MHz channel is broken down into 256 sub-channels or bands, each of which is 4 KHz. Each of the sub-channels has its own carrier frequency, and the signal to noise ratio for each of the sub-channels is monitored by the DMT system to determine how many bits per signal may be carried in each of the sub-channels. Each of the sub-channels transmits a number of information bits in a single symbol or signal period. The number of bits per signal (or symbol) in a sub-channel is typically referred to as the “loading” of the sub-channel. The DMT system dynamically adjusts the loading of each of the sub-channels in accordance with the noise characteristics of the sub-channel. Particularly noisy sub-channels may sometimes not be used altogether.
DMT typically has long symbol periods of 250 microseconds. As a result, DMT exhibits fairly good immunity with respect to time domain events, since the effect of a time domain event will be averaged out over the relatively long symbol period. In this way, impulse noise has less of an effect on DMT transmissions. Although the effect is reduced, there is nevertheless, still an adverse effect due to impulse noise. With respect to narrowband interference, this type of noise is typically stable and can be compensated for by adjusting the loading of the particular, affected sub-channels.
Variable Constellation Multitone (VCMT) modulation is a transmission scheme specifically designed to effectively combat the high ingress and burst impairments in cable TV channels, and also to maximize the throughput capacity of such channels. VCMT uses variable bit loading per tone, along with coding and interleaving. The tones are independently modulated from QPSK (quadrature phase shift keying) to 256-QAM (quadrature amplitude modulation), depending on the noise measured for each tone. The SNR (signal to noise ratio) across the channel is monitored for each tone, and the headend receiver accordingly instructs the upstream transmitter in the cable modem to modify the QAM constellation for each tone to maintain a desired BER (bit error rate).
VCMT also utilizes spectral shaping to reduce the frequency sidelobes of the tones, as compared with conventional multitone modulation, in order to reduce the effect of narrowband interference to only those affected tones. As with all multitone modulation schemes, VCMT utilizes long symbol periods to average the effect of burst and impulse noise. Interleaving the data over time and frequency may also be used to minimize the number of impaired tones for each user.
The VCMT nominal configuration is designed for a bandwidth of 1.6 MHz. However, the VCMT configuration may be adapted for any particular bandwidth through proper modification of the system parameters. In the case of the nominal configuration, VCMT uses the following parameters:
RF Bandwidth:
1.6 MHz.
Number of tones:
36
Modulation:
QPSK to 256-QAM (per tone)
Inter-tone spacing:
43.75 KHz.
Signaling rate:
40 kbaud (per tone)
Data rate:
Variable, depending on tone
modulation
Symbol shaping:
Modified square-root-raised-
cosine, roll-off
0.09
Symbol duration:
10 symbol periods
(250 microseconds)
Code Division Multiple Access (CDMA) modulation is a multi-user access transmission scheme in which different users overlap both in frequency and in time. This is in contrast to Frequency Division Multiple Access (FDMA) in which users overlap in time, but are assigned unique frequencies, and Time Division Multiple Access (TDMA) in which users overlap in frequency, but are assigned unique timeslots. According to CDMA, each user is assigned a unique code sequence that allows the user to spread its information over the entire channel bandwidth, as opposed to particular sub-channel(s) in FDMA. Thus, signals from all users are transmitted over the entire channel. To separate out the signals for a particular user at a receiver, cross correlation is performed on the received signal using the unique user code sequence. In CDMA systems, inter-user interference is minimized using one of two possible techniques.
The first technique for minimizing inter-user interference is to modulate the user signals using an orthogonal basis of wideband functions, e.g., Walsh basis. Specifically, at a given instant in time, each user selects (according to its unique code) a different basis function from the orthogonal basis, ensuring zero cross correlation between the different users. The, basis function used by each user may be changed on a symbol by symbol basis, while still ensuring that different users use different functions. The orthogonal basis itself may be changed on a symbol by symbol basis. The result is that each user occupies the entire channel bandwidth, while user cross correlation is kept to zero. The disadvantage of this approach is that in order to build a wideband (and frequency overlapping) orthogonal basis, the user data generally needs to be time synchronized since the orthogonal base construction assumes time synchronization.
A second technique for minimizing inter-user interference which does not require that the user data be time synchronized is to modulate each symbol using a pseudo randomly selected wideband waveform, which is selected according to the unique code for each user. The wideband waveform may be generated by multiplying the user data by a pseudo random sequence, referred to as a spreading sequence. This approach minimizes inter-user interference, but does not reduce the interference to zero. However, the lower the signaling rate (baud rate) is versus its bandwidth, the lower the cross correlation will be. Thus, in CDMA the signaling rate of each user is usually much smaller than the channel bandwidth it occupies in ord
Chin Stephen
Darby & Darby
Kim Kevin
Tioga Technologies Ltd.
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