CA2139069A1 - Local distribution for interactive multimedia television to the home - Google Patents

Abstract

An interactive multimedia television service to the home employs a downstream digital signal which has a predetermined unique spectral shape which facilitates at a receiver filtering out of undesired interfering signals without removing any significant amount of the energy from the downstream digital; signal.

Description

D. D. Harrnan 2-28 21~ 9 01~ 9 LOCAL DISTRIBUTION FOR INTERACTIVE
MULTIMEDIA TELEVISION TO THE HOME

B~-~k~round of the Invention Arrangements are known for supplying downstream digital television signals to the home and for supplying an upstream control channel from the home. One such arrangement utilizes coaxial cable for supplying the downstream video signal and the upstream control channel. Another arrangement is known which employs a twisted wire pair for supplying the downstream video signal to the home and the upstream control 0 channel from the home. Although these arrangements operate satisfactorily on their specific tr~n.cmi~sion medium, they do not function adequately on the other medium, that is, the coaxial arrangement doesn't operate satisfactorily on twisted wire pair and vice versa.
It has also been recognized that each of the tr~n~mi~ion mediums have channel impairments. Specifically, the coaxial cable suffers from noise at lower frequencies while the twisted wire pair suffers from high propagation loss at higher frequencies.
Additionally, the twisted wire pair may have problems with radiation limits if energy is transmitted at frequencies above 30 MHz. The most (l~m:~ging type of noise, i.e., interfering signal, in the home for coaxial cable arrangements is the impulsive noise generated by light dimmers. The light dimmer noise is coupled through common grounds in the coaxial cable medium and appears as a strong interference noise at the input of the receiver. This of course, is extremely undesirable. Additionally, it is known that use of the twisted wire pair medium limits the bandwidth that may be employed.
Summary of the Invention The problems and limitations of the prior arrangements employed in interactive multimedia television service to the home are overcome by employing a downstreamtransmitted digital signal having a predetermined unique spectral shape to minimi7e the effects of the undesired noise in the coaxial cable medium and propagation loss in the twisted wire pair medium. The spectral shaping is such that a remote receiver can easily filter out the interfering light dimmer noise without removing energy from the downstream signal while still being able to transmit the signal over a twisted wire pair and meeting the radiation limits.
Brief Description of the Fi.~ures FIG. 1 shows in simplified form a tr~n~mi~sion arrangement in which the invention may be employed;

D. D. Harman 2-28 ~ ~3 9 ~3 6 9 FIG. 2 shows in simplified block diagram form details of a transmitter used for the downstream channel;
FIG. 3 shows a signal constellation for the transmitter of FIG. 2;
FIG. 4 graphically illustrates the spectral shaping employed in the transmitter of this invention;
FIG. 5 shows in simplified block diagram form details of a receiver used for thedownstream channel; and FIG. 6 shows a signal constellation for the receiver of FIG. 5.
Detailed De~e. ;l~lion o FIG. 1 shows in simplified form a transmission arrangement in which the invention may be employed. Shown is curbside unit 101 which includes transmitter 102 hybrid 103, and receiver 104. Transmitter 102 is employed to transmit the downstream digital television signals *om the curbside via hybrid 103 and tr~nsmission media 105 to a home side unit 106. Also included in curbside unit 101 is receiver 104 for receiving the upstream control channel from home side unit 106 via tr~n~mis.~ion medium 105 and hybrid 103. In this example, transmitter 102 is a carrierless AM/PM (CAP) transmitter shown in FIG. 2 and described below. Receiver 104 is a standard quadrature phase shift keying (QPSK) receiver of a type well-known in the art. The downstream digital television signals and the upstream control channel are transported from the curbside to the home side and from the home side to the curbside, respectively, via tr~n~mission medium 105 which may include coaxial cable, twisted wire pair, or a combinationof both. Home side unit 106 includes hybrid 107, CAP receiver 108 and QPSK transmitter 109. The received downstream video signal is supplied from tr~nsmission medium 105 via hybrid 107 to receiver 108, and the upstream control channel is generated via transmitter 109 and supplied via hybrid 107 to tran~mission medium 105. CAP receiver 108 is shown in FIG. 3 and described below. It is noted that the digital television signal and a control channel may be transported on either coaxial cable or twisted wire pair within the home.
FIG. 2 shows in simplified block diagram form details of CAP transmitter 102. Itis noted that CAP is a bandwidth-efficient two-dimensional pass-band tr~nsmission scheme, which is closely related to the more f~mili~r quadrature amplitude modulation (QAM) tr~n~mi~ion scheme. Such transmitter arrangements are known it the art. See for example, an article by G-H Im an J.J. Wemer entitled "Bandwidth-Efficient Digital Tr~n.smi~sion up to 155 Mb/s over Unshielded Twisted Pair Wiring", Conference Record ICC '93, Geneva, 1993, pages 1797-1803 and a standards contribution authored by W.Y.
Chen, G-H Im and J.J. Werner entitled "Design of Digital Carrierless AM/PM

D. D. Harman 2-28 ~ (16 9 .
Transceivers", TlE1.4/92-149, August 19, 1992. The digital bit stream to be transmitted from curbside unit 101 (FIG. 1) is first passed through a scrambler (not shown) and then fed to encoder 201 of a type known in the art. Encoder 201 maps blocks of m bits into one of k=2m different complex symbols An = an + jbn A CAP transceiver that uses k 5 different complex symbols is called a k-CAP transceiver. Let 1/T be the rate at which the symbols are to be transmitted. The bit rate R is then equal to R = log2k 1/T = m 1/T.
The 2-dimensional display of the discrete values assumed by the symbols an and bn is called a signal constellation. An example of a 16-point signal constellation is shown in FIG. 3. The symbols an are then fed from encoder 201 to an in-phase shaping filter 202 o and symbols bn are fed from encoder 201 to quadrature shaping filter 203. Both of filters 202 and 203 are typically implemented as finite impulse response (FIR) filters of a type well-known in the art. The values of the tap coefficients of filters 202 and 203 can be computed by sampling the in-phase and quadrature impulse response, p(t) and p(t)described below at multiples of the sampling period T . The outputs of filters 202 and 203 are algebraically subtracted in algebraic combiner 204 and the resulting signal is passed through digital-to-analog (D/A) converter 205. Note that D/A converter 205, in this example, operates at a sampling rate of 51.84 MHz. The resulting analog signal is passed through interpolating low-pass filter (LPF) 206 which removes higher frequency images generated by the D/A 205. The digital shaping filters 202 and 203 and D/A 205 20 operate at a sampling rate of 1/T = i/T, where i is a suitably chosen integer, for example, i = 4, and 1/T = 12.96 Mbaud, for a 16-CAP transceiver providing a bit rate of 51.84 Mb/s.
The output signal of CAP transmitter 102 shown in FIG. 2 can be written as oo s(t) = n ~OO[anq(t- nT)- hnq(t- nT)], where T is the symbol period, an and bn are discrete multilevel symbols, which are sent in symbol period nT, and q(t) and q(t) are the impulse responses of the cascade of in-phase shaping filter 202 and quadrature pass band shaping filter 203, respectively, with D/A 205 and low-pass filter 206. Specifically, q(t) = p(t)($J d(t) and q(t) = p(t)~ d(t), 30 where ~ denotes convolution and d (t) is the impulse response of D/A 205 and low-pass filter 206. The band pass impulse responses, i.e., pulses, p(t) and p(t) are designed in the following manner: p(t) _ g(t) cos(27~fct) and p(t) _ g(t) sin(2~fct), where g(t) is a base band pulse and fc is a frequency that is larger than the largest frequency component in g(t). The two impulse responses p(t) and p(t) form a so-called Hilbert 35 pair, i.e., their Fourier transforms have the same amplitude characteristics and phase 213906~
D. D. Harman 2-28 characteristics that differ by 90 . The theoretical minimum for the bandwidth of the CAP
signal s(t) in the above equation is equal to the symbol rate 1 / T. The amount of bandwidth that is used in excess of this theoretical minimum is called the excess bandwidth a, which is formally defined as a ~ , where W is the bandwidth of the CAP signal's spectrum at the output of transmitter 102.
In one example, the design parameters chosen for a 16-CAP 51.84 Mb/s downstream transmitter to be employed in transmitter 102 are as follows:

Bits per symbol: m = 4 o Symbol rate: 1/T = 12.96 Mbaud Excess bandwidth: a = 0.5 (50%) . Center frequency: fc = 16.2 MHz Lowest frequency: fmin = 6.48 MHz Highest frequency: fmax = 25.92 MHz Bandwidth utilization: W = 19.44 MHz Shaping filters: square-root raised-cosine The resulting downstream spectral shape resulting from using the above parameters in transmitter 102 is shown in FIG. 4 between 6 MHz and 26 MHz. Note that very little downstream energy is transmitted below 6 MHz and above 26 MHz. This is important so that remote receiver 108 can filter out any noise below 5-6 MHz where the light dimmer noise resides without removing any substantial energy from the downstream digital signal and also allows for an upstream channel between 26 MHz and 30 MHzwithout having radiation limit problems on a twisted wire pair. Consequently, the downstream signal is not corrupted by the filtering in receiver 108. It should be noted that the above transmitter 102 parameters may be varied by the implementor as desired to get whatever spectral shape is necessary for the particular problem or problems they wish to address.
FIG. 5 shows in simplified form details of a CAP receiver 108. Shown is band-pass filter 501 which is employed to filter out the light dimmer noise and any portion of the upstream channel which may leak through hybrid 107 (FIG. 1). It is again noted that the unique spectral shaping effected in transmitter 102 facilitates the filtering of the light dimmer noise, i.e., interfering signal, via band-pass filter 501 without removing any significant amount of energy from the received downstream signal. The filtered signal is fed to analog-to-digital converter (A/D) 502 where it is converted to digital form and supplied to adaptive equalizers 503 and 504 which are utilized in generating received D.D.Harman2-28 21~9069 versions ân and bn of transmitted symbols an and bn~ respectively. One such arrangement for generating the received symbol versions ân and bn is disclosed in U.S.
patent 4,247,940 issued to K. H. Mueller and J. J. Werner on January 27, 1981, which is hereby incorporated by reference. Also see an article by K. H. Mueller and J. J. Werner entitled "A Hardware Efficient Pass-band Equalizer structure For Data Tr~nsmi~.~ion", IEEE Transactions on Communications, Vol.COM-30, March 1982, pages 538-541. It should be noted that the symbol outputs from adaptive equalizers 503 and 504 arecorrupted by noise. The received noisy versions of the symbols are supplied to decision device 505 which decides which transmitted symbols have been received. In turn, the 0 received versions of the symbols are supplied to decoder 506 which outputs a received version of the data inputted to transmitter 102 by mapping the received symbols into data bits. Note that A/D converter 502, in this example, operates at a sampling rate of 51.84 MHz and that the outputs of adaptive equalizers 503 and 504 need only be computed at the symbol rate, i.e., 12.96 MHz. Note that the outputs of adaptive equalizers 503 and 504 are sampled at the symbol rate 1/T which yields a signal constellation as shown in FIG. 6.
Upstream QPSK transmitter 109 (not shown in detail) is of a type well-known in the art. In this example, an upstream data rate of 1.62 Mb/s has been selected because it is a "nice" submultiple of the downstream data rate of 51.84 Mb/s, i.e., 51.84/1.62 = 32.
This facilitates the synchronization of the clocks used in the upstream and downstream channels. QPSK transmitter 109 transmits two bits of information in each symbol period by selecting one of four possible phases of a carrier signal. The carrier frequency chosen for QPSK transmitter 109, in this example, is fc = 29.16 MHz, which is 18 times the upstream bit rate. One example of parameters which may be employed in QPSK
transmitter 109 are as follows:
Bits per symbol: m = 2 Symbol rate: 1/T = 0.81 Mbaud . Excess bandwidth: a = 1 (100%) Center frequency: fc = 29.16 MHz . Lowest frequency: fmin = 28.35 MHz . Highest frequency: fmax = 29.97 MHz Bandwidth utilization: W = 1.62 MHz The resulting upstream spectral shape resulting from using the above parameters in QPSK transmitter 109 is also shown in FIG. 4 between 29-30 MHz. It should be noted that the above transmitter 109 parameters may be varied by the implementor as desired to D.D.Harman2-28 21~9069 -get whatever spectral shape is necessary for the particular problem or problems they wish to address.
QPSK receiver 104 is also of a type well known in the art. It is noted, however,that no D/A and A/D converters are employed, in this example, although they may be 5 employed in other embodiments and that all the signal processing is done on zero-crossing information.
Although the application of the invention has been described in the context of aCAP transceiver for transmitting video signals, it will be apparent to those skilled in the art that it may be employed to transmit and receive other digital signals and that QAM
o and other band-pass transceiver arrangements can equally be employed for transmitting video and other digital signals as desired.

Claims (10)

Claims:

1. Apparatus for use in transmitting downstream digital signals comprising:
encoder means for mapping bits into symbols; and shaping filter means supplied with the symbols for generating a predetermined frequency band limited downstream digital signal, said predetermined frequency band being such that substantially all the energy of the downstream digital signal is within the band and any interfering signals are at frequencies outside the band so that a remote receiver can filter out the interfering signals without removing any substantial energy from the a received downstream signal.

2. The apparatus as defined in claim 1 wherein the frequency band is at most from 5 MHz to 30 MHz.

3. The apparatus as defined in claim 1 wherein the encoder maps the bits into complex symbols having a real part value and imaginary part value, the shaping filter means includes an in-phase shaping filter supplied with the real part value of the symbols and a quadrature shaping filter supplied with the imaginary part value symbols and further including algebraic combining means for algebraically combining signals at the outputs of the in-phase shaping filter and the quadrature shaping filter for yielding the downstream band limited digital signal.

4. The apparatus as defined in claim 3 wherein the frequency band is at most from 5 MHz to 30 MHz.

5. A system for transmitting a downstream digital signal and for receiving the transmitted downstream digital signal at a remote location comprising:
a transmitter including encoder means for mapping bits into symbols; and shapingfilter means supplied with the symbols for generating a predetermined frequency band limited downstream digital signal, said predetermined frequency band being such that substantially all the energy of the downstream digital signal is within the band and frequencies including interfering signals are outside the band so that a remote receiver can filter out the interfering signals without removing any substantial energy from the a received downstream signal; and a receiver including a band-pass filter for filtering out interfering signals outside the frequency band of a received version of the band limited downstream signal, whereby the interfering signals are filtered out without removing any substantial energy from the received downstream signal.

6. The system as defined in claim 5 wherein the downstream signal frequency band is at most from 5 MHz to 30 MHz.

7. The system as defined in claim 5 wherein the encoder maps the bits into complex symbols having a real part value and imaginary part value, the shaping filter means includes an in-phase shaping filter supplied with the real part value of the symbols and a quadrature shaping filter supplied with the imaginary part value symbols and further including algebraic combining means for algebraically combining signals at the outputs of the in-phase shaping filter and the quadrature shaping filter for yielding the downstream band limited digital signal.

8. The system as defined in claim 7 wherein the frequency band of the downstream signal is at most from 5 MHz to 30 MHz.

9. The system as defined in claim 5 further including an upstream transmitter atthe remote location for transmitting a control channel, the control channel having a predetermine frequency band outside the frequency band of the downstream signal and a remote upstream receiver for receiving the control channel.

10. The system as defined in claim 9 wherein the upstream transmitter is a QPSK
transmitter and the upstream receiver is a QPSK receiver and wherein the control channel is in a frequency band above the frequency band of the downstream signal.