IEEE 802.14-95/128

TITLE: INTRA Modem PHY Proposal
AUTHOR: Bill Miller
ABSTRACT: The Information Transformation (INTRA) modulation is formally proposed as a candidate for the PHY layer for 802.14.

The following paper is based on Document # IEEE 802.14-95/128 prepared to assist and submitted to the IEEE 802.14 WG Cable TV Protocol Working Group November 6, 1995.

Summary

Modulation based on INformation TRAnsformations (INTRA) is proposed for upstream and downstream communication in CATV networks (see Notes). Proposed parameters are:

Table 1
RATE (Mbps)	CLASS	BW (Mhz)	TYPE
20	FM-DSB	6	Non-coherent
51.84	FM-SSB	6	Non-coherent
54	AM-SSB	6	Coherent

A sub-split system would use very robust FM-DSB for two-way interactive data, whereas a high-split system could optionally use the higher rate FM-SSB. The AM-SSB modem can carry HDTV in Asynchronous Transfer Mode (ATM) plus plant-management signaling (2Mbps). The INTRA AM modem generates data at baseband (0-6Mhhz) and then translates (AM-SSB) to any passband downstream CATV channel. While FM could be generated at passband, the proposed scheme generates FM digitally in 0-6Mhz and then translates to any CATV channel. Specific design equations and bandwidth efficiencies for INTRA FM, and the general properties of INTRA AM and its compatibility with both compressed and uncompressed video have been described previously in IEEE802.14-95/094 and are reviewed briefly below.

Theory of Linear INTRA

The concept of baseband INTRA can be visualized as an apparent coordinate rotation. The rotations are performed by baseband multi-rate filter banks. In modems the Information being conveyed exists in two distinct but equivalent forms; namely, as digital DATA and as a band-limited analog SIGNAL. A group of N samples of the SIGNAL at the D/A converter comprise the coordinates of an N-Dimensional vector. The DATA must be another representation of the same vector. That is, the DATA must be the same vector projected onto another coordinate system.

The modulation operator [M] rotates the representation of the Information vector from DATA coordinates into SIGNAL coordinates. The demodulation operator [D] rotates the A/D converter vector back to DATA. As with any modem, [D][M] = [I] , where [I] is Identity. By insisting that the operators are rotations, they must also commute so that [D][M] = [M][D] = [I]. At large N a band-limited Gaussian signal, G, can be demodulated and re-modulated, since [M][D]G =G. This proves that the modulator is capable of sending a SIGNAL with the maximum possible entropy, which is the prerequisite for an optimum modem receiving at Shannon®s limit.

Because modulation and demodulation are commuting operators in an INTRA modem, it is possible to send any band-limited analog signal through a demodulator to rotate it to DATA, then digitally encrypt that DATA, and finally use a modulator to rotate back into an analog SIGNAL. This provides the long sought means to encrypt without expanding the bandwidth ; or stated another way, to digitally encrypt without compression. The practical significance of this, aside from security, is that voice or NTSC video, for example, can be digitally networked, error protected, and so forth without compression. Compression-less transmission can be competitive in actual bandwidth to compression algorithms because the latter react poorly to bit errors unless there is considerable error protection.

Quadrature Mirror Filter Banks (QMF) have commuting polyphase matrices so they can perform the rotations. Note that the 2-Dimensional QMF performs the Discrete Wavelet Transform by Multi-Resolution Analysis. The Perfect Reconstruction (PR) property of QMF pairs make an exact Identity operator [I]; although Near Perfect (NPR) QMF banks, particularly those with linear phase, are a better choice for modems. Because the rotations are performed by sub-band filter banks, it is easy to determine which DATA coordinates will rotate into in-band frequencies for the SIGNAL representation. The filter overlap and rolloff is a design parameter in QMF®s (unlike an FFT which has a fixed 13db stop-band attenuation) so the commuting property can exist over any in-band region to make an optimal modem.

AM Modems

An AM Modem is a linear mapping from baseband to passband. For CATV a 6Mhz linear baseband modulator can be followed by a Headend Translator in a manner similar to channel translation of baseband NTSC to any downstream CATV channel. With all forward channels phase-locked, any video carrier can be used by an INTRA AM Modem receiver as a frequency reference for coherent demodulation back to baseband . The design of translator modules is straightforward so only the linear baseband modem is examined here. The AM Modem would carry digital video and data, possibly in an ATM format, plus Network Management. It could also provide uncompressed NTSC through the same modem, if desired.

The full 6Mhz baseband channel can be defined by a 12Mhz sample rate. An 8-Dimensional rotation has 6 coordinates that are useable without either a DC response or aliasing. That is, the inner 6 sub-bands will provide an overall response profile that fits within the translator®s SAW filter. Since the symbol rate is 12/8 of the sample rate, the bit rate for B bits per channel is 6*B*1.5 Mbps. Using B=6 there are 36 bits per symbol at 54Mbps. The 3db points of the outermost sub-bands will be at 1/8 and 7/8 or 750khz and 5.25Mhz. The AM Profile is then given by the filter response curves. (Using 12 dimensions would allow 51Mbps with B=5 and 6 db less S/N). An optional sync bit in the upper sub-band provides sync at 5.25Mhz. The lower sub-band can transmit a sync at 625khz and/or at 750khz. The sample rate would be phase-locked to the passband translator so the receiver can also use any video carrier as a pilot. The nominal delay is equal to 36 bits at 54Mbps for framing, plus 72 samples in the 12Msps rotation filter or D = 36/54 + 72/12 = 6.67 microseconds.

AM Channel Profile
-3db BW	750khz to 5.25Mhz
-60db BW	375khz to 5.625Mhz

 

Theory of Non-linear INTRA

The FM Modem is an example of a non-linear mapping of baseband into passband that has good immunity to impulse noise and narrow-band interference. An INTRA FM Modem can have gain in its non-coherent receiver (compared to say, GMSK, which is often called FM). This gain permits an FM receiver to use more signaling levels than an AM receiver for the same Carrier-to-Noise ratio and BER. The gain comes from three factors:

1. Modulation Gain	= 6(m+1)(m^2)/(PAR)^2
2. De-Emphasis Gain	= Gs(N^3)/(N-1)	Gs = (H(I)^2)/(3I^2+3I+1)
3. Noise-Reduction Gain	= F(Mu)

Modulation gain comes from collapsing the inherent band spreading of FM. For the FM-SSB Modem the spreading is negligible. The other two gains employ the principle that a baseband nonlinear amplification (log-amp) at the transmitter will have no net effect on the signal if there is a matching (inverse-log) attenuation at the receiver. Noise at the receiver will be attenuated, thereby giving an improvement in the Signal-to-Noise ratio, i.e. gain. Noise-Reduction uses a log-amp in the SIGNAL coordinate representation to suppress link noise, whereas De-Emphasis uses logarithmically related amplifiers in the DATA coordinate representation to suppress parabolic noise out of the FM discriminator.

SIGNAL Domain Non-Linearity

For Noise-Reduction a nonlinear SIGNAL look-up table, the Mu-Law function, is proposed with Mu=1023 where the Mu-Law OUTPUT is related to the INPUT by

OUTPUT = SIGN(INPUT)*Vc*LOG2{1+Mu*ABS{INPUT/Vp))/lLOG2{1+Mu}

The peak magnitude of the INPUT is Vp and the peak magnitude of the OUTPUT is Vc so that fewer bits of resolution are needed for the analog components than for the rotation.. The Mu-Law provides Noise Reduction; and since it amplifies small signals more than large signals , it improves the Peak-to-Average Ratio of the baseband signal which serendipitously increases the modulation gain.

DATA Domain Non-Linearity

For the DATA representation non-linearity the following pre-emphasis gain table, H(I), is proposed

TABLE 1
Sub-Split (20Mbps)
I	0	1	2	3
H(I)	1	2	4	1
B(I)	7	6	5	0

High-Split (51Mbps)
I	0	1	2	3	4	5	6	7
H(I)	1	2	4	8	8	8	16	1
B(I)	9	8	7	6	6	6	5	0

The binary shifts required by H(I) in TABLE 1 are easily implemented. A SYNC bit is carried in sub-band 3 with no pre-emphasis (H(3)=1).

FM Modems

The FM-DSB efficiency for N=4 at E=25 db is 3.564, so 5.61Mhz is needed for 20Mbps. The profile for FM-DSB is specified by its Bessel functions. Less than 1% per side of its power is outside the Carson BW.

FM-DSB Channel Profile
-20db BW (98%)	195khz to 5.81Mhz

The efficiency of an FM-SSB modem is 2B/[N(m+1)]. Examples from IEEE8-2.14-95/94 are repeated here

TABLE 2 (N=8)
10LOG(E) (db)	FM-SSB (efficiency)	Benchmark
0	2.81	QPSK (4-QAM)
7.0	3.85	16-QAM
13.2	4.93	64-QAM
35	9.36	approx. 16k-QAM

Switched Carrier

Some multiple access protocols require switched carrier operation. Upstream TDMA is an example that is under consideration. FM modems are particularly well suited because of:

1. Non-Coherence	---no carrier acquisition time
2. FM Capture Effect	---turn-on and turn-off can overlap

Sub-Split Systems

Sub-split systems have difficult narrow-band noise in the upstream direction. FM-DSB is proposed for such systems because it should be more robust than any other modulation. Impulse noise or ingress from shortwave radio should have little effect on constant-envelope FM.

High-Split Systems

For High-Split systems the upstream channel can use the less robust FM-SSB and still have the benefits of FM for switched carrier operation. High-split upstream channels are presumed to have a noise floor of 45db. This permits 51Mbps in a 6Mhz upstream channel.

Range and BER Measurement

To obtain range timing one end must turn around the signal in hardware to provide a fixed delay. Proposed MAC layer TDMA protocols place ranging measurement in the headend so the turnaround is done by the NIU. FM links can use that method but another possibility is to have the headend translate the upstream traffic to a downstream 6Mhz channel. In that case all modems can "see" there own encrypted upstream transmissions and each modem can measure not only its slot timing but also the effects of ingress noise and request-slot collisions. Local measurements by the NIU of the upstream noise funneled to the headend (and translated back to the NIU) should simplify closed-loop adaptive noise cancellation in the NIU. This arrangement also permits each modem to monitor the upstream bit-error rate and report it to the MAC layer.

Channel Selection

Each modem would be capable of tuning to any 6Mhz channel in the designated direction. The lowest upstream channel would be T-7 (5.75-11.75Mhz). Installation of the PHY layer would begin by a search of downstream channels for a recognized protocol and once recognized, the downstream protocol would identify the upstream channel number.

Management

All modems would be capable of digital and RF loop-back, power adjustment, and BER measurement in response to network management commands on the forward channel recognized at power-on initialization or, in response to the MAC layer. The use of robust FM Modems in 6 Mhz upstream channels simplifies the cable plant and avoids the need for extensive dynamic channel assignment due to ingress in sub-split systems.

Block Diagrams

A rotation is equivalent to a VECTOR FILTER. The shift registers contain vectors. The weights are matrices. This is shown in Diagram 1 for the 4-Dimensional FM-Modem. In that proposed FM-DSB design there are 8 matrices and each is 4x4. Because of symmetries there are actually only 32 numbers needed to construct these 8 matrices and another 8 matrices that are used for the counter-rotation filter. The 8-Dimensional Modems can use shift registers with 9 matrices of 8x8. Both vector filters are linear phase in their overall response even though the polyphase (down-sampled) coefficients are not symmetric.

Diagram 1 "Vector Filter"

For the AM Modem the input data is grouped 8 coordinates as 6 words of 6 bits each, plus two 1-bit sync words. For the FM-DSB Modem the input data at the transmitter is grouped into 18-bits divided into 3 words of 7,6,and 5 bits according to TABLE 1 and a toggling sync bit for the fourth coordinate. The word for each coordinate is then converted to PAM form; namely, +-1,+-3,+-5,+-7,». The conversion from serial to parallel, subdivision into words, and conversion to PAM are called "PARTITIONING" in Diagram 2.

Diagram 2. "Partitioning"

PRE-EMPHASIS is obtained by multiplying the coordinates of the DATA after PARTITIONING by the amplification factor H(I) from Table 1. The multiplication can be done in hardware by a binary shift of LOG2{H(I)}. The resulting vector is then rotated by a VECTOR FILTER into its SIGNAL representation. The SIGNAL coordinates can be converted from parallel to serial to form a sequence prior to being non-linearly amplified by a Mu-LAW look-up, and the result is input to the FM modulator as a digital sequence.

Diagram 3. "Mu-Law Companding"

The building blocks are combined to make an FM Modem Transmitter in Diagram 4. The FM modulator is represented in Diagram 4 as an all digital voltage controlled oscillator (VCO) followed by a channel TRANSLATOR . The VCO could be implemented digitally with a phase accumulator. A Hilbert transformer and exponential lookup table can be appended to the VCO to generate SSB digitally.

Diagram 4. "FM Modem Transmitter"

Diagram 5. "AM Modem Transmitter"

Diagram 6. "PHY Interface"

Note: If included in an IEEE 802.14 standard the Information Transformation (INTRA) technology and INTRA Modems described herein will be licensed on reasonable and non-discriminatory terms and conditions.