IEEE 802.14-96/097

TITLE: Analog Interface for Upstream INTRA
AUTHOR: Bill Miller

ABSTRACT: Upstream FM INTRA designs will operate with a 6-bit D/A and wideband cable driver. A previously proposed linear INTRA modulator adapted for 0-36MHz requires no multipliers, no frequency guard bands, and no guard-time or ramp-up gap between bursts when used for upstream with a 6-bit D/A.

The following paper is based on Document # IEEE 802.14-96/097 prepared to assist and submitted to the IEEE 802.14 WG Cable TV Protocol Working Group March 8, 1996.

Upstream Interface

An upstream INTRA modulator consists of a 75-ohm wideband (40Mhz) amplifier, a D/A converter with alias filter, and the digital circuitry and its clock circuit. A 6-bit D/A is adequate for either non-linear (FM-DSB or FM-SSB) or linear INTRA. A block diagram for a 12-Dimensional linear INTRA modulator is shown in Figure 1.

 

Block diagrams for a non-linear FM modem were given previously in IEEE 95-128 and 96-015. The FM simulation results in the -015 paper show the required digital accuracy at the baseband input to the VCO to be under 12-bits for a simplified 10.5 Mb/s FM modem. If the FM were received by an analog FM circuit at RF ---which would have a high component count--- then a 12-Bit A/D converter would be needed for the baseband signal but with a sample rate of only 3 MHz. As noted in that paper, an alternative for the modulator is to digitally calculate the FM signal for the 5-35 MHz passband and then the only analog circuitry required is a 6-bit D/A and cable driver. In the latter case the sample rate is over 70Mhz but the resolution required is much less since the amplitude of the FM signal conveys no information about the data.

The FM-DSB data in Table 1 is for the constant envelope 10.5 MB/s modem given in 96-015 assuming the cable noise is at -40 DB. Even single sideband FM can use 6-bits as Table 1 shows in line 2. Margins of less than 17.8% will give 10^-8 so this Table, which shows small increases in margin with a decrease in quantization levels, indicates a good BER at 6 bits resolution for guassian noise.

In Table 1 the FM-SSB transmissions were generated by amplitude modulating the FM with where h is the Hilbert transform of the baseband FM input; consequently, FM-SSB is not a constant envelope signal, but a 6-bit A/D is still sufficient.

Unlike other RF modem proposals, the linear 12-Diensional design in Table 1 (see Figure 1) does not use a carrier. Instead it creates a signal in a 0-36MHz passband by baseband filtering the digital data as described further below.

Table 1. A 6-bit D/A is adequate
D/A Resolution	5 bits	6 bits	7 bits	8 bits
FM-DSB margin	3.72%	2.93%	2.81%	2.71%
FM-SSB margin	8.14%	5.95%	5.91%	5.67%
12-D margin	3.44%	2.06%	1.49%	1.27%

 

 

Linear INTRA for Upstream

A linear INTRA modem for upstream is depicted in Figure 1. In this example, the 12 dimensional INTRA modulator appearing in 96-014 for downstream has been used for upstream. All 12 sub-bands together span the range from 0 to 36MHz, or 3MHZ per sub-band. Two modem transmitter spectra for two adjacent sub-bands is shown superimposed in Figure 2 to show how two modems overlap in frequency. An 802.14 MAC would normally use only one sub-band.

In the downstream proposal 96-014, the set-top receiver? digital complexity in millions of multiplications/sec was given for a 12-D demodulator; but for upstream modulation no multiplier is required at all, and a 6-bit converter is adequate. Table 2 shows the error margin versus output (D/A) sample resolution for a 12MB/s modem in white noise.

 

Figure 2 Two Overlapping 12 MB/s Upstream Modems

When -40db guassian noise was added to one modem from Figure 2 the error margin was 1.5% as shown in Table 1. In Figure 4 the simulated cable noise spectrum in Figure 3 was added to one modem output from Figure 2. The error margin was then 3.2%, indicating only a minor (1.7%) effect from the RFI appearing in the modem? sub-band in Figure 3.

The 12-D linear INTRA modulator is just a 12-band multi-rate FIR filter bank. It was shown at the Montreal meeting that the QMF bank can be re-drawn as a vector filter. For the linear phase filter bank used for the simulations, the vector filter has 5-taps and each tap weight is a 12x12 scalar matrix.. For upstream, there is only 1 sub-band to be generated ----elsewhere, other unrelated services on the cable may use the remaining sub-bands. If the modem is operating at 4 bits/sec/Hz (note: a practical 16-QAM is less efficient) then a sub-band carries 2 bits so the entire vector filter contains only 10 bits. Instead of computing each output sample from the sum of the 5 taps, each sample can be obtained by using a 10-bit address into a look-up table to immediately obtain the sample for the D/A. In high interference, only 1 bit per sub-band could be used for an efficiency of 2 b/s/Hz (compared to QPSK at 1.4-1.8). A pilot can accompany the burst transmission to aid the receiver.

A feature of wavelet-based modulations like INTRA is that adjacent sub-bands can overlap in frequency. This overlap enables INTRA to exceed the efficiency of n-QAM, which loses bandwidth efficiency due to the rolloff of QAM? raised cosine filtering. In 96-014, it was shown that the coefficients of the receiver? vector filter can be adaptively equalized to account for perturbations to the orthoganality of the sub-bands.

The INTRA modem can also overlap in time so that no guard & ramp-up time is needed for TDMA, which is particularly useful for ultra short packets in a burst-mode voice protocol. If user A finishes its packet and inserts "no-data" into the 5-tap vector filter, there will be a non-zero D/A output due to the 4 vectors (4x12 samples) still in the filter. With no prior data in its vector filter, User B can insert its data bits into its own vector filter at the same instant that user A inserted "no-data". At the headend User A and User B RF signals will add as though the signal came from one FIR filter. Thus the received data streams are contiguous with no gap. Since each data symbol persists in the filter for 60 samples, the power ramp-up/down for each burst is seamless. Note that the data bits are end-to-end delayed by the length of the modulation filter (1 microsec , or 60=5x12 samples in this case).

Conventional modulations do not spread out each bit over many samples like INTRA. This feature was seen in 96-014 to make linear INTRA robust to impulse noise (or clipping) that might corrupt some of the samples in a symbol. The combined effect of robustness, overlapping bands and contiguous time slots makes INTRA a very efficient alternative for 802.14.

 

Figure 3 Upstream Ingress Noise

 

Figure 4 12 MB/s Linear INTRA plus Ingress