Paradise P300 specifications + Fec

For some applications, particularly when operating with lower data rates, a reduction in the processing delay associated with Reed-Solomon FEC is the prime goal. In such circumstances use the `Custom Features` feature and select shorter codewords to reduce the encoder / decoder & interleaver / de-interleaver delay. For example switching from INTELSAT standard values of n, k, t = 126, 112, 7 to 64, 56, 4 provides approximately the same correction ability (7 in 126 = 5.5% and 4 in 64 = 6.25% respectively), with similar overheads (126/112=12.5%, 64/56=14.3%), but with interleaving & decoder delays reduced from 5632 to 3104 bits (156ms to 85ms at 32kbps). If using Sequential FEC, see also `Fault Mode = 3` in Appendix E.

8.5NOTES ON DATA RATES & SYMBOL RATES

The normal constraint for a modem is the range of Symbol Rates over which the modem can operate correctly. The Symbol Rate range however is not of initial interest to most modem users, as they first need to know if the modem can operate at the terrestrial Data Rate they require. This section briefly explains the relationship between Data Rates and Symbol Rates.

The Data Rate is normally the frequency of the clock used on the terrestrial port however if Drop/Insert is active the Data Rate is 64kbps times the number of timeslots dropped/Inserted.

As the P300 Series Modem use PSK (Phase Shift Keying) the Symbol Rate is the rate of phase changes on the I.F. signal. Each phase change represents a new symbol to the demodulator, and depending on the modulation scheme that symbol may convey one (BPSK), two (QPSK), or three (8PSK) bits of information.

The diagram below shows how the symbol rate is built up from the data rate.

Terrestrial	Data Rate	Data Rate	Data Rate	Baseband signal	IF Signal Modulated
Data Rate	+ Framing	+ Framing + RS	+ Framing + RS	at Symbol Rate	at Symbol Rate
			+ FEC

Clock

Data

Data Rate Increase:

FRAMING

(if any)

IBS/SMS: x 1615

IDR: +96kbps

REED-

SOLOMON OUTER FEC CODEC

(if any)

Specified as (N.K)

e.g. (219,201)

x NK

FORWARD

ERROR

CORRRECTION

(FEC)

1/2 Rate: x 2

2/3 Rate: x 23

3/4 Rate: x 34

7/8 Rate: x 78

Uncoded: x 1

MODULATOR

BPSK: 1

QPSK: 2

8PSK: 3

IF STAGES

Occupied Bandwidth:

1.2 x Symbol Rate.

Intelsat Recommended Carrier Spacing: 1.4 x Symbol Rate

Example:	+ IBS/SMS framing:	+ (126,112) RS Coding:	+ 1/2 Rate FEC:	+ QPSK:	Occupied b/w= 92.16KHz
64 kbps	=68.266kbps	=76.8kbps	=153.6kbps	=76.8kSym/s	Carrier Spacing: 107.52KHz

You will see that with ½ rate FEC and Q PSK (a common combination), the Symbol Rate is the same as the Data Rate (assuming no Framing or Reed-Solomon). This is a useful combination to check on any data sheet, as this will give you the Symbol Rate limits of the modem, from which the Data Rate limits in all other modes and combinations can normally be derived.

P300H P300 Series Modem Installation and Operating Handbook Page 145