1. Common Terminology Summary
NASA CCSDS Optical Communications Working Group
May 8, 2015
May 8, 2015

2. Definitions of Terms Term Definition Bit
A unit of information transferred between data link protocol sublayer and the sync and channel coding sublayer
Code symbol
An indivisible entity transferred between the sync and channel coding sublayer and the physical layer
Channel symbol
An optical waveform used to transmit information within a unit interval (i.e., indivisible output of physical layer)
Off time
Unit time interval between channel symbols or groups of channel symbols in which no information is conveyed via an optical transmission
Burst period
Periodic interval including a single off time and an associated preceding channel symbol or group of channel symbols
Data rate
The average rate of bit transmission
Channel symbol rate
The average rate of channel symbol transmission
Bandwidth expansion
The ratio of the channel symbol rate to the data rate
Channel clock rate
The maximum instantaneous rate of channel symbol transmission
May 8, 2015

3. Suggested Notation Notation Definition 𝑇 𝐶𝐻𝑆 Channel symbol duration
𝑇 𝑂𝑇
Off time duration
𝑇 𝐵
Burst period duration*
𝑁 𝐶𝐻𝑆
Number of channel symbols per burst period*
𝑅 𝐶𝐻𝑆
Channel symbol rate (≡ 𝑁 𝐶𝐻𝑆 / 𝑇 𝐵 )
𝐵𝑊𝐸
Bandwidth expansion ( ≡𝑅 𝐶𝐻𝑆 / 𝑅 𝐷 )
𝑅 𝐶𝐻𝐶
Channel clock rate (≡1/ 𝑇 𝐶𝐻𝑆 )
* For non-burst-modulations such as BPSK and OOK , 𝑇 𝐵 = 𝑇 𝐶𝐻𝑆 and 𝑁 𝐶𝐻𝑆 =1.
May 8, 2015

4. Next Steps
Each proposed recommended standard, and associated documentation, should be updated to use this terminology
Examples of needed updates
Fix incorrect use of “bit” (all proposals for all regimes)
The word “bit” is used in many places to refer to the make-up of the ASM, unique word, control data, and header (all proposals)
According to our definition, these aren’t bits since they are not passed from data link protocol sublayer, they originate in the coding sublayer, and they do not convey information to the end user (the markers are removed before data makes it to the user) and do not contribute to data rate
The interleaver in NASA’s proposal is described as operating on “bits,” but it really operates on code symbols.
“Mbps” is used in many places in ESA/DLR proposal where bits are not present; may refer instead to symbol rate, e.g., “Msps”. Also, LPC is described as operating on “bits”. (Also, CCSDS books use “octets” not “bytes” to avoid historical ambiguity with length of byte.)
All proposals could use suggested notation, for clarity across proposals
May 8, 2015

5. Backup

6. Suggested Definitions
Bits occur at the boundary between the Data link protocol sublayer and the Synchronization and channel coding sublayer
Code symbols occur at the boundary between the Synchronization and channel coding sublayer and the physical layer
Channel symbols occur at the output of the physical layer, and they have a physical manifestation (light).
Bits
Code symbols
Channel
symbols

7. Consistency with Prior CCSDS Terminology
Previous definitions are consistent with CCSDS Prox-1 Blue Book (211.2-B-1):
Data rate = bit rate = Rd (bits / second)
Code symbol rate = Rcs (code symbols / second)
Channel symbol rate = Rchs (channel symbols / second)
Photon efficiency: photons/bit
Bandwidth expansion (BWE): Rchs / Rd (channel symbols / bit)

8. Channel Symbols in Current White Books1 1 1 1 
BM-RZ-DPSK*
Off Time 
BPSK
OOK
PPM + Guard Slots*
*Note: parameters for burst duration, off time, and guard slots were selected for illustration purposes only. They do not necessarily reflect parameter proposals in the current White Books for these signaling formats.
Guard Slots

9. Proposed Definitions (1 of 2)
Channel Symbol = An optical waveform used to transmit information within a unit interval 1 1 1 1 
BM-RZ-DPSK
TCHS = channel symbol duration
Off Time = Unit time interval between channel symbols or groups of channel symbols in which no information is conveyed via an optical transmission
TCHS
TOT
TB (NCHS=4) 
BPSK
TOT = off time duration
TCHS
Burst Period = Periodic interval including a single off time and an associated preceding channel symbol or group of channel symbols
OOK
TCHS
TB = Burst period duration (=TCHS for non-burst modulations)
PPM + Guard Slots
NCHS = number of channel symbols transmitted per burst period (=1 for non-burst modulations)
TCHS
TOT
TB (NCHS=16)

10. Proposed Definitions (2 of 2)
The channel symbol rate is the average rate of channel symbol transmission
Bandwidth expansion is the ratio of the channel symbol rate to the data rate
The channel clock rate is the maximum instantaneous rate of channel symbol transmission
Rationale
Channel symbol rate and bandwidth expansion definitions are consistent with widely-accepted approaches for analyzing spectral and power efficiency
For optical links, cost in terms spectral efficiency of adding off time or guard slots is negligible
New metric, channel clock rate, can be used to assess the cost of high-rate electronic processing required for optical signal processing
RCHS = NCHS/TB = NCHS/(NCHS*TCHS + TOT)
BWE = RCHS/RD
RCHC = 1/TCHS

11. Channel Symbol Rate Examples
RCHS = NCHS/TB 1 1 1 1
RCHS = 4/(4*TCHS + 4*TCHS) = 1 / (2*TCHS) 
BM-RZ-DPSK
TCHS
TOT
TB (NCHS=4) 
BPSK
RCHS = 1 / TCHS
TCHS
OOK
RCHS = 1 / TCHS
TCHS
PPM + Guard Slots
RCHS = 16/(16*TCHS + 2*TCHS) = 8 / (9*TCHS)
TCHS
TOT
TB (NCHS=16)

12. Channel Symbol Rate Example: BM-RZ-DPSK
RCHS = NCHS/TOB
NASA Proposed High Data Throughput Modes (for Rate ½ DVB-S2 FEC)
Mode Name
User Data Rate (Mbps)
Duty Cycle
Q Repetition
RCHS (GHz)
BWE RCHS/RD
U-1244
1244 1
2.88
2.27
U-622
622
1/2
1.44
U-311
311
1/4
0.72
U-155
155.5
1/8
0.36
U-51.8
51.8
1/24
0.12
U-16 16
1/40 2
0.072
4.38
U-8 8 4
8.76
U-4
17.52
U-2
35.04

13. Channel Symbol Rate Example: BM-RZ-DPSK Photon Efficiency vs
Channel Symbol Rate Example: BM-RZ-DPSK Photon Efficiency vs. Bandwidth Expansion
U-2
U-4
U-1244
U-622
U-311
U-155
U-51.8
U-8
U-16

14. Alternate Definition for Channel Symbol Rate, RCHS
The channel symbol rate is the maximum instantaneous rate of channel symbol transmission
RCHS = 1/TCHS

15. Alternate Definition Examples
RCHS = 1/TCHS 1 1 1 1
RCHS = 1/TCHS 
BM-RZ-DPSK
TCHS
TOT
NCHS=4 
BPSK
RCHS = 1 / TCHS
TCHS
OOK
RCHS = 1 / TCHS
TCHS
PPM + Guard Slots
RCHS = 1/TCHS
TCHS
TOT
NCHS=16

16. Alternate Definition Example: BM-RZ-DPSK
RCHS = 1/TCHS
NASA Proposed High Data Throughput Modes (for Rate ½ DVB-S2 FEC)
Mode Name
User Data Rate (Mbps)
Q Repetition
Def 2 RCHS (GHz)
Def 2 BWE RCHS/RD
U-1244
1244 (Max) 1
2.88
2.27
U-622
622 (Max/2)
4.54
U-311
311 (Max/4)
9.08
U-155
155.5 (Max/8)
18.17
U-51.8
51.8 (Max/24)
54.50
U-16 16 2
175.18
U-8 8 4
350.35
U-4
700.70
U-2

17. Alternate Definition Example: BM-RZ-DPSK Photon Efficiency vs
Alternate Definition Example: BM-RZ-DPSK Photon Efficiency vs. Bandwidth Expansion
U-4
U-8
U-16
U-1244
U-622
U-155
U-51.8
U-311

18. Alternate Definition Example: BM-RZ-DPSK Photon Efficiency vs
Alternate Definition Example: BM-RZ-DPSK Photon Efficiency vs. Bandwidth Expansion
U-4
U-8
U-16
U-1244
U-622
U-155
U-51.8
U-311