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Use of Code Division Multiple Access (CDMA) to Support Multiple Spacecraft per Antenna: 

Use of Code Division Multiple Access (CDMA) to Support Multiple Spacecraft per Antenna

The Requirements for Multiple Spacecraft per Antenna: 

The Requirements for Multiple Spacecraft per Antenna The plans for lunar exploration involve multiple spacecraft orbiting or on the surface of the Moon Due to the Earth-Moon geometry, a significant number of these missions will have line of site to the Earth simultaneously. Near continuous ranging for early lunar missions is required due to the lack of Moon gravity model

Why Code Division Multiple Access (CDMA)?: 

Why Code Division Multiple Access (CDMA)? CDMA more easily enables 24/7 TT&C support A single frequency allocation can be simultaneously used by multiple platforms Simultaneous ranging, commanding and telemetry for multiple users CDMA requires much less spectrum than FDMA for the expected maximum lunar constellation size (could be up to 20) Compatibility with TDRSS can be maintained Overall ground implementation may be less expensive and less cumbersome Each service equipment chain virtually identical Low power combining perhaps possible on uplink There is no hard cut-off for the number of active platforms which can be supported Allows for demand access service

Achievable Data rates using CDMA: 

Achievable Data rates using CDMA The achievable data rates using CDMA are bounded by the basic requirement for a 10:1 ratio of PN chip rate to symbol rate. (3 Mcps/10 = 300 ksps for TDRSS). The current use of concatenated codes (Reed-Solomon and Convolutional coding) lead to a maximum possible information data rate of ~130 kbps. Use of high code rate Turbo or Low Density Parity Check codes would allow up to 280 kbps to be transmitted over the same channel. There are some interference effects between users as the number of users increases. Methods to avoid interference include: Limiting the data rate per user Use of different type Use of orthogonal Walsh codes Cellular industry PN codes which reduce correlation between signals to zero Use of multi-user detection interference mitigation Use of time-shared CDMA approach (require some scheduling)

Achievable Downlink Data Rate Simulation Results: 

Achievable Downlink Data Rate Simulation Results Using simulation techniques and considering PN/carrier/symbol synch acquisition and tracking, the downlink maximum recommended data rate versus lunar constellation size is as follows: Note: • For data rates < 40 kb/sec, BER and symbol synch simulations could not be performed – excessively long simulation run-time

Achievable Uplink Data Rate Simulation Results: 

Achievable Uplink Data Rate Simulation Results Using simulation techniques and considering PN/carrier/symbol synch acquisition and tracking, the uplink maximum recommended data rate versus lunar constellation size is as follows: Note: • For data rates < 40 kb/sec, BER and symbol synch simulations could not be performed – excessively long simulation run-time

Simultaneous Ranging: 

Simultaneous Ranging All user transponders receive the same uplink signal Common uplink PN code User command data is addressed to individual users Single link could also include broadcast information for all users Each user transmits unique downlink signal Each user has same carrier frequency Different PN code for each user Downlink doppler and PN code epoch delay different for each user

Simultaneous Ranging (cont’d): 

Simultaneous Ranging (cont’d) Epoch Uplink User 1 Epoch User 2 Epoch User 3 Epoch User 4 Epoch Each user transponder synchronizes its return link PN code epoch to the received forward link long code epoch The delay between the transmit epoch and the received return link epochs will give the range measurement to each user

CDMA versus FDMA: 

CDMA versus FDMA An FDMA S-band Command and Telemetry communications system requires approximately 3 MHz of spectrum per lunar platform Assumes a 1.024 MHz subcarrier LRO which is tentatively using a 1.7 MHz subcarrier is pursuing a 5 MHz frequency allocation A total frequency allocation of up to 120 MHz is required to support the uplinks and downlinks for 20 lunar platforms using an FDMA approach Using a CDMA approach, these 20 lunar platforms can be supported using a total frequency allocation of 12 MHz Ground terminal Power Flux Density (PFD) levels must be characterized and compared to allowable limits Utilization of CDMA will put all high power signals to and from the Moon at the same frequency

Conclusions: 

Conclusions The use of CDMA/spread spectrum has provided many benefits to TDRSS users Simultaneous ranging, command, and telemetry at higher data rates than tone ranging Frequency re-use Demand access service Multiple users at or in the vicinity of the Moon will all be in the same antenna beam at S-Band (12 m antenna), so operations comparable to TDRSS multiple access will be possible Use of CDMA greatly reduces the need for S-band frequency allocations while still providing for simultaneous user support.

Back-up Material: 

Back-up Material

TDRSS-Compatible PN Spread Signal Format: 

TDRSS-Compatible PN Spread Signal Format Forward Service (Command) I-Channel (Command Channel) Short PN Code: PN Code Modulo-2 added asynchronously to Command Data PN Code Family: Gold Codes per 451-PN CODE-SNIP PN Code Length: 210-1 chips PN Chip Rate: ≈ 3 Mcps Coherent with carrier frequency: 31/(221x96) x Carrier Frequency Q-Channel (Range Channel) Long PN Code: PN Code Only (No data on this channel) PN Code Family: Truncated 18-state shift register sequences per 451-PN CODE-SNIP PN Code Length: (210-1) x 256 Chips PN Chip Rate: ≈ 3 Mcps Coherent with carrier frequency: 31/(221x96) x Carrier Frequency Therefore, long code chip rate synchronized to short code chip rate

TDRSS-Compatible PN Spread Signal Format: 

TDRSS-Compatible PN Spread Signal Format Q-Channel (Range Channel) Long PN Code (cont’d): Long Code Repeat Rate: 85 ms Long Code Epoch Reference: All 1’s condition synchronized to command channel PN code Epoch

TDRSS-Compatible PN Spread Signal Format: 

TDRSS-Compatible PN Spread Signal Format Return Service (Telemetry) Data & Long PN Code on both I-Channel & Q-Channel: Long PN Code Modulo-2 added asynchronously to Data on both I-Channel & Q-Channel PN Code Family: Truncated 18-state shift register sequences per 451-PN CODE-SNIP PN Code Length: (210-1) x 256 Chips PN Chip Rate: ≈ 3 Mcps Coherent with carrier frequency: 31/(240x96) x Carrier Frequency I & Q Channel Offset: PN Code epoch on Q-Channel delayed x+1/2 chips relative to I-Channel PN code epoch (SQPN) PN Code Epoch Reference: All 1’s condition on I-Channel synchronized to the uplink range channel (Q-Channel) PN code Epoch