After signals leave a high efficiency, 300m diameter dish antenna pointed at deep space and amplified further with an LNA and filtered, they are downconverted in 245 KHz chunks and fed into 489 Ks/s 8-bit ADCs. 8 Gs/s digitizers are available, if we wanted to sample the entire bandwidth at once, they are exceptionally expensive just for terrestrial use. Due to the challenges of being in a harsh space environment such as the moon, that kind of sampling rate may not be available in a space-certified device no matter the price. CMOS, the technology many ADCs are based around, often has problems in a space environment. An even bigger problem than hardware cost, however, is how that data is going to be sent to Earth [5]. By taking smaller bites, so to speak, we can use CDMA to spread out our signals to the bandwidth we would have required to start with (or thereabouts) yet achieve massive processing gain. Compare the 10 MW required to send our signal with 3 digitizers with CDMA to our current scheme with 12288 digitizers that is able to complete the job with just one watt. Furthermore, low sampling rates are easy to achieve, making the design feasible, even if there are massive arrays involved.
The block diagram without the RF chain is shown in the figure below.
Output Datarate
While we may be taking small bites of the bitrate, the total amount of bandwidth still needs to be optimized for feasible operation. The bandwidth, symbol, and coding considerations will be further explored and calculated in depth in the NEXT section, Communication Schema.
