With a total bitrate around 48 Gbps, bandwidth is obviously an issue that we will need to deal with. Therefore, while the design will implement a raised cosine pulse, the roll-off factor will be a very conservative alpha = 0.2.
Modulation Scheme
In an effort to reduce our bandwidth significantly, and in anticipation that our Moon Satellite to Earth Satellite link will be exceptionally clear with low-noise and no inteference, we are planning on implementing 1024-QAM as a modulation scheme. Choosing QAM will divide each datastreams into Quadrature and In-phase streams for transmit, doubling the number of total streams with which we are dealing.
Coding
Before transmitting our admittedly very tightly packed I's & Q's for all channels, our design calls for each stream to be Turbocoded at rate (1/2). Turbocodes are currently the one coding scheme that allows us to approach the Shannon capacity of a channel. Under good conditions with several rounds of coding, Turbocodes will give a 10^-6 BER at just 0.7 dB SNR. In our case, it's likely that the number of iterations required to go so low will be impossible due to processing requirements. However, we approximate that a rate (1/2) code should yield an acceptable BER at 1 dB SNR.
Using a raised cosine of alpha=0.2, an optimistic QAM-1024, and a rate (1/2) turbocode, calculations below show that we will be consuming an RF Bandwidth of about GHz per channel.
CDMA: Spreading Codes
While a bandwidth of 1.875 MHz per channel for such a high datarate is not so bad, we now have 24576 channels (when you split the I/Q) of equally high bandwidth to transmit. In order to get these transferred without resorting to a monstrous FDM, we have decided to use CDMA. 24576 channels, or "users", requires a minimum maximal length sequence of 24576 which means we need at least N=15 shift registers to yield a maximal length sequence of 32770. As shown below, this will yield a huge spreading bandwidth of 61.44 GHz. Note that this is comparable to the bandwidth required for fewer digitizers at an equal total bitrate.
