Noise-Loop Multiple Access

A fundamental problem for signal processing in wireless communications is the separation of multiple signals/ users from their mixture. The problem would be simpler if they are non-overlapping in either time domain or frequency domain. Two signals may be overlapping in both time and frequency, but they still can be easily separated if they are uncorrelated (orthogonal), like in Code Division Multiple Access system (CDMA). It may be interesting to know that two signals may be overlapping in both the frequency and time domain and also not orthogonal to each other, but they can still be separated if each autocorrelation function at the receiver is separable in the time-lag domain. In this paper, we will show how the Noise-Loop (NL) modulation, originally thought to produce security at the physical layer, can be used to effectively separate uncoordinated/non-orthogonal users sharing the same time and frequency interval. In order to show the potential of noiseloop multiple access (NLMA) scheme, the theoretical analysis is carried out and compared with simulations. In particular, together with the error probability, a new figure of merit is derived: the multiuser efficiency index. It can be shown that the NLMA is able to distinguish the bit stream of the users regardless of the their number. In addition, the capacity region of the NLMA is derived and compared to conventional CDMA scheme. All the above features together with the intrinsic security make the NL an interesting scheme for wireless technology.