Randomness and metastability in CDMA paradigms

Code Division Multiple Access (CDMA) in which the signature code assignment to users contains a random element has recently become a cornerstone of CDMA research. The random element in the construction is particularly attractive in that it provides robustness and flexibility in application, whilst not making significant sacrifices in terms of multiuser efficiency. We present results for sparse random codes of two types, with and without modulation. Simple microscopic consideration on system samples would suggest large differences in the phase space of the two models, but we demonstrate that the thermodynamic results and metastable states are equivalent. This may have consequences for developing algorithmic methods to escape metastable states, thus improving decoding performance.