CODE-ONLY DEPENDENT ASYNCHRONOUS

Abstract The use of CDMA communication systems over fre-quency selective channels requires channel estimationor complex receiver structures to mitigate multi-user interference (MUI). Different classes of codes such as{Zold codes or wave let packets have been adopted. Butrecently new codes, called Lagrange codes, have beendesigned which include OFDM and multi-carrier trans-missions as special cases. With simple linear (so calledVandermonde) receivers, Lagrange codes offer completeMUI cancellation independent of the channels’ impulseresponses, by converting the effect of frequency selec-i!ive channels into flat fading. In this work we analyzei!he performance of CDMA-Lagrange transmitters andVandermonde receivers, when each user transmits dif-~ferentially encoded binary symbols. With differentialdemodulation at the receiver, we circumvent residual jlat fading effects, while deep channel fades are han-dled via a root hopping technique. ‘1 Introduction In ideal propagation conditions, code division mul-tiple access (CDMA) offers advantages in terms of ef-ficiency and flexibility, with respect to other multiusercommunication systems. However, asynchronism andpropagation through frequency selective channels canseriously limit efficient use of the available bandwidth.Therefore, the design of simple linear receivers (zero-forcing or mean-square error), capable of accommodat-ing asynchronous users and offering resistance to near-far effects and multipath propagation has been a con-spicuous field of research (see e.g., [6], [4], [10]). Inwireless communications, the cost related to the use oftraining sequences increases considerably. Blind meth-ods offer one way to overcome frequent restraining, [9],15], [8]. But oftentimes, blind algorithms entail restric-tive assumptions, large data records and computationalcomplexity, or, reduced capability to suppress MUI. Onthe other hand, recognizing that CDMA can be inter-preted as a multirate system, several novel approacheshave emerged recently (e.g., [8]-[11]).A novel CDMA system was introduced in [3], wherejudicious design of codes (named Lagrange codes) offersrobustness to multipath and asynchronism, and leadsto low-complexity receivers which are pre-computableand capable of eliminating MUI completely, in the pres-ence of