Electronic processing for generation and detection of multi Gbit/s CDMA over fibre Invited Paper Miguel Pimenta and I. Darwazeh Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, UK e-mail: {m.pimenta,i.darwazeh}@ee.ucl.ac.uk Abstract The use of distributed electronic transversal filter circuit techniques for the generation and detection of OCDMA signals is investigated. New circuit designs are proposed with simulation results indicating suitability for 40 GChip/s OCDMA systems. Introduction Optical Code Division Multiple Access (OCDMA) network proposals are becoming increasingly popular for local area and access network applications. OCDMA offers secure communication and simple architectures as it allows asynchronous operation and can be operated with passive star topologies. One of the key limitations of OCDMA networks is associated with complex terminal equipment and the need to employ user specific optical encoders and decoders. Over the past few years we have been developing OCDMA electronic circuits that can overcome the requirements for terminal specific optical components for signal generation and decoding and more recently for dispersion compensation in multi wavelength OCDMA links. This paper presents concepts of electronic CDMA encoding and decoding. Time-Domain OCDMA Encoding/Decoding For an OCDMA network (Fig. 1), the transmitter encoder and the receiver decoder may be implemented using the distributed structure of Fig. 2. The Distributed transversal filter (DTF) [1,2] may be implemented to effect high rate encoding and decoding. Such structures found several applications in high speed optical communications, ranging from filtering and equalisation to pulse shaping [3] and more recently high chip rate CDMA applications [4,5]. In [4], a DTF is proposed to encode and decode bipolar CDMA signals at chip rates up to 40 GChip/s. Such design required the number of active devices (stages) to be equal to the spreading gain. The design was modified in [5] to allow a reduction of the number of the active devices. Fig. 1. Simplified block diagram of the proposed Optical CDMA network Fig. 2. Distributed Transversal Filter in reverse-mode operation In a time-domain Optical CDMA network, each user is assigned a unique binary unipolar sequence characterized by its length n and the weight w; each element of this sequence is called a chip. This (0,1) sequence can be described by the vector: c = (c 1 ,c 2 ,…,c k ,…,c w ) with c k+1 > c k and c k ≤ n, where each element represents the position of a “positive” chip relative to the beginning of the sequence. For example, if the user sequence is 10010100, the vector c is (0,3,5), n = 8 and w = 3. The user transmits data bit “one” by sending the sequence of the intended user in the time T bit and transmits nothing for bit zero [6]. The chip period T chip is equal to T bit /n. The encoder of the Optical CDMA system generates a unipolar sequence when a pulse x in (t) with width T chip is applied. Therefore, the time-domain response of the encoder may be represented as:     w k chip k in outENC T c t x t x 1 ) ( ) ( Conversely, the OCDMA decoder should be able to generate a correlation peak whose amplitude is proportional to the number of "positive" chips, when the correct sequence is applied. To achieve this, the time- domain output response of the decoder is given by:        w k chip k w w in outDEC T c c t x t x 1 1 ) ) ( ( ) ( The Distributed Transversal Filter (DTF) structure shown in Fig. 3 is an attractive component to encode and decode unipolar sequences. For such purpose, the number of transversal filter stages is equal to the sequence weight w and the delay between stages is a multiple of the time T chip , depending on the distance between positive chips in the codeword. The gain blocks are implemented with two transistors in cascode configuration. Additional delay is achieved using LC- elements which comprise MIM capacitors and thin microstrip lines.