Code Division Multiplexing System Research Articles

Optical code-division multiplexing systems using Manchester coded Walsh codes

The authors propose an optical code-division multiplexing (OCDM) system using a Manchester coded Walsh code (MCWC) as the spreading code. The MCWC, which is a modification of the traditional Walsh code, has some special properties that are suitable for both single-rate and multirate synchronous OCDM systems. In such systems, the multiple-access interference (MAI) is directly proportional to the number of users; therefore, as long as the number of users is determined, the MAI can be estimated and cancelled. According to the performance analysis of the proposed system, the numerical results show that the bit error rate of the proposed OCDM system is much lower than that of other OCDM systems.

Performance of MISO-OFCDM systems with imperfect channel estimation

Performance degradation due to imperfect channel estimation is addressed in orthogonal frequency and code division multiplexing (OFCDM) systems with multiple transmit antennas and a single receiver antenna. For the performance evaluation, the average bit error rate (BER) expression is derived in the presence of imperfect channel information. Derived results show that the transmit diversity OFCDM systems experience a severe BER degradation regardless of the number of users.

Design of efficient all-optical code-division multiplexing systems supporting multiple-bit-rate and equal-bit-rate transmissions

We present the design of efficient all-optical code-division multiplexing (AOCDM) systems that can transmit multiple-bit-rate (MBR) data signals over a common optical fiber. This is achieved when the proposed strict optical orthogonal code (OOC) of autocorrelation and cross-correlation constraints of 1 are used but without performance degradation compared with the use of conventional OOC. We describe the design of various strict OOC's by employing the useful concept of slot distances, and methods of code construction are also presented. Moreover, we give the principle of MBR data transmissions in an AOCDM system. It is shown that AOCDM systems using the proposed OOC can effectively transmit multiuser MBR and equal-bit-rate (EBR) data with no increase of system complexity. In principle, optimal strict OOC's need the same or a slightly larger system bandwidth compared with optimal conventional OOC's for EBR operation, whereas the former can require a smaller system bandwidth and have a better system performance than the latter for MBR transmissions. A new, to our knowledge, family of strict OOC's is also introduced, whose code words can have nonconstant weights to support multiuser communications with different transmission quality. Furthermore, we design low-cost AOCDM transmitters that are based on an efficient gain-switching scheme that does not require an electro-optic intensity modulator to on-off modulate an optical clock pulse stream at each transmitter. The basic operation principle is also experimentally demonstrated.

Optical code division multiplex system with multiphase mapping

Optical code division multiplex system with multiphase mapping

Very-high-speed All-optical Code-division Multiplexing Systems Using a 2^n Prime Code

We demonstrate experimental all-optical code-division multiplexing (AO-CDM) systems using 64-ps optical pulses and a 2" prime code of n = 3. A distinguishing feature of this experiment is that the modulation of an ultrashort optical clock stream by electrical data is realized without using any optical intensity modulator at each transmitter. Moreover, only low-cost optical 2 x 2 couplers and fiber delay lines are employed to implement all-serial encoders and decoders for a 2n prime code.

Spread Spectrum‐Based Subcarrier Recovery Method for Multi‐Carrier Code Division Multiplexing System

AbstractA time domain Spread Spectrum (SS)‐based subcarrier recovery method is proposed for down‐link Multi‐Carrier Code Division Multiplexing (MC‐CDM) system, which periodically inserts SS pilot signals into data signal stream in a time division manner at the transmitter. Instantaneous channel impulse response is first estimated by correlating the received pilot signal at the receiver, and then corresponding instantaneous frequency response essential for subcarrier recovery is calculated from the Discrete Fourier Transform (DFT) of the estimated impulse response. The SS pilot‐assisted method is also applicable to Direct Sequence (DS)‐CDM system as a means to estimate instantaneous channel impulse response for Rake combining, and taking the same approach in both MC‐CDM and DS‐CDM systems, there is no large difference between them in terms of transmission efficiency and receiver complexity. The proposed MC‐CDM system must be a promising down‐link multiplexing protocol, because it can achieve better bit error rate (BER) performance than the DS‐CDM system with the same SS pilot‐assisted Rake combiner in frequency selective fast Rayleigh fading channel.

Error-free 100 km transmission at 10 Gbit/s inoptical codedivision multiplexing system using BPSK picosecond-pulse codesequence with novel time-gating detection

Error-free 100 km transmission at 10 Gbit/s in optical code division multiplexing (OCDM) system with time-gating detection is experimentally demonstrated. The numerical analysis confirms that OCDM using short-pulse sequences can be used to overcome the fibre dispersion effect though time-gated correlation detection.

Experiments on high-speed all-optical code-division multiplexing systems using all-serial encoders and decoders for 2/sup n/ prime code

In this paper, we demonstrate experimental all-optical code-division multiplexing (AO-CDM) systems using 64-ps optical pulses and a 2/sup n/ prime code of n=3. A distinguishing feature of this experiment is that the modulation of ultrashort optical clock stream by electrical data is realized without using any optical intensity modulator at each transmitter. Moreover, only low-cost standard optical 2/spl times/2 couplers and fiber delay lines are employed to implement all-serial encoders and decoders for a 2/sup n/ prime code. As a result, this new system is more cost- and power-effective than a conventional AO-CDM system. Furthermore, the use of AO-CDM systems can offer parallel communications over a common fiber channel, which in turn can support real-time computer interconnections for image and data communications.

Demonstration of novel optical code‐division multiplexing systems for multirate and variable‐rate data communications

We demonstrate optical code-division multiplexing (OCDM) systems using a new optical orthogonal code (OOC) to support multirate and variable-rate data communications. An important characteristic of the proposed codes is that their autocorrelation and cross-correlation operations are dependent only on each individual code word rather than a periodic pulse sequence at the input of OCDM receivers. Consequently, multirate and variable-rate data communications are offered without violating cross-correlation and autocorrelation constraints of 1. This is achieved without increasing any system complexity. Compared with conventional OOCs, our proposed codes not only can have shorter code-word lengths for multirate and variable-rate data communications, but also have high flexibility and performance. Experimental OCDM systems using the proposed code are also carried out.

Variable-bit-rate video transmission systems using optical fiber code-division multiplexing scheme

Code-division multiplexing (CDM) systems using a novel optical orthogonal code (OOC) are proposed for multichannel variable-bit-rate (VBR) video transmission over an optical fiber channel. Compared with a conventional OOC, the proposed code fully guarantees the peaks of its cross-correlation functions and the sidelobes of its autocorrelation function to equal 1 which is the minimum correlation constraint for incoherent optical signal processing, and therefore, high-performance VBR video transmission can be realized. We also give an approach for designing novel OOCs and present the principle of VBR video transmission systems using optical fiber CDM. To verify the correlation characteristics of the proposed OOC, we also demonstrate an experiment on the two-channel optical fiber CDM system. It is shown that using this novel type of OOCs one can implement a more cost-effective CDM system for VBR video transmission than the use of conventional OOCs.

A code division multiplex using manchester-coded orthogonal sequences

There has been increasing interest recently in code division multiplex (CDM) systems. The authors reported a CDM system using pseudo-orthogonal Manchester-coded M-sequence pairs. For this CDM system, the effective energy per bit in the received signals is reduced (M + 1)/2M times as much as the transmitted energy per bit. Moreover, the bit error rate (BER) of the 3M-plex system, which uses the “a-chip-shift” multiplex operation alone, does not converge on zero even if Eb/N0 increases. This paper proposes a CDM system using Manchester-coded orthogonal sequences which consists of an M-sequence M chip and an additional chip. The proposed 3M-plex system is a combination of the “a-chip-shift” multiplex operation and the amplitude multiplex operation. The proposed systems do not have loss of the effective received signal energy. The BER of the proposed systems converges to zero with increasing Eb/N0. Moreover, the proposed system out performs the Manchester-coded multilevel transmission system.

On the capacity of asynchronous code-division multiplexing systems

In this letter the capacity of asynchronous code-division multiplexing systems with unquantized output is considered. The minimax capacity is evaluated and the reliability function is briefly discussed.