Cyclic code‐shift extension keying for multi‐user optical wireless communications

Abstract

An optical code-division multiple access (OCDMA) scheme based on cyclic shift extension of a base code is proposed to enable asynchronous multi-user optical wireless communications for very populated scenarios. This multiple access method is convenient for lowmedium data rate communications of a large number of simultaneous transmitters. An expression to obtain a tight upper bound of the biterror probability of the new scheme is provided which is also confirmed by experimental results. A variant of the proposed communication method which allows the encoding of several bits per symbol is also presented, although its analysis demonstrates that the first simpler strategy reaches similar performance with a significantly lower receiver complexity.Introduction: In the past years, many research efforts on visible light communications (VLCs) have been oriented to the multi-user communication issue [1][2][3][4]. The preferred multiple access methods for VLC are frequency-division [1], colour-shift keying [2] and optical code-division multiple access (OCDMA). However, the implementation of the first two schemes is usually complex and/or they are only feasible for a limited number of users. OCDMA is inherently simpler and it is able to accommodate a large number of users [3,4].In [3], an OCDMA scheme which provides high-data rates by multilevel signalling in a synchronous scenario is presented mainly for the VLC downlink. When synchronous transmission is infeasible or impractical, a system which can deal with asynchronous communications is required. In [4], a very simple OCDMA scheme based on random optical codes [5] is proposed for asynchronous multi-user communications for low-medium data rates. Asynchronous communications are enabled by incorporating a synchronisation preamble prior to transmitting data. However, from [5] we can see that a bit-error rate (BER) below 10 −4 requires code lengths L > 10 × s, where s is the number of simultaneous users and L is the number of slots (chips), in which the bittransmission interval is divided. Moreover, effective synchronisation preambles must last m × L chips, with m ≥ 3 [6]. Therefore, a large number s of users requires a large L with an increasing synchronisation complexity. Additionally, any subsequent chip-desynchronisation or bad synchronisation would make the system incur an uncontrollable burst of errors. In this Letter, we propose a new codification method which works with shorter codes and enables improved performance by extending the base code with cyclic shifted replicas of itself. This cyclic code-shift extension (CCSE) also provides auto-synchronisation capability, thus preventing bursty errors. A multi-bit variant is also evaluated to enable higher data rates.

Sinusoidal-Based Multiple Access Scheme for Visible Light Decentralized Asynchronous Systems

“…The selection of E CCSE is flexible but E CCSE affects the trade-off between BER and system throughput. For a larger CCSE , the system throughput from (79) decreases but the BER would be reduced, as shown in [15,Eqn. (1)].…”

“…Several asynchronous multiple access schemes were proposed for VLC [10][11][12][13][14][15][16]. For the system in [10] with N transmitters, each transmitter is allocated a unique codeword of length 2 N and a receiver can completely eliminate multiple access interference (MAI) and decode the transmitted data.…”

“…Thus, the system throughput is less than or equal to 1 40 bits/chip, which is relatively low. Cyclic Code-Shift Extension (CCSE) [15] was proposed to enhance the random optical codes of [13] in terms of crosscorrelation property and system throughput. Due to MAI, the decoding of transmitted data still has errors even if the system is assumed to be noiseless.…”

“…In the following, we investigate the performance of the proposed multiple access scheme Sine-MA against CCSE [15], which is known suitable for DAS and effective for OCDMA and related [12][13][14].…”

Minimal Noise Variance Decoder for Uncoordinated Multiple Access in VLC

“…As a result, the scheme delay grows exponentially with the number of transmitters. In [10], asynchronous OCDMA is proposed for communications. To obtain a probability of error below 10 −3 for a system with 20 transmitters, the code length L ≥ 150 chips is needed so that the system throughput ≤ 0.133 bits/chip.…”