Using equalisers to increase the bandwidth of VLC systems

Li, Xicong (2023) Using equalisers to increase the bandwidth of VLC systems. Doctoral thesis, Northumbria University.

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Abstract

This thesis investigates and proposes the equalisation techniques for visible light communications (VLC) to increase a VLC system’s bandwidth and achievable data rates from a practical standpoint. The equalisation techniques are mainly classified into two categories. The first category is the analogue equaliser, which is implemented at either transmitter or receiver side using analogue devices with a reverse frequency response of the raw system to extend the overall system bandwidth. The second category is the digital equaliser, which is highly based on advanced digital signal processing (DSP) algorithms.

The digital approach provides significant flexibility and adaptivity to combat the frequency response variance in VLC wireless channels and analogue components compared with the analogue method. On the other hand, the analogue equalisers can be easily designed and implemented cost-effectively, demonstrating fast improvement in baseband modulation schemes like on-off keying (OOK). Although data transmission of Gbit/s OOK has been demonstrated by simply using analogue pre-equalisers, the cost of such a pre-analogue equaliser is avoidable massive power attenuation, which is limiting its use in practical applications. In contrast, the digital post-equaliser does not have such a power loss issue and shows its advantages over the pre-analogue equaliser.

Therefore, this thesis firstly studies the fundamentals of the analogue pre-equaliser and proposes a universal analogue equaliser design method to validate that such a pre equaliser actually decreases system capacity because of the SNR penalty introduced by the analogue pre-equaliser. The analogue pre-equaliser should be avoided if the system is aiming at the highest data rates. The better solutions can be single-carrier modulation with complex digital post equalisers or multi-carrier modulation with simpler digital post-equalisers.

Then this work investigates how the post-equaliser can be implemented in a practical single-carrier CAP receiver. The CAP modulation scheme was selected because it has been one of the most popular schemes proposed for VLC for its low implementation cost. However, real-time oriented CAP receiver has been a topic rarely touched. For these reasons, this work proposes a full-digital CAP receiver architecture with both equalisation and synchronisation included. The inclusion of synchronisation adds more problems because the coupling between synchronisation and equalisation creates a “chick-and-egg” problem. It is the use of the blind equalisation algorithm, i.e., constant modulus algorithm (CMA), that decouples the equalisation and synchronisation and makes a full-digital receiver possible.

The thesis finally investigates the possibility of applying multi-carrier CAP modulation with low-complexity post equalisation in a real-world office environment by upgrading a large white LED panel light with the VLC function. Benefited by not losing any power, the multi-carrier CAP modulation enables the light panel with a raw bandwidth of only 1 MHz to deliver up to 40 Mbit/s at a standard illuminance height.

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