Optimization Of Modulation Scheme In Visible Light Communication

Optical wireless communications (OWC) has the potential to become a remedy for the shortage of the radio frequency (RF) spectrum. Particularly, OWC could enable wireless home networking systems which offload data traffic from existing RF systems. In OWC, data is transmitted by modulating the intensity of light sources, typically incoherent light emitting diodes (LEDs). Therefore, OWC systems employ intensity modulation (IM) and direct detection (DD) of the optical carrier. Since off-the-shelf LEDs have a limited modulation capability, the transmission bandwidth of practical OWC systems is restricted. Consequently, the available bandwidth has to be used efficiently.The objective of this dissertation is to develop a new modulation scheme that is well-suited for inter cell interference environment and can considerably improve the channel capacity and bit error rate. It also improve the system performance by merging with existing modulation schemes. The proposed scheme is suitable to improve bandwidth, transmission capacity, and power efficiency of the system. It precisely provides the maximum area of coverage.Since the received power is an important factor to improve BER, channel capacity, data rate, and coverage area, we investigated the relation between the received power, distance, and coverage area, and find out the right location of LEDs to cover maximum area of the room by considering required acceptable power and minimum the interference. The maximum area can be the reason of inter cell interference (ICI). It deteriorates the BER and channel capacity of the system. The proposed modulation scheme can mitigate the interference power at cell edges from neighboring cells by using different orthogonal pulses (DOP). The interference power can be easily mitigated through orthogonal function at the receiver’s matched filter. Each cell can use different DOP. The orthogonal pulses (OP) can be generated through the Hadamard code matrix (HCM). The large numbers of OP are generated with higher order HCM which require high bandwidth LEDs. The number of OP is subject to the availability of high bandwidth LEDs. Therefore, the number of OPs has minimized by reusing OP (ROP) at certain distances in the room, which reduces the utilization of more bandwidth and complexity of transmitters and receivers. The channel capacity cannot be similar for different cells because the high bandwidth pulses produce the high capacity and low bandwidth pulses produce the low capacity, therefore the capacity is maintained by using derivation. The optimum power has derived and compared for DOP, ROP and NOP to maintain required channel capacity without interference. The interference is also dependent upon the distance between those cells which are using same signaling schemes. In case of NOP scheme we use the same signal having the same bandwidth for all cells in the room. The transmission power is allowed to be independently allocated on each active physical resource block (PRB) that has been assigned to users in the network. Hence, dynamic or fixed power allocation can be performed depending on interference power at a particular location of the room. The results show that the BER and capacity performance can be significantly improved by using OPs particularly in interference areas of the room.The proposed scheme has merged with existing baseband modulation schemes, for example, pulse position modulation (PPM), differential PPM (DPPM), differential amplitude PPM (DAPPM), and multiple PPM (MPPM). The merged schemes PPSM, DPPSM, DPISM and DAPPSM improve the spectral efficiency, transmission capacity and bandwidth efficiency significantly.The higher order modulation has also investigated by mapping HCM into M bits. The transmission capacity increases without increasing the transmitter power. The data rate significantly improves particularly in the interference region of a multi-cell environment. Moreover, the signal to noise ratio (SNR) is derived to provide the similar capacity distribution between cells, and achieve an optimum data rate in high interference area of the room. Theoretical and simulation results of bit error rate (BER) and capacity performances are derived. The three arrangements of OPs in cells are compared, which are defined as, all cells use the different sets of OPs (DSOP), all cells use the same set of OPs (SSOP), and reuse OPs in diagonal (ROPD) cells of the room. In contrary to ROPD and SSOP, DSOP requires less power to achieve the required data rate particularly in high interference area.