Predistortion in Optical Wireless Transmission using OFDM Hany Elgala † , Raed Mesleh † † Department of Electrical Engineering and Computer Science Jacobs University Bremen, Campus Ring 1 28759 Bremen, Germany h.elgala & r.mesleh@jacobs-university.de Harald Haas †* * Institute for Digital Communications The University of Edinburgh Edinburgh EH9 3JL, UK h.haas@ed.ac.uk Abstract—The nonlinear characteristic of an LED (light emit- ting diode) imposes limitations on the performance of indoor optical wireless (OW) systems when using intensity modulation in combination with OFDM (orthogonal frequency division multiplexing). First, the impact of the nonlinear characteristic on bit-error performance is analyzed using a commercially available LED (OSRAM, SFH 4230). Second, the paper proposes a predistorter to overcome the nonlinearities. Key features of the predistorter reside in the use of the LED inverse characteristics as nonlinear distortion compensator. A DC biased optical OFDM (DCO-OFDM) system is considered and the performance without compensation and after compensation is analyzed via simulations in an AWGN (additive white Gaussian noise) environment. In this context, the bit-error performance is determined for different bias points and power back-off values applied to the OFDM signal modulating the LED intensity. It is shown that LED nonlinearity can significantly degrade the performance. However, it is demonstrated that this degradation can greatly be mitigated by using the proposed predistortion technique. Index Terms—OFDM, LED, optical wireless communication, nonlinearity distortion, PAPR. I. I NTRODUCTION Due to the ever increasing demand for wireless data rates, especially indoors, new frequency bands are explored such as the license-free 60GHz band [1]. Recently also the optical spectrum has enjoyed growing interest for use in indoor wireless data transmission [2–4]. The multi-carrier OFDM modulation is a promising modulation scheme to realize indoor OW links [5]. OFDM offers high data rate capabilities as well as high bandwidth efficiency and inherently provides a means to combat ISI (inter-symbol-interference) resulting from multipath propagation. However, the performance of OFDM with its high PAPR (peak-to-average power ratio) can potentially be severely affected by the nonlinear behavior of the LED. In RF (radio frequency) systems, the main source of non- linearity is the power amplifier (PA) as shown in Fig. 1(a). The PA operates near the saturation point in order to achieve the maximum power efficiency. In this operation region, undesirable nonlinear effects due to amplitude and phase distortions are introduced. Additionally, signal clipping at the PA saturation level is a critical source of distortion, in particular for OFDM because of its high PAPR [6]. Backing- off the average power of the input signal ensures that the PA operates in a quasi-linear region of operation and avoids saturation. Alternatively, the reduction of the PAPR through Input amplitude Saturation level O u t p u t a m p l i t u d e Input amplitude O u t p u t a m p l i t u d e Max. permissible AC/pulsed current Max. permissible DC current (a) (b) DC bias point operation region PA V-I response Linearized response LED V-I response Linearized response The OFDM signal Fig. 1. (a) Nonlinear and linearized PA transfer characteristic. (b) Nonlinear and linearized LED transfer characteristic. The nonlinear transfer charac- teristic distorts the OFDM signal in RF as well as in optical applications. Linearization through predistortion helps to improve system performance. methods such as clipping, filtering, constrained coding, and selective mapping are considered [7]. However, neither power back-off nor PAPR reduction techniques necessarily result in an improvement in system performance and trade-offs must be considered [8, 9]. In addition, linearization through predis- tortion can be applied to compensate for the PA nonlinear distortion as shown in Fig. 1(a). In optical systems, the LED is the main source of non- linearity (see Fig. 1(b)). A real value baseband OFDM signal is used to modulate the instantaneous power of the optical carrier resulting in IM. DCO-OFDM and asymmetrically clipped opti- cal OFDM (ACO-OFDM) are two forms of OFDM using IM. In DCO-OFDM, the bipolar OFDM signal is superimposed on a bias point to produce a positive signal. In ACO-OFDM, however, the OFDM signal is made unipolar by modulating the odd sub-carriers only and clipping the signal at the zero level [10]. In this paper, the DCO-OFDM is considered. Several bias points are selected to investigate the bias point influence on the generated distortion. Power back-off values are applied to the OFDM signal to control the distortion levels by operating the LED in a quasi-linear segment of its characteristic around the chosen bias point. The paper also focuses on applying a digital