Coherent Optical Orthogonal Frequency Division Multiplexing CO-OFDM

Principle of orthogonal frequency-division multiplexing (OFDM) The principles of orthogonal frequency division multiplexing (OFDM) modulation have been in existence for several decades. However, in recent years these techniques have quickly moved out of textbooks and research laboratories and into practice in modern communications systems. The techniques are employed in data delivery systems over the phone line, digital radio and television, and wireless networking systems. OFDM is a special form of a broader class of multi-carrier modulation (MCM), a generic implementation of which is depicted in Fig. 1.

The structure of a complex mixer (IQ modulator/demodulator), which is commonly used in MCM systems, is also shown in the figure. The MCM transmitted signal s(t) is represented as where cki is the ith information symbol at the kth subcarrier, k s is the waveform for the kth subcarrier, Nsc is the number of subcarriers, fk is the frequency of the subcarrier, and Ts is the symbol period. The optimum detector for each subcarrier could use a filter that matches the subcarrier waveform, or a correlation matched to the subcarrier as shown in Fig. 1. Therefore, the detected information symbol c′ ik at the output of the correlator is given by :

The classical MCM uses non-overlapped band limited signals, and can be implemented with a bank of large number of oscillators and filters at both transmit and receive end. The major disadvantage of MCM is that it requires excessive bandwidth. This is because in order to design the filters and oscillators cost-efficiently, the channel spacing has to be multiple of the symbol rate, greatly reducing the spectral efficiency.

A fundamental challenge with the OFDM is that a large number of subcarriers are needed so that the transmission channel affects each subcarrier as a flat channel. This leads to an extremely complex architecture involving many oscillators and filters at both transmit and receive end. A generic optical OFDM system can be divided into five functional blocks including (i) the RF OFDM transmitter, (ii) the RF-to-optical (RTO) up-converter, (iii) the optical channel, (iv) the optical-to-RF (OTR) down-converter, and (v) the RF OFDM receiver.

Optical transmitter for CO-OFDM The primary design goal for CO-OFDM is to construct a linear transformation system. The Mach-Zehnder modulators (MZM) characteristic has been extensively investigated.

Figures 5(a) and 5(b) show respectively a CO-OFDM system which uses direct up/down conversion architecture and intermediate frequency (IF) architecture. In the direct up conversion architecture [Fig. 5(a)], the optical transmitter uses an optical I/Q modulator which comprises two MZMs to up convert the real/imaginary parts of the s(t) [Eq. (1)]. In the direct down-conversion architecture, the OFDM optical receiver uses. Two pairs of balanced receivers and an optical 90° hybrid to perform optical I/Q detection. The RF OFDM receiver performs OFDM base-band processing to recover the data. The advantages for such a direct-conversion architecture are (i) elimination of a need for image rejection filter in both transmitter and receiver, and (ii) reduction of the required electrical bandwidth for both transmitter and receiver.

Optical spectral efficiency for CO-OFDM In CO-OFDM systems, Nsc subcarriers are transmitted in every OFDM symbol period of Ts. Thus the total symbol rate R for CO-OFDM systems is given by Figure 3(a) shows the spectrum of wavelength-division-multiplexed (WDM) channels each with CO-OFDM modulation. We use the bandwidth of the first null to denote the boundary of each wavelength channel. The OFDM bandwidth, BOFDM is thus given by

where ts is the observation period. Assuming a large number of subcarriers used, the bandwidth efficiency of OFDM η is found to be

Channel model for CO-OFDM The channel model describes the behavior of communications systems, thus fundamentally determining the performance of the systems.

Nonlinearity compensation with receiver digital processing for CO-OFDM Because of the large peak-to-average-power-ratio (PAPR) inherent for CO-OFDM signals, we expect that the CO-OFDM is sensitive to the fiber nonlinearity.

some modern applications • Long-haul 100 Gbps and higher data rate transmission systems. • High-speed multi-mode fiber transmission for interconnects in data centers and high performance computing. • Optical Ethernet in Automotive.