Frequency Quadrature Amplitude Modulation Research Articles

Improvement of optical fiber channel capacity using compressed non-orthogonal frequency quadrature amplitude modulation

Abstract A new modulation scheme, that can increase the channel capacity of optical fiber link, is proposed in this paper. It is implemented by using a non-orthogonal frequency shift keying (FSK), quadrature amplitude modulation (QAM), and sparse sinusoidal sampling based on a compressed sensing. The proposed technique can allow us to have the improved transmission capacity because frequency spacing among digitally sample RF subcarriers becomes much narrower than the channel bandwidth by the non-orthogonality and the total signal length can be reduced by using the sparse sinusoidal sampling. For experimental verification, a 64-non-orthogonal frequency quadrature amplitude modulation (NoFQAM) signal is generated using non-orthogonal 4-FSK and 16-QAM. The total length of 64-NoFQAM signal is reduced up to 60% at the bit error rate (BER) of 4 × 10−5 by the sparse sinusoidal sampling. The BER of 2 × 10−5 was measured at a frequency spacing between digitally sampled RF subcarriers corresponding to 10% of channel bandwidth (500 MHz). The 7.5-Gb/s transmission of 64-NoFQAM signal with the channel bandwidth of 500 MHz was demonstrated successfully by using 50-km and 300-km optical fiber link.

Optical access transmission with improved channel capacity using non-orthogonal frequency quadrature amplitude modulation

A new modulation scheme that improves the bandwidth efficiency of an optical access link is proposed in this paper. It is implemented using non-orthogonal frequency shift keying (FSK) and quadrature amplitude modulation (QAM) simultaneously. We call the proposed technique non-orthogonal frequency quadrature amplitude modulation (NOFQAM). Especially, non-orthogonal FSK based on digital signal processing (DSP) is proposed for the first time. DSP-aided non-orthogonal FSK allows us to select RF carrier frequency irrespective of the channel bandwidth to allocate FSK symbols, unlike the existing orthogonal FSK. The non-orthogonality is implemented using a sequential correlation, where a received NOFQAM signal is correlated with only one RF carrier at a time by using DSP. After the sequential correlation is completed, both the FSK and the QAM symbols are recovered successfully and merged to generate the NOFQAM symbols. For experimental verification, a 20-km optical access link, which can transmit a 64-NOFQAM signal sampled at 10 Gsample/s, is implemented. We observed no increase in occupied channel bandwidth and a power penalty <0.5 dB compared to the 16-QAM scheme. A bit error rate lower than 10−11 was obtained for the frequency spacing considered herein, which corresponds to 3% of the used RF carrier (1.5 GHz) when there are 50 sampling points per 64-NOFQAM symbol.