512 QAM transmission over 240 km using frequency-domain equalization in a digital coherent receiver

Abstract

We demonstrate a marked performance improvement in a 512 QAM transmission by employing frequency-domain equalization (FDE) instead of an FIR filter. FDE enables us to compensate for distortions due to hardware imperfections in the transmitter with higher precision, which successfully reduced the power penalty by 4 dB in a 54 Gbit/s (3 Gsymbol/s)-160 km transmission. FDE also allows the transmission distance to be extended up to 240 km.

Adaptive multidimensional modulation and multiplexing for next generation optical networks

“…Let us compare the spectral efficiency of multicarrier formats with the spectral efficiency of single carrier modulation formats. When the OFDM/DMT format is applied on the top of 2D (let' us assume M-QAM formats; M=2 p ), the spectral efficiency becomes [5] sc SC SC (17) where p presents the number of bits per symbol in M-ary QAM modulation scheme and R SC is bit rate per subcarrier. Eqn.…”

“…Eqn. (17) assumes that subcarrier spacing is defined as f sc =1/(T s T G ), which means that it is dependent on the system design. Since the guard interval relates to total dispersion accumulated during transmission, the spectral efficiency can be calculated only after a detail design considerations.…”

“…The O-OFDM is therefore a format that combines multiple optical carriers while providing their spectrum overlapping per Nyquist criterion. A number of optical carriers (sometimes also known as optical subcarriers) generated by the O-OFDM scheme represent a superchannel, which appears to be the most attractive technology for ultra high-speed optical transmission [16][17][18][19][20][21][22][23]. The data rate of the formed superchannel can be increased either by adding additional optically generated subcarriers with specified bit rate per carrier, or by increasing the spectral efficiency per already deployed subcarriers.…”

High Speed All Optical Nyquist Signal Generation and Full-band Coherent Detection

“…The technology options for transmission with higher bandwidth efficiency are being intensively studied, which comes in two major categories: reducing the signal spectrum bandwidth requirement or increasing the modulation levels11121314151617181920212223242526. The former uses orthogonal multiplexing technologies based on optical or electrical waveform generation, known as the orthogonal-frequency-division-multiplexing (OFDM) in frequency domain or Nyquist pulse technology in time domain1112131415161718192021; the latter uses multi-level advanced modulation formats enabled by coherent detection, such as QPSK, 16-QAM, 64-QAM and even higher order QAMs2223242526. Until now, OFDM with high-order modulation formats has been widely investigated, using both electrical and all optical OFDM signal generation12131415.…”

“…The high transmission capacity with high spectrum efficiency has also been demonstrated by the single-laser 32.5 Tbit/s Nyquist WDM transmission based on 12.5GBaud PDM-16QAM with 325 optical carriers in27. However, most Nyquist pulse-shaping methods for each carrier reported are either too complex16 or restricted by the electronics speed because of the limited sampling rate and limited processor capacities1718192021222324252627.…”