Technical Bulletin N°10

Optical Complex Spectrum Analyzer AP2440A application note
Bit rate and symbol rate in M-ary modulation formats
Binary modulation : In optical digital communication, bits can be transmitted using intensity (NRZ, RZ) or phase modualtion (DPSK). These modulations are using two states to represent numeric values (0;1). In the case of NRZ modulation the '1s' are represented by the presence of signal and the '0s' by the absence of signal. In DPSK modulation, the '1s' and '0s' are represented by a 180° phase difference. This numeral system is called binary. Back to the top M-ary modulation : Some new modulation formats are under study to transmit more than two numeric values. For example in QPSK modulation, 4 different phase states can be transmitted, thus two bits are transmitted for each symbol. This numeral system is called quaternary. Back to the top Symbol rate : With these new modulation formats it is necessary to dissociate the symbol rate and the bit rate. The symbol rate is the bit rate divided by the number of bits transmitted in each symbol. Symbol rate is measured in symbol-per-seconds or baud (Bd). The symbol rate is distinct from the bit rate because one symbol may carry more than one bit of information. For example with 4 phases, QPSK can encode two bits per symbol. So, a 40Gb/s QPSK signal, which is transmitting symbols that each carry 2 bits should be described as operating at 20Gsymbol/s. This is a table resuming the pattern length acceptable for a measurement with the AP2440A series OCSA depending on the symbol rate (for other symbol rate please contact us): Symbol rate 10Gsymbol/s 20Gsymbol/s 40Gsymbol/s 80Gsymbol/s 160Gsymbol/s Pattern length 4bits or 16bits 8bits or 32bits 16bits or 64bits 32bits or 128bits 64bits or 256bits Back to the top 8PSK : With its eight phase states, 8PSK can encode 3 bits per symbol, so a 30Gb/s 8PSK signal should be described as operating at 10Gsymbol/s. Back to the top
Every measurement displayed on this technical bulletin have been measured with an AP2440A Optical Complex Spectrum Analyzer from APEX Technologies
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