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Orthogonal frequency-division multiplexing
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== Efficiency comparison between single carrier and multicarrier == The performance of any communication system can be measured in terms of its power efficiency and bandwidth efficiency. The power efficiency describes the ability of communication system to preserve bit error rate ([[bit error rate|BER]]) of the transmitted signal at low power levels. [[Bandwidth efficiency]] reflects how efficiently the allocated bandwidth is used and is defined as the throughput data rate per hertz in a given bandwidth. If the large number of subcarriers are used, the bandwidth efficiency of multicarrier system such as OFDM with using [[optical fiber channe]]l is defined as<ref>{{cite book | author=William Shieh, Ivan Djordjevic. | title="OFDM for Optical Communications". | location= 525 B Street, Suite 1900, San Diego, California 92101-4495, USA | publisher=Academic Press | year=2010 }}</ref> : <math>\eta = 2 \frac{R_s}{B_\text{OFDM}}</math> where <math>R_s</math> is the symbol rate in giga-symbols per second (Gsps), <math>B_\text{OFDM}</math> is the bandwidth of OFDM signal, and the factor of 2 is due to the two [[polarization (waves)|polarization]] states in the fiber. There is saving of bandwidth by using multicarrier modulation with orthogonal frequency-division multiplexing. So the bandwidth for multicarrier system is less in comparison with single carrier system and hence bandwidth efficiency of multicarrier system is larger than single carrier system. {| class="wikitable" ! S. no. !! Transmission type !! M in M-QAM !! No. of subcarriers !! Bit rate !! Fiber length !! Received power, at BER of 10<sup>β9</sup> !! Bandwidth efficiency |- | 1 || Single carrier || 64 || 1 || {{nowrap|10 Gbit/s}} || {{nowrap|20 km}} || β37.3{{nbsp}}[[dBm]] || 6.0000 |- | 2 || Multicarrier || 64 || 128 || {{nowrap|10 Gbit/s}} || {{nowrap|20 km}} || β36.3{{nbsp}}dBm || 10.6022 |} There is only 1{{nbsp}}dB increase in receiver power, but we get 76.7% improvement in bandwidth efficiency with using multicarrier transmission technique.
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