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Difference Between single Carrier and OFDM

This page describes difference between single carrier and OFDM systems.

The single carrier transmission means one Radio Frequency carrier is used to carry the information. Hence information in the form of bits is carried by one single RF carrier. OFDM, also known as multicarrier transmission or modulation, uses multiple carrier signals at different frequencies, sending some of the bits on each channel. This is similar to FDM(Frequency Division Multiplexing) however in the case of OFDM; all of the sub channels are dedicated to a single data source. For OFDM case IFFT is used at the transmitter to accomplish this, which does not exist in Single carrier case.

Single carrier versus OFDM

As shown in the figure, single carrier(SC) system information in the form of voice or data is modulated on single RF carrier frequency. This modulated IF signal is converted to the modulated RF frequency.This is amplified using RF power amplifier before being transmitted over the air using the antenna. One carrier carry data bits based on modulation scheme employed in the modem. For BPSK 1 bit is mapped on this carrier, for QPSK 2 bits , for 16QAM 4bits and for 64QAM 16 bis and so on.

Satellite communication systems,GSM,CDMA,HF and other radio systems use single carrier for transmission and reception.

Unlike SC system, OFDM uses multiple carriers spaced very closed over the band. Each of this carriers carry data bits as per modulation scheme employed. Hence OFDM delivers data rate is higher than the SC system.OFDM technique is used in wlan and wimax broadband technologies. Its variant OFDMA is used in LTE and mobile-wimax systems.

FFT structure in OFDM

Figure explains IFFT structure for 256 point FFT size, here the inputs are complex data from the baseband modulator(i.e. BPSK/QPSK/16QAM/64QAM). Each of the complex data represent bits according to the modulation technique employed. Following is the equation for K point FFT used to map data over multiple baseband carriers, which gets modulated later over RF carrier resulting into multiple radio frequency carriers carrying multiple data bits.

FFT equation

OFDM data rate calculation

Raw Data rate for OFDM (bit rate)= Ndatasc*bm*Cr / Ts,
Where,
Ndatasc is the number of Data Subcarriers.
bm is the coded bits/subcarriers
Cr is the code rate i.e. input bits to FEC/coded output bits
Ts is the symbol time with or without CP, where Ts=Tb+Tg, Tg=1/4 or 1/8 or 1/16 or 1/32 of Tb period
Tb= 1/delF, Where, delF=Fs/Nfft, where Fs is the n*BW and Nfft is the FFT size say 256 point.

EXAMPLE: Calculate data rate for wimax system having 256 point FFT with 192 data subcarriers. Bandwidth is 20MHz and modulation code rate is 64QAM 3/4, Tg=1/16.
From the equation above data rate = (192*6*3/4)/11.8µs=73.2 Mbps

Merits

As explained above in difference between SC and OFDM,OFDM is used to achieve high data rate over single carrier system.

De-merits

Due to multiple carriers OFDM leads to high PAPR(Peak to Average Power Ratio) To overcome PAPR scrambler(randomizer) is used in OFDM based systems which spreads the energy across wide bandwidth. There are various techniques to reduce the PAPR the same is explained in PAPR article.

RELATED LINKS

OFDM Transmitter modules as per wimax standard, READ MORE.

OFDM Transmitter modules as per WLAN-11a standard, READ MORE.

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