RFSingle Carrier vs. OFDM: Understanding the Key Differences
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This page describes the differences between single-carrier and OFDM systems. 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 multi-carrier 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, IFFT is used at the transmitter to accomplish this, which does not exist in the 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 a single RF carrier frequency. This modulated IF signal is converted to the modulated RF frequency. This is amplified using an RF power amplifier before being transmitted over the air using the antenna. One carrier carries data bits based on the modulation scheme employed in the modem. For BPSK, 1 bit is mapped on this carrier, for QPSK 2 bits, for 16QAM 4 bits, and for 64QAM 6 bits, and so on.
Satellite communication systems, GSM, CDMA, HF, and other radio systems use a single carrier for transmission and reception.
Unlike SC systems, OFDM uses multiple carriers spaced very closely over the band. Each of these carriers carries data bits as per the modulation scheme employed. Hence, the OFDM data rate is higher than the SC system. OFDM technology is used in WLAN and WiMAX broadband technologies. Its variant OFDMA is used in LTE and mobile-WiMAX systems.
FFT structure in OFDM
The figure explains the IFFT structure for a 256-point FFT size, where 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 in multiple radio frequency carriers carrying multiple data bits.
FFT equation
The equation above mentions generic DFT calculation.
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 the data rate for a WiMAX system having a 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 the difference between SC and OFDM, OFDM is used to achieve a high data rate over a single carrier system.
De-merits
Due to multiple carriers, OFDM leads to high PAPR (Peak to Average Power Ratio).
To overcome PAPR, a scrambler (randomizer) is used in OFDM-based systems, which spreads the energy across a wide bandwidth. There are various techniques to reduce the PAPR, the same is explained in the PAPR article.
