OFDM modulation

OFDM (Orthogonal Frequency-Division Multiplexing) – is a modulation technique for simultaneous transmission of multiple data streams on multiple orthogonal carrier frequencies. A high-rate data stream is distributed over multiple low-rate data streams to allow system operation in the channels that can be affected by multi-path propagation, which due to low bit rate is negligible. When a signal is propagated over multiple paths, it reaches the receiver at different times, delaying the signal against other signals. The delay due to multi-path propagation may result in the symbol sent via a longer path to “leak” to another symbol, resulting in the intersymbol interferences.

OFDM modulation eliminates this problem. The transmitted data are divided into sub-carriers, each carrying a certain information. Data transmission rate for a specific sub-carrier is low, thus reducing the interferences due to multi-path propagation. Using multiple sub-carriers means that only a small amount of data can be corrupted, and can be repaired using special error detection techniques, e.g. via a re-transmission.

The most commonly used sub-carrier modulations in OFDM are phase-shift keying (PSK) and quadrature amplitude modulation (QAM).

One of the key parameters for OFDM implementation is a channel bandwidth that determines the number of sub-carriers that can be used in the OFDM modulation. For example, LTE technology defines several channel bandwidths: 1.4 MHz to 20 MHz. The higher the channel bandwidth, the higher its capacity. 1.4 MHz channel is distributed into 72 sub-carriers, whereas 20 MHz channel is distributed into 1,200 sub-carriers. A 15 kHz interval is used between each sub-carrier. The symbol duration must be 66.7 µs to keep the sub-carriers orthogonal. Each sub-carrier can transfer data at 15 ks/s (kilosymbols per second). At 20 MHz, the symbol rate is 19 Ms/s. Using 64QAM encoding, each symbol is sent using 6 bits at total bit rate of 108 Mb/s.

Less efficient modulation techniques (QPSK) do not require high S/N ratio (signal-to-noise ratio), and do not offer high bit rates. Higher bit rates can be achieved at higher S/N ratios.

However, OFDM required a high-precision matching of the carrier transmission frequency and the local oscillator frequency. Otherwise, it will result in interchannel crosstalk and increased number of errors.

OFDM modulation is used in broadband digital systems, e.g.: