What is OFDM?

Orthogonal Frequency Division Multiplexing or OFDM is a digital, multi-carrier data transmission scheme where a single stream of data or binary stream of data is split among multiple closely spaced narrowband carrier frequencies or sub-carrier frequencies instead of transmitting the data on a single carrier frequency and over the entire bandwidth. It is considered to be an extension of frequency division multiplexing (FDM).

Rather than transmitting a high data rate signal on a single carrier frequency, OFDM splits the signal over several closely spaced sub-carriers and transmits all the sub-carriers in parallel over the same bandwidth, thereby improving the spectral efficiency compared to transmission on a single carrier frequency. OFDM has therefore become a popular scheme for wideband communication, making it ideal for applications such as wireless networks, digital television, wireless LAN, wireless personal area networks (WPAN), mobile broadband wireless access, and in 4G/LTE mobile communication systems.

OFDM vs. Traditional Single Carrier Modulated Signals

Bandwidth Efficiency and Reduced Noise

In a traditional single-carrier or single-channel modulation schemes where the information stream modulates a single wideband high frequency carrier signal, each data bit is transmitted serially i.e. one after the other. In an OFDM scheme, a single information stream is split and can be simultaneously transmitted in separate, closely spaced, narrowband sub-carrier frequencies. As a result, each sub-carrier (narrowband) data rate will be relatively lower compared to a single information stream containing all the bits. Therefore, these narrowband sub-carrier signals are relatively less susceptible to interference and channel noise compared to a wideband signal. Furthermore, since all the data bits are transmitted in parallel rather than sequentially, the OFDM scheme provides a relatively efficient use of the channel bandwidth, resulting in improved spectral efficiency compared to traditional single-carrier modulation.

A unique feature of OFDM is that the sub-carrier waveforms are orthogonally combined such that the null (or zero amplitude) of one sub-carrier coincides with the peak of other sub-carriers as shown in the OFDM signal figure (above). This is ensured by keeping the sub-carrier frequency separation equal to the reciprocal of the symbol time. Thus, even though they are closely spaced, each sub-carrier is independent of the other and will not interfere with each other. As a result, the receiver will still be able to decode all the data bits effectively, thereby maintaining an improved spectral efficiency.

This orthogonality enables OFDM to eliminate the use of a guard band, resulting in improved spectral efficiency compared to its earlier version, FDM.

Multipath Reflections and Inter-Symbol Interference (ISI)

In a traditional wideband single carrier transmission scheme, the information stream is transmitted over the entire bandwidth. As we go up in frequency, the bandwidth of the signal also increases. In situations when the signal bandwidth is greater than the channel bandwidth (therefore, signal time is less than the channel time), multiple copies of the symbol of a signal will interfere with the next consecutive symbol. This interference resulting from multipath reflections is referred to as inter-symbol interference (ISI). The figure given below illustrates this point.

ISI Effects due to multipath reflections

In the above figure, the delay spread, or delay time is the channel time and is defined as the time difference between the first and last reflection of the symbol. When this delay spread is lesser than the symbol time, it will result in ISI and hence, the receiver will find it more difficult to decode the symbols effectively. This in turn increases the bit error rate (BER) and probability of error. ISI is an undesirable effect that leads to distortion of the original symbol or signal.

OFDM is a principle where the available bandwidth is divided into multiple sub-carriers, each having a narrow bandwidth. Thus, the symbol bandwidth is also reduced compared to the single carrier scheme. This increases the symbol time. Engineers carefully design OFDM based wireless systems by taking the delay spread that can be expected for a given channel. This means that the symbol time is greater than the delay spread, which accordingly reduces the likelihood of the symbols interfering with other symbols of the information. OFDM greatly reduces the impact of ISI on the signal. Additionally, OFDM scheme inserts guard time intervals between symbols to further minimize the risk of ISI. Therefore, OFDM contributes to a significant reduction in the BER,  cross-talk, and improves the overall performance compared to traditional single carrier scheme.

Frequency Selective Fading

In OFDM, the signal bandwidth is lower (narrowband) compared to the channel bandwidth (wideband). Another term, coherence bandwidth is used to better explain the behavior of the channel. Coherence bandwidth is defined as the range of frequencies which experience nearly the same amount of fading i.e. flat fading.

If the signal bandwidth is greater than the coherence bandwidth as in single carrier scheme, not all the frequency components experience the same amount of fading. Different frequency components of the signal experience different levels of fading and are said to experience frequency selective fading. Resolving this problem is a big challenge as it is difficult to determine how the channel affected the signal in every portion and apply signal processing techniques for every portion of the spectrum. While these errors can be compensated, it involves more complex hardware and signal processing techniques, resulting in increased processing time and cost.

The OFDM scheme transmits narrowband sub-carrier signals over the bandwidth and hence, every sub-carrier experiences flat fading, resulting in improved performance compared to traditional single-carrier systems.

It is also worth pointing out that the OFDM scheme makes an optimal choice of the relationship between signal bandwidth and expected channel delay spread, thereby eliminating the effects of frequency selective fading and ISI altogether, offering significantly better performance. This optimal choice will be different for different applications, frequency of operation, and channel conditions.

Key Limitations of OFDM Compared to Single Carrier Scheme

High Peak-to-Average Power Ratio (PAPR) and Reduced Efficiency

Signals transmitted through an OFDM system usually contain all the sub-carriers that are out-of-phase with each other. The signals have different amplitude values from each other at certain phase values. However, if all the signals simultaneously achieve a maximum value at a given phase, it causes the output to shoot up, resulting in a peak in the output envelope. Since the OFDM system contains independent sub-carriers, the peak value of the entire system can be higher than the average power of the system. This results in a higher peak-to-average power ratio (PAPR).

A higher PAPR is an undesirable effect and it reduces the efficiency of the power amplifier within the OFDM system. Power amplifiers operate in the linear region for up to a pre-determined level of output power. Sudden peak surges will drive the amplifier into the non-linear region, thereby resulting in inefficiency. This occasional surge in peak is one of the biggest disadvantages of the OFDM system.

Sensitive to Sub-Carrier Synchronization Issues

OFDM scheme requires both the transmitter and receiver to be perfectly synchronized to the sub-carrier frequencies. As we go up in the frequency, maintaining perfect sub-carrier frequency synchronization will be more critical as a slight drift or offset will remove the orthogonality of sub-carrier frequencies, which increases the ISI and BER.

Sensitive to Doppler Shifts

OFDM is also sensitive to Doppler shifts that cause offsets in sub-carrier frequencies. The impact is more severe when the speed of the vehicle increases, resulting in an increased offset. As a result, the OFDM technique is not currently used in applications involving high-speed vehicular systems.

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