What is DFT-S-OFDM?

DFT-S-OFDM or Discrete Fourier Transform-Spread-Orthogonal Frequency Division Multiplexing is a single carrier-based transmission scheme that is utilized in the uplink of LTE and 5G NR wireless systems. It is similar to single carrier frequency division multiple access (SC-FDMA) where each user or transmitter is allocated a single carrier and a finite portion of the channel bandwidth, and every user is separated from the adjacent users with a finite amount of spacing to prevent interference. However, the DFT-S-OFDM scheme eliminates the need for spacing between users and combines all the users orthogonally such that the peak of one user coincides with the null of other users. Thus, even though the users are spaced very closely to each other, there will be no interference due to each other. This scheme is also similar to the OFDMA where all the users are orthogonally combined together while completely eliminating the interference due to multiple users. As a result, this scheme can utilize the spectrum more efficiently than SC-FDMA.

In addition to this, it also uses the concept of Discrete Fourier Transform (DFT) – a technique that converts a discrete set of input signal sequences in the time domain into discrete components in the frequency domain. In a traditional OFDM system, the incoming symbols are directly mapped in the sub-carrier symbol mapping block as shown in the below figure.

OFDM Transmitter System, Image Credit: Telcosought

In the DFT-S-OFDM, a transform precoding step is performed before mapping the signal symbols on every sub-carrier. This is shown in the below figure.

Transform precoding is nothing but applying the DFT technique to the symbols. When the DFT is applied to the data symbols, it spreads the symbols in the frequency domain and encodes the symbols with Fourier coefficients. In this technique, each user or transmitter is designed such that they utilize Fourier coefficients that are unique only to a specific transmitter. Therefore, when the receiver receives the OFDM signal, it uses the Fourier coefficients from the desired transmitter to correctly decode the symbols in order. Since the coefficients are unique to every DFT-S-OFDM transmitter, it is possible to demap the symbols more effectively than traditional OFDM systems which do not use DFT blocks. Thus, this technique can further reduce multiple-user interference, an effect that normally impacts the performance of traditional OFDM systems. Furthermore, once the sub-carrier mapping is performed, the scheme also introduces cyclic prefix i.e. a technique to prefix or place the last few samples of the symbol at the first of the OFDM symbol while retaining these symbols at the end. This is performed for every symbol that is mapped and hence, every symbol is separated by a guard interval that also contributes to the elimination of intersymbol interference (ISI). ISI is an effect of multipath reflections that causes one OFDM symbol to overlap with another in the time domain, causing a spread in time or, delay spread.

Furthermore, this technique also addresses one important problem that traditional OFDM systems inevitably face. Since OFDM systems utilize multiple sub-carriers to transmit the symbols, each sub-carrier frequency is different from the other and hence are out-of-phase with each other. Therefore, it is possible that at certain phase values, multiple sub-carrier symbols can overlap with each other, causing peak amplitude values to shoot up compared to the average symbol sub-carrier value. This results in a higher peak-to-average power ratio (PAPR), which limits the performance of traditional OFDM systems.

The DFT-S-OFDM technique spreads the data symbols over the entire signal bandwidth, which results in an increase in the bandwidth, and a proportional decrease in the signal power. This eventually reduces the PAPR value and improves the signal-to-noise ratio (SNR) compared to conventional OFDM scheme.

However, this comes at the cost of wastage of signal bandwidth as the amount of useful data that can be transmitted will be inevitably reduced, thereby reducing the throughput. And due to the reduction in power, this technique can be used only in power-limited scenarios.

In conclusion, DFT-S-OFDM offers a relatively better solution to further mitigate the interference due to multiple users that occupy the same channel bandwidth by spreading the symbols and encoding unique Fourier coefficients to each transmitter. By utilizing this technique, a reduction in PAPR can be observed, which enables devices such as power amplifiers to operate efficiently in their linear region, while preventing over-driving of amplifiers and possible damages. It also contributes to the further elimination of ISI effects due to multipath reflections and correct decoding of the symbols, owing to the cyclic prefix, which is not otherwise possible in conventional OFDM systems. While this technique does contribute to reduced throughput due to the inevitable wastage of useful bandwidth, it does offer more advantages that tend to outweigh the limitations and can be very useful in standard LTE/5G infrastructure and low-power applications.

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