By In a previous article on OFDM, we have seen that single-carrier systems are simpler and less power-hungry. On the other hand, OFDM slices the allocated wireless spectrum, hence making it easier to equalize in high multipath scenarios. This is why it has been the dominant technique in most high data-rate applications such as WiFi and cellular networks.
However, multipath problem is relatively less severe for Unmanned Aerial Vehicles (UAVs) in non-urban settings. In this situation, UAV communication links can use both single-carrier and OFDM-based schemes. Yet, modern systems increasingly favor OFDM even though the multipath channel and frequency selectivity are not severe in such open environments. What we explore next are the reasons behind this.
- Robustness to Frequency Selective Fading: This is the typical OFDM advantage but from a slightly different angle. While multipath is less rich in non-urban environments and the delay spread is low, Air-to-Ground (A2G) links suffer from ground reflections. The ground creates a two-ray model where the reflected wave interferes with the direct wave. This has the potential to cause deep notches or nulls at specific frequencies in the given band. While in a single-carrier system, a null in the middle of the band can crash the entire link, only a few subcarriers at most are lost in an OFDM system. Forward Error Correction (FEC) then recovers that original data from the remaining subcarriers.
- Adaptive Modulation and Coding: It is theoretically as well as practically easier to scale an OFDM system. If the system wants to drop from 30 Mbps in 5 MHz bandwidth to 5 Mbps in 1.25 MHz bandwidth, e.g., to increase range when the UAV flies further away, all it has to do is turn off some subcarriers. What happens if the outer subcarriers are switched off?
- All the available transmit power in concentrated into a smaller frequency window, thus boosting the Power Spectral Density (PSD) of the signal.
- Thermal noise picked by the receiver is directly proportional to bandwidth. Therefore, narrowing the Rx filter from 5 MHz to 1.25 MHz results in 75% less background noise entering the processing chain. We can say that cutting the bandwidth by 4x reduces the noise floor by 6 dB.
- The combined effect is that the system link budget sees a gain of roughly 12 dB (6 dB from power concentration + 6 dB from noise reduction). In free space, every 6 dB of gain doubles the communication distance.
Also, the modulation on the subcarriers can be switched from, say, 64-QAM to 4-QAM enabling the station to still decode the signal.
On the other hand, it becomes possible to send different modulation signals on different subcarriers. For example, while subcarriers experiencing a bad channel can have a lower-order modulation like 4-QAM, subcarriers experiencing a good channel can be used to transmit a higher-order modulation like 64-QAM that translates into more bits within the same time.
- Resistance to Pulsed Interference: In urban settings, UAVs operate in crowded or dirty RF environments such as 2.4 GHz or 5.8 GHz bands. OFDM handles narrowband interference better than single-carrier systems in terms of complexity and performance.
- Standardization: UAVs often rely on existing cellular (LTE, 5G, 6G) or Wi-Fi technologies, all of which use OFDM. Therefore, the communication link becomes compatible with the broader infrastructure. Moreover, modern UAV communication systems often use MIMO for beamforming and spatial diversity. OFDM possesses a natural compatibility with MIMO which simplifies system design.
Keep in mind that in power-constrained scenarios, SC-FDE (Single-Carrier Frequency Domain Equalization) is still the preferred option. It provides almost similar frequency-domain benefits as OFDM but has a lower PAPR (Peak-to-Average Power Ratio). A low PAPR enables the UAV power amplifier to operate in the power efficient region thus saving battery life.
