What is OFDM? (Orthogonal Frequency Division Multiplexing)

Orthogonal Frequency Division Multiplexing (OFDM) is a signal modulation technique that divides a high data rate stream into multiple slower substreams, each transmitted over closely spaced, narrowband subcarriers. This makes OFDM highly resistant to frequency-selective fading and multipath interference.

OFDM has been widely adopted in modern wireless and telecommunications standards due to its robustness and spectral efficiency. It is used in WiFi standards such as 802.11a, 802.11n, and 802.11ac, as well as in LTE/LTE-Advanced, WiMAX, and other broadband wireless technologies.

In broadcasting, OFDM is employed in standards like DAB Digital Radio, Digital Video Broadcasting (DVB), and Digital Radio Mondiale (DRM), covering a range of frequency bands including long, medium, and short wave.

While OFDM is more complex than earlier modulation schemes, it offers significant advantages- particularly for high data rate applications over wide bandwidths- making it essential for next-generation communication systems.

What is OFDM? – The concept

OFDM is a multicarrier modulation technique where multiple closely spaced carriers transmit data simultaneously. Unlike traditional systems that require guard bands between channels to avoid interference, OFDM carriers can overlap because they are mathematically orthogonal—meaning their sidebands do not interfere with one another. This orthogonality is achieved by spacing carriers at intervals equal to the reciprocal of the symbol period.

At the receiver, a bank of demodulators separates and processes each carrier by integrating over the symbol period, allowing the original data to be recovered. As the carrier spacing is set to the reciprocal of the symbol period, each carrier completes a whole number of cycles within that period, resulting in their contributions summing to zero, so no interference occurs. For OFDM to work effectively, the transmission system must be linear; non-linearity introduces intermodulation distortion that disrupts orthogonality and causes interference.

However, OFDM signals have a high peak-to-average power ratio, requiring the RF final amplifier on the output of the transmitter to handle high peaks efficiently, which leads to power inefficiency. Some systems clip these peaks, accepting distortion and an increased level of data errors that are later corrected by error correction mechanisms.

Data on OFDM

In an OFDM signal, the data to be transmitted is distributed across multiple carriers, with each carrier handling a portion of the payload, thereby reducing the data rate per carrier. This lower data rate minimises the impact of interference from reflections. A guard band time or guard interval is incorporated into the system, ensuring that data is sampled only when the signal is stable, preventing newly arriving delayed signals that could affect the signal’s timing and phase.

The distribution of the data across a large number of carriers in the OFDM signal offers further advantages. Nulls caused by multi-path effects or interference on a given frequency only impact a small number of the carriers, the remaining ones being received correctly. By using error-coding techniques, which involve adding further data to the transmitted signal, it enables most or all of the corrupted data to be reconstructed within the receiver. This can be done because the error correction code is transmitted in a different part of the signal.

OFDM advantages & disadvantages

OFDM advantages

OFDM has been used in many high data rate wireless systems because of the many advantages it provides.

• Immunity to selective fading: OFDM splits the channel into multiple narrowband sub-channels, which each experience flat fading, making it more resistant to frequency selective fading.
• Resilience to interference: Interference typically only affects a few sub-channels, as it may be bandwidth-limited, so not all data is lost.
• High spectrum efficiency: Closely spaced, overlapping sub-carriers allow efficient use of the available spectrum.
• Resilience to ISI: Low data rates per sub-channel reduce inter-symbol and inter-frame interference.
• Resilience to narrow-band effects: Channel coding and interleaving help recover data lost due to narrow-band interference.
• Simpler channel equalisation:  Equalisation is easier since it is applied per sub-channel rather than across the entire bandwidth.

OFDM disadvantages

Whilst OFDM has been widely used, there are still a few disadvantages to its use which need to be considered.

• High peak to average power ratio: OFDM signals have large amplitude variations, requiring linear RF amplifiers, which reduces power efficiency.
• Sensitive to carrier offset and drift: OFDM is more sensitive to frequency offset and drift compared to single-carrier systems.

OFDM variants

There are several other variants of OFDM for which the initials are seen in the technical literature. These follow the basic format for OFDM, but have additional attributes or variations:

• COFDM: Coded OFDM. Error correction coding is incorporated into the signal.
• Flash OFDM: This is a fast hopped variant of OFDM, developed by Flarion. It uses multiple tones and fast hopping to spread signals over a given spectrum band.
• OFDMA: Orthogonal Frequency Division Multiple Access. It provides a multiple access capability for applications such as cellular telecommunications when using OFDM technologies.
• VOFDM: Vector OFDM. Developed by CISCO Systems, it uses the concept of MIMO (Multiple Input Multiple Output) technology, using multiple antennas for transmission and reception. VOFDM utilises multi-path effects to improve signal reception and support higher transmission speeds.
• WOFDM: Wideband OFDM. It uses a large enough degree of spacing between channels to avoid frequency errors between the transmitter and receiver affecting the performance. This is particularly applied in WiFi systems.

Each of these forms of OFDM utilise the same basic concept of using close spaced orthogonal carriers each carrying low data rate signals. During the demodulation phase the data is then combined to provide the complete signal.

OFDM has gained a significant presence in the wireless market place. The combination of high data capacity, high spectral efficiency, and its resilience to interference as a result of multi-path effects means that it is ideal for the high data applications that have become a major factor in today’s communications scene.

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