OFDM et ses applications

Orthogonal Frequency Division Multiplexing (OFDM)

• OFDM significantly expanded wireless networks, improving efficiency in bits per Hertz (bps/Hz)
• It became a crucial technology for broadband wireless access, particularly in WiMAX and 802.11 standards (like Wi-Fi 802.11n).
• OFDM operates by splitting a data stream into multiple parallel data streams—called subcarriers. Each subcarrier has a specific bandwidth.
• This allows data transmission on numerous closely spaced frequencies.

How OFDM Works

• OFDM is also known as multicarrier modulation.
• Starting with a data stream of R bits per second, with a suitable bandwidth Nf, enables sending data with a bit duration of 1/R.
• OFDM divides the data stream into N parallel data streams, each of which is a subcarrier
• Each subcarrier has its own bandwidth (f), enabling a data rate of R/N (bits per second) per subcarrier, and the bit time becomes N/R.

Orthogonality

• The spacing of carrier frequencies (f) allows for tight packing of signals despite overlaps, something traditionally not possible with FDM (frequency-division multiplexing).
• Signals are spaced at intervals of f, 2f, 3f, and so on.
• Importantly, these frequencies and bit rates are chosen to make individual signals orthogonal, preventing interference in time.
• The average value of the product of two orthogonal signals over a bit period is zero.

Orthogonality (Continued)

• Traditional FDM maintains separate frequencies to avoid overlaps. In contrast, OFDM allows overlap, significantly increasing capacity.
• A crucial aspect of OFDM is that the subcarrier frequencies (f) are meticulously chosen to be spaced in multiples of the reciprocal of the bit time (1/T), enabling orthogonality.
• This orthogonality is illustrated in the time domain where the signals overlap in the frequency domain and do not interfere.

Orthogonality (Technical Details)

• A subcarrier bit time of T necessitates f to be a multiple of 1/T.
• For example, in IEEE 802.11n wireless networks, a 20MHz total bandwidth limits usable bandwidth to 15MHz, leading to 48 subcarriers.
• Here, f is 0.3125 MHz. The signal is translated to 2.4GHz or 5GHz bands.

Benefits of OFDM

• Frequency selective fading, a common problem in wireless communication that affects different frequencies in a signal, affects only some OFDM subcarriers. This is readily addressed by error correcting codes.
• OFDM is especially effective at combating intersymbol interference (ISI) which results from signals reflected from different distances (multipath signals) arriving at later bits.
• The impact of ISI is reduced considerably due to the greatly increased (by a factor of N) bit time of OFDM, making equalization less critical.

OFDM Implementation

• OFDM uses the Inverse Fast Fourier Transform (IFFT) to combine subcarrier signals, making use of N oscillators for N subcarriers.
• Discrete Fourier Transform (DFT), especially its faster version, the FFT, significantly improves computational speed when N is a power of two.
• The transmitter assembles an OFDM symbol from the data of various subcarriers.
• The IFFT calculates the data stream that needs to be transmitted.
• The OFDM symbol provides weights for the subcarriers, simplifying transmission using only one oscillator.

Cyclic Prefix

• OFDM's extended bit times reduce intersymbol interference (ISI).
• To further combat remaining ISI, a cyclic prefix (CP) is added to each OFDM symbol.
• The CP is a portion of the signal duplicated from the end of the OFDM symbol, working as a guard interval which ensures the signals do not interfere before the data portion begins.

Difficulties of OFDM (Peak-to-Average Power Ratio (PAPR))

• OFDM signals have a significantly higher peak-to-average power ratio (PAPR) than single-carrier signals, because the signal may contain large peaks.
• This requires amplifiers with suitably wide linear ranges, potentially making them costly.

Difficulties of OFDM (Amplifier Linearity)

• Practical amplifiers don't exhibit perfect linearity over the entire signal range.
• Amplifying a signal exceeding the linear range will result in distortion.

Difficulties of OFDM (Intercarrier Interference (ICI))

• OFDM subcarriers are precisely spaced for orthogonality.
• Channel impairments can corrupt this, creating intercarrier interference.
• Cyclic prefix (CP) helps control and limit this interference, effectively improving spectral efficiency by limiting CP time.

OFDMA (Orthogonal Frequency-Division Multiple Access)

• OFDMA builds on OFDM to share frequencies amongst multiple users rather than just one, by dividing subchannels onto resource blocks.
• This is managed using opportunistic scheduling.

Opportunistic Scheduling

• Subchannels and power levels are dynamically adjusted, depending on channel conditions and user data needs, to make the best use of available resources.

Opportunistic Scheduling (Criteria)

• Opportunistic scheduling considers various criteria, typically simultaneously, like system efficiency, fairness, user demand, priority for some users (e.g., public safety).

Single-Carrier FDMA (SC-FDMA)

• SC-FDMA has structure and performance similar to OFDMA but with lower PAPR.
• This benefits mobile users by improving things like power efficiency and battery life.
• Data is spread over all the subcarriers to support uplinks.

Ce quiz traite de l'importance de la modulation par répartition en fréquence orthogonale (OFDM) dans les réseaux sans fil, notamment dans WiMAX et Wi-Fi. L'OFDM améliore l'efficacité de transmission en divisant un flux de données en plusieurs sous-porteuses parallèles. Faites le test pour approfondir vos connaissances sur ce sujet essentiel.