OFDM Flashcards
What is orthogonal frequency-division multiplexing?
OFDM is a type of digital transmission used in digital modulation for encoding digital data on multiple carrier frequencies. In OFDM, the incoming bitstream representing the data to be sent is divided into multiple streams.
What’s the main advantage of OFDM over single-carrier schemes
It has the ability of cope with severe channel conditions* without the need for complex equalisation filters.
*i.e. attenuation of high frequencies, narrowband interference, frequency-selective fading due to multipath)
What does the low symbol rate allow?
The low symbol rate makes the use of a guard interval between symbols affordable, making it possibile to eliminate intersymbol interference (ISI) and use echoes and time-spreading to achieve a diversity gain.
Give the summaries of the advantages of OFDM
- High spectral efficiency in comparison to other DS modulation schemes,
- Can easily adapt to severe channel conditions without complex time-domain equalisation.
- Robust against narrow-band co-channel interference.
- Robust against ISI and fading caused by multipath propagation.
- Efficient implementation using FFT
- Low sensitivity to time synchronisation errors
Give the summaries of disadvantages of OFDM
- Sensitive to doppler shift
- Sensitive to frequency synchronisation problems
- High peak-to-average-power ratio, requiring linear transmitter circuitry, which suffers from poor power efficiency
- Loss of efficiency caused by cyclic prefix/guard interval.
How are subcarrier frequencies chosen?
The subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other, meaning that crosstalk between the sub-channels is eliminated and inter-carrier guard bands are not required.
It also allows high spectral efficiency (total symbol rate near the NQ rate for the equivalent BB signal)
cross-talk: signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel
What type of synchronisation does OFDM need at a very accurate level and why is that?
OFDM requires very accurate frequency synchronisation between the receiver and transmitter. Without this synchronisation, a deviation in frequency causes the subcarriers to no longer be orthogonal, causing inter-carrier interference.
Frequency offsets are typically caused by mismatched transmitter and receiver oscillators, or by Doppler shift due to movement.
Explain the use of guard interval for elimination of intersymbol interference
- It is a key principle in OFDM; low symbol rate modulation schemes* suffer less from ISI caused by multipath propagation.
- It is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream.
- Since the duration of each symbol is long, it is feasible to insert a guard interval between the OFDM symbols, thus elimination the intersymbol interference.
- The guard interval also eliminates the need for a pulse-shaping filter.
- Reduces sensitivity to time synchronisation problems.
What is the cyclic prefix and what does it do?
The cyclic prefix, which is transmitted during the guard interval, consists of the end of the OFDM symbol copied into the guard interval.
The guard interval is then transmitted followed by the OFDM symbol.
What happens when DPSK or QPSK is applied to each subcarrier
Equalisation can be totally omitted, since these non-coherent schemes are insensitive to slowly changing amplitude and phase distortion.
Does OFDM use interleaving? If so, what’s influence does it have to OFDM?
Almost always uses frequency or time interleaving. Frequency interleaving increases resistance to frequency-selective channel conditions such as fading.
Explain the use of space diversity in OFDM
- Receivers can benefit from receiving signals from several spatially dispersed transmitters simultaneously.
- Transmitters will only destructively interfere with each other on a limited number of subcarriers, where in general they will actually reinforce coverage over a wide area.
- Single Frequency Networks* result in a diversity gain in receivers situated midway between the transmitters.
- Broadcasting systems, deliberately use a long guard interval in order to allow the transmitters to be spaced further apart in an SFN.
Explain the difference between single-carrier and multiple-carrier modulation
Single-carrier modulation systems exploit only one signal frequency to transmit data symbols. Differently, multicarrier modulation systems divide the whole frequency channel into many subcarriers
Explain the modulation and the demodulation of OFDM
Modulation:
Multiple closely spaced orthogonal subcarrier signals with overlapping spectra are transmitted with each carrier modulated with bits from the incoming stream so multiple bits are being transmitted in parallel.
Demodulation:
Demodulation is based on FFT algorithms.
