Ofdm why orthogonal

This can be achieved far more effectively within a low data rate substream. The distribution of the data across a large number of carriers in the OFDM signal has some further advantages. Nulls caused by multi-path effects or interference on a given frequency only affect a small number of the carriers, the remaining ones being received correctly.

By using error-coding techniques, which does mean adding further data to the transmitted signal, it enables many 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 has been used in many high data rate wireless systems because of the many advantages it provides.

Whilst OFDM has been widely used, there are still a few disadvantages to its use which need to be addressed when considering its use. OFDM, orthogonal frequency division multiplexing 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.

Development of OFDM The use of OFDM and multicarrier modulation in general has come to the fore in recent years as it provides an ideal platform for wireless data communications transmissions. What is OFDM? Traditional-slection if signals on different chanels To see how OFDM works, it is necessary to look at the receiver.

Data on OFDM The traditional format for sending data over a radio channel is to send it serially, one bit after another. A guard interval is added to each symbol to minimize the channel delay spread and intersymbol interference. Immunity to selective fading: One of the main advantages of OFDM is that is more resistant to frequency selective fading than single carrier systems because it divides the overall channel into multiple narrowband signals that are affected individually as flat fading sub-channels.

Resilience to interference: Interference appearing on a channel may be bandwidth limited and in this way will not affect all the sub-channels. This means that not all the data is lost. Spectrum efficiency: Using close-spaced overlapping sub-carriers, a significant OFDM advantage is that it makes efficient use of the available spectrum.

This results from the low data rate on each of the sub-channels. Resilient to narrow-band effects: Using adequate channel coding and interleaving it is possible to recover symbols lost due to the frequency selectivity of the channel and narrow band interference.

Not all the data is lost. Simpler channel equalisation: One of the issues with CDMA systems was the complexity of the channel equalisation which had to be applied across the whole channel. An advantage of OFDM is that using multiple sub-channels, the channel equalization becomes much simpler. OFDM disadvantages Whilst OFDM has been widely used, there are still a few disadvantages to its use which need to be addressed when considering its use.

Note that the output is updated at a periodic interval T that forms the symbol period. To maintain orthogonality, T must be the reciprocal of the subcarrier spacing. In the frequency domain, each transmitted subcarrier results in a sinc function spectrum with side lobes that produce overlapping spectra between subcarriers, see "OFDM Signal Frequency Spectra" figure below. This results in subcarrier interference except at orthogonally spaced frequencies. At orthogonal frequencies, the individual peaks of subcarriers all line up with the nulls of the other subcarriers.

The receiver multiplies i. The use of orthogonal subcarriers allows more subcarriers per bandwidth resulting in an increase in spectral efficiency.

In FDM systems, any overlap in the spectrums of adjacent signals will result in interference. In OFDM systems, the subcarriers will interfere with each other only if there is a loss of orthogonality. For example, frequency error will cause the subcarrier frequencies to shift so that the spectral nulls will no longer be aligned resulting in inter-subcarrier-interference. These transforms are important from the OFDM perspective because they can be viewed as mapping digitally modulated input data data symbols onto orthogonal subcarriers.

In principle, the IFFT takes frequency-domain input data complex numbers representing the modulated subcarriers and converts it to the time-domain output data analog OFDM symbol waveform. In a digitally implemented OFDM system, the input bits are grouped and mapped to source data symbols that are a complex number representing the modulation constellation point e.

These complex source symbols are treated by the transmitter as though they are in the frequency-domain and are the inputs to an IFFT block that transforms the data into the time-domain. Each of these N input symbols has a symbol period of T seconds. In contrary, multi-carrier transmission is analogous to shipment goods using several smaller cars, each carrying a single package right side of the figure. This is analogical to a single deep fade, by which only one subcarrier part of data is destroyed i.

To see other posts on network and wireless fundamentals — for example about pathloss , shadowing or MIMO — see our explained section. To subscribe to our mailing list for our online platform where you can learn all this and more for example all the details on OFDM transmission visit GrandmetricWatch. Marcin Dryjanski received his Ph. You can reach Marcin at marcin. Your email address will not be published. Sign up to our newsletter! Save my name, email, and website in this browser for the next time I comment.

Toggle navigation. How does OFDM work? Author: Marcin Dryjanski, Ph. Inter-Symbol Interference Lets look at the example of the actual impairments see figure below.