Why ofdm is better

Possesses a high peak to average power ratio — this requires the use of linear power amplifiers which are less efficient than non-linear ones and this results in higher battery consumption.

The cyclic prefix used causes a lowering of the overall spectral efficiency. When used in a cellular system, it is possible to minimise interference from neighbouring cells by using different carrier permutations between the two cells. Again when used with a cellular system, interference within the cell are averaged by using allocation with cyclic permutations. A single frequency network can be used to provide excellent coverage and good frequency re-use.

Offers frequency diversity by spreading the carriers all over the used spectrum It has a relatively high sensitivity to frequency offsets as this degrades the orthogonality between the carriers It is sensitive to phase noise on the oscillators as this degrade the orthogonaility between the carriers Requires complex electronics to run the software — DSP including FFT algorithms needed for the forward error correction.

This is always active regardless of data rate, although when no data is being transmitted the system can hibernate. However power consumption can be an issue. If only a few carriers are assigned to each user the resistance to selective fading will be degraded or lost. Initially the use of OFDM required large levels of processing and accordingly it was not viable for general use. Some of the first systems to adopt OFDM were digital broadcasting - here OFDM was able to provide a highly reliable form of data transport over a variety of signal path conditions.

Once example was DAB digital radio that was introduced in Europe and other countries. OFDM was also used for digital television. Later processing power increased as a result of rising integration levels enabling OFDM to be considered for the 4G mobile communications systems which started to be deployed from around OFDM is a form of multicarrier modulation.

An OFDM signal consists of a number of closely spaced modulated carriers. When modulation of any form - voice, data, etc. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data. As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them.

This is not the case with OFDM. Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another.

This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of demodulators, translating each carrier down to DC.

The resulting signal is integrated over the symbol period to regenerate the data from that carrier. The same demodulator also demodulates the other carriers. Pilot Subcarriers: The pilot subcarriers do not carry modulated data; however, they are used for synchronization purposes between the receiver and transmitter. Unused Subcarriers: The remaining unused subcarriers are mainly used as guard carriers or null subcarriers against interference from adjacent channels or sub-channels.

These tones are grouped into smaller sub-channels, known as resource units RUs. By subdividing the channel, parallel transmissions of small frames to multiple users can happen simultaneously.

The data and pilot subcarriers within each resource unit are both adjacent and contiguous within an OFDMA channel. Wi-Fi 6. With previous technology like FDM, the barista could only handle one customer inside of the cafe at a time, forcing the other customers to wait outside to prevent overwhelm.

With OFDM, on the other hand, the barista can handle a line of people coming into the coffee shop at once, also allowing them to stand more closely together, take their orders sequentially, and use the space more efficiently. Rather, faster speeds for devices sharing a single channel is a consequence of its efficiency.

The organized traffic over signals helps to reduce conflict between devices. Even though OFDM helped receivers efficiently separate each signal, the lingering limitation is that wireless access points can only process one user's data transmission at a time for each channel.

Basically, if multiple users are on one channel, each one has to wait its turn. OFDMA is the latest implementation of structuring data transmissions with a new innovation.

It takes the traditional channels of OFDM and subdivides them into smaller channel sections, carrying data through resources units RUs. Even a 20MHz channel can split itself into sections to be shared by up to nine simultaneous users as long as the access point utilizes the smallest possible resource units.

The access point then schedules the units to make sure each one is processed efficiently and with minimal conflict. Even better, this works for both downlink and uplink connections. Sticking with our coffee shop analogy, think of the frustration you feel when there's several people in front of you and you know waiting for their orders is going to slow you down.