Why ofdm

The input symbols are complex values representing the mapped constellation point and therefore specify both the amplitude and phase of the sinusoid for that subcarrier. After some additional processing, the time-domain signal that results from the IFFT is transmitted across the radio channel. At the receiver, an FFT block is used to process the received signal and bring it into the frequency domain which is used to recover the original data bits.

An To begin the OFDM signal creation process, the input data bit stream is encoded with convolutional coding and Interleaving. Note that the bit-rate will be different depending on the modulation format, a QAM constellation 6 bits at a time can have a bit rate of 54 Mbps while a QPSK constellation 2 bits at time may only be 12 Mbps.

Then 52 bins of the IFFT block are loaded. The samples are clocked out at 20 Msps to create a 3. To complete the OFDM symbol, a 0. This produces a "single" OFDM symbol with a time duration of 4 us in length, 3. The process is repeated to create additional OFDM symbols for the remaining input data bits. In this blog series, we will use both terms interchangeably.

Each OFDM subcarrier is Subcarrier spacing is equal to the reciprocal of the symbol time. As a result of the longer symbol time, the subcarrier size and spacing decreases from The narrow subcarrier spacing allows better equalization and therefore enhanced channel robustness.

Because of the Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them. The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload. This reduces the data rate taken by each carrier. The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system.

This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal. 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. 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. As the carrier spacing equal to the reciprocal of the symbol period means that they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution.

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission. In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency.

In some systems the peaks are limited.