What is OFDMA and how it works?

What is OFDMA and how it works?

OFDMA is a multi-user version of OFDM enabling concurrent AP communication (uplink & downlink) with multiple clients by assigning subsets of subcarriers, called Resource Units (RUs) to the individual clients. OFDMA allows multiple users with varying bandwidth needs to be served simultaneously.

What is the difference between OFDMA and FDMA?

The most obvious difference between the two schemes is that OFDMA transmits the four QPSK data symbols in parallel, one per subcarrier, while SC-FDMA transmits the four QPSK data symbols in series at four times the rate, with each data symbol occupying a wider M x 15 kHz bandwidth.

What is the difference between OFDM and OFDMA?

The main difference between an OFDM and an OFDMA (Orthogonal Frequency Division Multiple Access) system if the fact that in the OFDM the user are allocated on the time domain only while using an OFDMA system the user would be allocated by both time and frequency. For the uplink SC-FDMA will be used.

What is the advantage of SC-FDMA over OFDMA?

Owing to its inherent single carrier structure, a prominent advantage of SC-FDMA over OFDM and OFDMA is that its transmit signal has a lower peak-to-average power ratio (PAPR), resulting in relaxed design parameters in the transmit path of a subscriber unit.

Why is OFDM better than CDMA?

OFDM CDMA comparison Both OFDM and CDMA have significant benefits. OFDM can combat multipath interference with greater robustness and less complexity. Equalisation can be undertaken on a carrier by carrier basis. OFDMA can achieve higher spectral efficiency with MIMO than CDMA using a RAKE receiver.

How orthogonality is achieved in OFDM?

In OFDM, multiple closely spaced orthogonal subcarrier signals with overlapping spectra are transmitted to carry data in parallel. In coded orthogonal frequency-division multiplexing (COFDM), forward error correction (convolutional coding) and time/frequency interleaving are applied to the signal being transmitted.

How is OFDM different from FDM your answer?

It is the multiplexing technique which is used in analog system. In FDM, Guard band is required and the spectral efficiency of FDM is low….Orthogonal Frequency Division Multiplexing (OFDM):

3. The spectral efficiency of FDM is low. While the spectral efficiency of OFDM is high.

Why OFDM is used in downlink?

To overcome the effect of multi path fading problem available in UMTS, LTE uses Orthogonal Frequency Division Multiplexing (OFDM) for the downlink – that is, from the base station to the terminal to transmit the data over many narrow band careers of 180 KHz each instead of spreading one signal over the complete 5MHz …

Why OFDM is called orthogonal?

In OFDM, 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.

What are pilot symbols in OFDM?

Training OFDM symbols or equivalently OFDM preambles are transmitted at the beginning of the transmission record, while pilot symbols (complex exponentials in time) are embedded in each OFDM symbol, and they are separated from information symbols in the frequency-domain [1–3].

Why SC-FDMA has low PAPR?

SC-FDMA spreads the energy of one subcarrier over all subcarriers before the IFFT. This way spectral nulls in the channel is reduced with averaging. Hence, PAPR is reduced.

Why we use SC-FDMA in uplink?

SC-FDMA is preferred for LTE uplink mainly due to its advantage of having low peak-to-average power ratio (PAPR) during signal transmission. It is found that by increasing spreading factor symbol error rate is reduced for SC-FDMA system and for spreading factor of 64 for SNR=20 dB, SER is of the order of 10 -4 .

What is ofofdma and how does it work?

OFDMA stands for Orthogonal Frequency Division Multiple Access. It is considered as a modulation and multiple access technique for next generation wireless networks for such as Mobile WiMAX and LTE.

What is the physical layer of OFDMA?

We will see physical layer with the example of 1024 FFT for Downlink chain which is used by Base Station to send information to Mobile subscribers. This is also called as mobile wimax physcial layer. OFDMA stands for Orthogonal Frequency Division Multiple Access.

What is orthogonal frequency division multiple access (OFDMA)?

This type of multiaccess scheme is called orthogonal frequency division multiple access (OFDMA), which gives the mobile WiMAX PHY its name. Subchannels may be constituted using either contiguous subcarriers or subcarriers pseudorandomly distributed across the frequency spectrum.


9. 4.0 Scalable OFDMA (SOFDMA) Scalable OFDMA is the OFDMA mode is used in Mobile Wi-MAX defined in IEEE 802.16e. Scalability is supported by adjusting the size of FFT size while fixing the sub- carrier frequency spacing in 10.94 kHz. It supports channel bandwidths ranging from 1.25 MHz to 20 MHz. SOFDMA adds scalability to OFDMA.

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