What is Ofdma – Troubleinthepeace


OFDM (Orthogonal frequency multiplexing) and OFDMA (Orthogonal frequency division Multiple access) are both broadband digital communication technologies with slight differences between them. However, both are based on the same concept of webgiaidap.com which combines multiple sub-carriers with special characteristics into one large block and still propagates separately on the transmission medium. However, when it comes to webgiaidap.com providing simultaneous multi-user access, the two technologies have significant differences in channel allocation mechanisms.

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What is OFDM?

OFDM is a Frequency Division Multiplexing (FDM) mechanism, which works by dividing a single wideband signal into a large set of narrowband subcarriers in such a way that all subcarriers are orthogonal. together. In other words, OFDM divides a high-speed signal into many slow signals to be stronger at the receiver so that the sub-channels can then transmit the data without suffering the same intensity of multi-stream distortion that transmits the wave. single bearing face. Multiple subcarriers are then collected at the receiver and recombined to form a high-speed transmission line.

The subcarrier orthogonality provides high Spectral efficiency and low Intercarrier Interference (ICI). Since each subcarrier is treated as a different narrowband signal, where each carrier is individually modulated, it is easy to combat frequency selective blurring due to multipath. In other words, simplified channel equalization is required due to the narrow subcarrier nature. In addition, the low data rate (Symbol Rate) of each sub-carrier significantly reduces Symbol Interference (ISI) and results in a very high Signal-to-Noise Ratio (SNR) of the system. Due to all the above advantages, it is possible to implement a Single Frequency Network (SFN) and solve the spectrum limitation problems in the commercial implementation of that system.

In OFDM systems, only one user can transmit on all subcarriers at any given time. To accommodate multiple users, a strict OFDM system must use Time Division Multiple Access (TDMA) (separate time frames) or Frequency Division Multiple Access (FDMA) (separate channels) ). Both of these techniques are time or frequency efficient. The big drawback to these static access schemes is that different users see different wireless channels (Sub-Carriers) that are not being used. OFDM technologies typically account for nomadic, fixed, and one-way transmission standards ranging from TV transmission to Wi-Fi as well as fixed WiMAX and newer omnidirectional wireless systems such as Forward Link Only (FLO). ) by Qualcomm.

What is OFDMA?

OFDMA is a multi-user OFDM technology where users can be assigned on both TDMA and FDMA bases where a single user does not necessarily occupy all subcarriers at any given time. In other words, a subset of subcarriers are assigned to a particular user. This allows for simultaneous low data rate transmission from several users as well as can be dynamically assigned to the best fade-free, lowest noise channels for a particular user and avoids bad sub-carriers. Nominated. Point-to-multipoint fixed and mobile systems use OFDMA and most emerging systems use OFDMA such as Mobile WiMAX and LTE.

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What is the difference between OFDM and OFDMA?


• OFDM only supports multiple users (Multiple Access) via TDMA base, while OFDMA only supports TDMA or FDMA basis or both at the same time.

• OFDMA supports simultaneous low data rate transmission from multiple users, but OFDM can only support one user at a time.

• Further improvement for OFDMA in OFDM’s robustness for fading and interference as it can assign a subset of subcarriers to each user by avoiding the assignment of bad channels.

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• OFDMA supports per channel or subcarrier power while OFDM needs to maintain the same power for all subcarriers.

Source: internet

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