OFDM vs OFDMA
OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) are both wideband digital communication technologies with a slight difference between them. However, both are based on the same concept of bundling evenly spaced multiple sub-carriers with special characteristics into one big chunk and still transmitting separately over the transmission media. However, when it comes to providing simultaneous multi-user access, two technologies have a significant difference in channel allocation mechanism.
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 sub-carriers in such a way that, all of the sub-carriers to be orthogonal to each other are evenly spaced . In other words, OFDM divides one high-speed signal into numerous slow signals to be more robust at the receiver’s end so that the sub-channels can then transmit data without being subject to the same intensity of multipath distortion faced by single carrier transmission. The numerous sub-carriers are then collected at the receiver and recombined to form one high-speed transmission.
The orthogonally of subcarriers provide high Spectral efficiency and low Inter-Carrier-Interference (ICI). Since each and every subcarrier is treated as a different narrowband signal where each of them modulated individually, makes it easy to combat with frequency selective fading due to multipath. In other words, simplified channel equalization is required due to narrowband sub-carrier nature. Moreover, low data rate (Symbol Rate) of each sub-carrier reduces Inter Symbol Interference (ISI) greatly and that results in very high Signal to Noise Ratio (SNR) of the system. As a result of all above advantages, it is made possible to implement Single Frequency Network (SFN) and solve spectrum limitation issues in commercial implementation of such system.
In OFDM systems, only a single user can transmit on all of the sub-carriers at any given time. To accommodate multiple users, a strictly OFDM system must employ Time Division Multiple Access (TDMA) (separate time frames) or Frequency Division Multiple Access (FDMA) (separate channels). Neither of these techniques is time or frequency efficient. The major drawback to this static multiple access schemes is the fact that different users seeing the wireless channels (Sub-carriers) differently is not being utilized. OFDM technologies typically occupy nomadic, fixed and one-way transmission standards, ranging from TV transmission to Wi-Fi as well as fixed WiMAX and newer multicast wireless systems like Qualcomm’s Forward Link Only (FLO).
What is OFDMA?
OFDMA is the multi-user OFDM technology where users can be assigned on both TDMA and FDMA basis where a single user does not necessarily need to occupy all the sub-carriers at any given time. In other words, a subset of subcarriers is assigned to a particular user. This allows simultaneous low data rate transmission from several users as well as it can be dynamically assigned to the best non-fading, low interference channels for a particular user and avoid bad sub-carriers to be assigned. Point-to-Multipoint fixed and mobile systems use OFDMA and most emerging systems are using OFDMA such as Mobile WiMAX and LTE.
What is the difference between OFDM and OFDMA?
• OFDM support multiple users (Multiple Access) via TDMA basis only, while OFDMA support either on TDMA or FDMA basis or both at the same time.
• OFDMA supports simultaneous low data rate transmission from several users, but OFDM can only support one user at given moment.
• Further improvement to OFDMA over OFDM robustness to fading and interference since it can assign subset of subcarrier per user by avoiding assigning bad channels.
• OFDMA supports per channel or sub-carrier power while OFDM needs to maintain the same power for all sub-carriers.