RFOFDM vs. OFDMA: A Detailed Comparison
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This article compares OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) modulation schemes, highlighting their differences and outlining a tabular comparison.
OFDM
In OFDM systems, only one user can transmit on all sub-carriers at any given time. To support multiple users, time and/or frequency division access techniques are employed. A significant drawback of this static multiple access scheme is that it doesn’t leverage the fact that different users experience the wireless channel differently.
OFDM is used in Fixed WiMAX systems for broadband internet service.
Figure 1: Depicts the OFDM frame structure used in a fixed WiMAX system. The downlink subframe is transmitted by the base station to subscriber stations, while the uplink subframe is transmitted from multiple subscriber stations back to the base station. Both frames are composed of multiple OFDM symbols, each consisting of data and pilot subcarriers. Data subcarriers carry the user’s data.
Figure-1 : OFDM modulation frame structure as per 16d standard
In the Fixed WiMAX system, there are 192 data subcarriers. The base station assigns one or more symbols to each subscriber station, and all data carriers (i.e., 192) within those symbols are occupied by that subscriber station. As illustrated in Figure 1, the entire 256 carriers are allocated to a single user statically in the TDD frame.
To fully understand the difference between OFDM and OFDMA, it’s essential to grasp the basic differences between OFDM and FDM multiplexing techniques, as well as the physical layers of OFDM and OFDMA as defined by Fixed and Mobile WiMAX standards.
OFDMA
OFDMA, on the other hand, is used in Mobile WiMAX and LTE systems. It allows multiple users to transmit simultaneously on different sub-carriers within each OFDM symbol.
In OFDMA, the total subcarriers are permuted and assigned to sub-channels. This allows multiple subscriber stations (SSs) to occupy the same sub-channel, but using different sub-carriers to transmit their information.
Figure 2: Illustrates the OFDMA frame used in Mobile WiMAX systems. It shows that one symbol is composed of more than one sub-channel, and each sub-channel contains distributed sub-carriers.
Figure-2 : OFDMA modulation frame structure as per 16e standard
Each symbol is used by multiple subscriber stations to transmit and receive information, as depicted by Burst 1 and Burst 2 in the figure. As mentioned, OFDMA divides sub-carriers among users at the same time.
Figure 2 refers to the 2048 FFT case, where a total of 2048 sub-carriers are divided among 60 sub-channels. Each sub-channel has its own pilot and data sub-carriers.
Note: The frame structures presented here are for demonstrating the concept. Actual WiMAX systems might have different frame structures.
Both OFDM and OFDMA are used to achieve high data rate transmission over the air. However, OFDMA can support more subscribers due to the sub-channelization concept. Both are implemented using IFFT and FFT operations at the transmitter and receiver, respectively.
In OFDM, the entire input of the IFFT is fully occupied by either a subscriber station or the base station. In OFDMA, only a portion of the input values (consecutively) is occupied by a subscriber station, and the rest of the input positions are filled with zeros or nulls. This process is repeated for other subscribers.
Further exploration of OFDM vs. OFDMA can be done by studying the comparison between WiMAX and LTE standards.
Difference between OFDM and OFDMA
The following table summarizes the key differences between OFDM and OFDMA modulation types:
| Features | OFDM | OFDMA |
|---|---|---|
| Full Form | Orthogonal Frequency Division Multiplexing | Orthogonal Frequency Division Multiple Access |
| Basic Principle | Divides a high-speed data stream into multiple parallel substreams, each modulated onto different orthogonal subcarriers. | Builds upon OFDM by allowing multiple users to access and share the same spectrum simultaneously by assigning subsets of subcarriers to different users. |
| Multiple Access | Single User (SU) | Multiple Users (MU) |
| Resource Allocation | Fixed | Dynamic |
| Subcarrier Assignment | Assigned to a single user | Assigned dynamically to multiple users |
| Flexibility | Limited | High |
| Efficiency | Lower | Higher |
| Scalability | Limited | High |
| Complexity | Lower | Higher |
| Interference Management | Less effective in managing interference when multiple users are involved. | More effective in managing interference as users’ subcarriers can be spatially separated and allocated dynamically. |
| Performance in Congestion | More prone to congestion as resources are not dynamically allocated among users. | More resilient to congestion as resources are dynamically assigned based on demand. |
| Robustness against fading/interference | Less robust to fading as well as interference. | More robust to fading as well as interference compared to OFDM. |
| Applications | Wired systems such as DSL, WiFi, Fixed WiMAX PHY | Mobile WiMAX PHY, 5G etc. |
Conclusion
In conclusion, while both OFDM and OFDMA are based on dividing the available spectrum into orthogonal subcarriers, their key difference lies in multiple access and resource allocation. OFDM primarily caters to single-user communication with fixed resource allocation. OFDMA extends this concept to accommodate multiple users by dynamically assigning subsets of subcarriers based on user demand. This enables OFDMA to achieve higher spectral efficiency, scalability, and interference management, making it the preferred choice for modern multi-user wireless communication systems such as 5G and WiMAX.
