OFDM vs OFDMA | difference between OFDM and OFDMA
This page on the OFDM vs OFDMA describes difference between OFDM and OFDMA modulation schemes. The tabular comparison between OFDM and OFDMA are also outlined.
In OFDM systems, only a single user can transmit on all of the sub-carriers at any given time. In order to support multiple users time and/or frequency division access techniques are used in OFDM. The major setback to this static multiple access scheme is the fact that the different users see the wireless channel differently is not being utilized. OFDMA, on the other hand, allows multiple users to transmit simultaneously on the different sub-carriers per OFDM symbol.
OFDM is employed in Fixed WiMAX system deployed around the world for broadband internet service. Figure 1 depicts OFDM frame structure employed in fixed WiMAX system.
Here Downlink sub frame is transmitted by Base station to subscriber stations and Uplink sub frame is transmitted by multiple subscriber stations to the Base Station. Both the frame is composed of more than one OFDM symbols and each symbol is made up of subcarriers, which fall in data and pilot subcarriers, where data subcarriers carry the user data. There are 192 data sub carriers in Fixed WiMAX System. The point here is Subscriber station has been assigned one or more symbols by BS and all the data carriers(i.e. 192) of the symbols are occupied by one SS. It is depicted in the figure-1 that entire 256 carriers are allocated to the one user statically in TDD frame.
Figure-1 : OFDM modulation frame structure as per 16d standard
To undertand difference between OFDM and OFDMA, one should understand basic difference between OFDM and FDM multiplexing techniques in addition to OFDM physical layer and OFDMA physical layer as per fixed wimax and mobile wimax standards.
In the case of OFDMA, which is employed in Mobile WiMAX system deployed around the world and also employed in LTE system being deployed, total subcarriers are permuted and assigned to sub channel. Hence many SSs can occupy the same sub channel but use different subcarriers to transmit the information.
Figure 2 describes OFDMA frame used in Mobile WiMAX System. It clearly mentions that one symbol is composed of
more than one sub channel and each sub channel is composed of distributed subcarriers.
The point here is each symbol is used by more number of SSs to transmit and receive the
information which is depicted by Burst 1 and Burst 2 in the figure.
As mentioned in OFDMA subcarriers are divided among users at the same time instant.
Figure mentions 2048 FFT case here. Total 2048 subcarriers of FFT is divided among 60 subchannels.
Each subchannels will have their own pilot and data subcarriers.
Figure-2 : OFDMA modulation frame structure as per 16e standard
The Frame structures mentioned here only for demonstrating the concept and it differs in the actual wimax system.
Both OFDM and OFDMA is used to achieve high data rate transmission over the air. With OFDMA system can support more subscribers with sub channelization concept compare to OFDM.
Both OFDM and OFDMA is implemented using IFFT and FFT operation at transmitter and receiver respectively. For OFDM entire input of IFFT is occupied fully by either subscriber staion or Base Station. For OFDMA part of input values (consecutively) is occupied by Subscriber station and at rest of the input positions zeros or nulls are inserted. Same is done with other subscribers and so on.
Further OFDM vs OFDMA can be explored by studying comparison between WiMAX and LTE standards. Refer wimax vs lte page in terminology section.
Following table summarize important difference between an OFDM and an OFDMA modulation types.
|Full Form||Orthogonal Frequency Division Multiplexing||Orthogonal Frequency Division Multiple Access|
|Capacity Allocation||One user is assigned full OFDM symbol with all the data subcarriers as well as pilot subcarriers, Data subcarriers are used for data transmission and pilot subcarriers are used to carry known symbols. The pilot subcariers help in channel estimation and equalization. For example, in fixed wimax 1 OFDM symbol consists of total 256 subcarriers. This 256 subcarriers are divided into 192 data, 8 pilots, 1 DC, 28 left guard and 27 right guard subcarriers. Here resource allocation to user is TDMA based.||Here one user is assigned unique one or more subchannels. The subchannel is composed of distributed or contiguous subcarriers based on OFDMA type. These subcarriers are spread across multiple OFDM symbols. Here resource allocation to user is both TDMA and FDMA based.|
|Robustness against fading/interference||Less robust to fading as well as interference.||More robust to fading as well as interference compare to OFDM.|
|Application||Fixed wimax PHY, refer OFDM physical layer➤||Mobile wimax PHY, refer OFDMA physical layer➤|
OFDM and OFDMA RELATED LINKS
Refer following links which mentions OFCDM, OFDM and OFDMA Physical layers(PHY) as per IEEE 802.11a(WLAN), 802.16d(Fixed WiMAX) and IEEE 802.11ad.
It mentions difference between OFDM and other terms which include OFDM vs DMT, OFDM vs FBMC, SC-FDMA vs OFDMA, OFDM vs CDMA etc.
OFDMA PHYSICAL LAYER OFDM vs CDMA SC-FDMA vs OFDMA OFDM vs DMT 802.11ad OFDM PHY Hybrid OFDM OFDM vs FBMC WiMAX OFDM PHY WLAN 11a PHY