Question

Research on the fundamentals of LTE OFDM Technology (OFDM is being used for many of the latest wireless and telecommunications standards, such as Wi-Fi 802.11ac, both 4G LTE and 5G cellular technologies, WiMAX, and many more) and create a comparative look on how it differ from other type of multiple access.

Why does it have many advantages over FDM?

Answer 1

OFDM:ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING:

• Orthogonal frequency division multiplexing is a form of signal modulation that divides a high data rate modulating stream placing them onto many slowly modulated narrowband close-spaced subcarriers, and in this way is less sensitive to frequency selective fading.
• Orthogonal Frequency Division Multiplexing or OFDM is a modulation format that is being used for many of the latest wireless and telecommunications standards.
• OFDM has been adopted in the Wi-Fi arena where the standards like 802.11a, 802.11n, 802.11ac and more. It has also been chosen for the cellular telecommunications standard LTE / LTE-A, and in addition to this it has been adopted by other standards such as WiMAX and many more.
• Orthogonal frequency division multiplexing has also been adopted for a number of broadcast standards from DAB Digital Radio to the Digital Video Broadcast standards, DVB. It has also been adopted for other broadcast systems as well including Digital Radio Mondiale used for the long medium and short wave bands.
• Although OFDM, orthogonal frequency division multiplexing is more complicated than earlier forms of signal format, it provides some distinct advantages in terms of data transmission, especially where high data rates are needed along with relatively wide bandwidths.
• Above diagram shows the fundamentals of OFDM
• Frequency Division Multiplexing (FDM) is one answer, where the transmitted signal is a composite of several modulated signals with slightly different center frequencies (see Figure ). One high-speed data stream is divided into multiple low-speed streams, each of which is transmitted in its own channel. The data rate of each channel is low enough to survive transmission at the desired frequency and motion speed. The receiver demodulates each channel and recombines the data to achieve the original high data rate.FDM has been used for many, many years in wireline and microwave communications. Rather than a single high-speed data stream, however, it has most often been used to carry many independent low-data-rate signals— telephone voice communications. FDM is also used in Digital Subscriber Line service, with the high-speed to multiple low-speed data rate conversion implemented for a different reason. Because DSL operates in the <50 MHz region, the use of multiple channels avoids the need for exceptionally wide percentage bandwidth modulation. It also allows selection of channel frequencies that minimize interference with commonly-used radio services that may be used in proximity to DSL lines. FDM requires separation of the channels. In radio-based FDM, there must be a space, or guard band between channels to avoid interference from adjacent channels. These empty spaces are not efficient use of the spectrum. Also, when channels are placed as close together as possible, the required narrowband channel filters increase the complexity of the hardware.

How OFDM is different from other type of multiple accesses:

• In OFDM (Orthogonal Frequency Division Multiplexing) systems the original bandwidth is subdivided into multiple subcarriers. Each of this subcarriers can them be individually modulated. Typically in OFDM systems we can have hundreds of subcarriers with a content spacing between them (15KHz on the LTE case). Since the multiple subcarriers in OFDM are transmitted in parallel, it’s possible for each one to transmit with a lower symbol rate. That improves robustness on the technology for mobile propagation conditions.

• The chain to generate an OFDM signal starts by paralyzing the symbols that need to be transmitted, after they are modulated (in LTE the modulation can be QPSK, 16AQM, 64QAM). Them they are used as input bands for an inverse fast Fourier transform operation. This operation produces OFDM symbols, which will be transmitted. Notice that a conversion from the frequency to the time domain was made when the IFFT was used. Before the transmission, however, a cyclic prefix is includes in the OFDM symbols in order to avoid intersymbol interference. This cyclic prefix in LTE has 5.2us on the first symbol, 4.7us for the rest of them and an extended cyclic prefix for larger cells.

• On the receiving side of the OFDMA system we should expect an FFT operation that will them convert the symbol to the frequency domain again.
• 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. This is useful for LTE since it makes possible to exploit frequency dependence scheduling. For instance, it would be possible to exploit the fact that user 1 might have a better radio link quality on some specific bandwidth area of the available bandwidth.OFDMA is used on the downling, but since it presents a high Peak-to-average Power Ratio it is not possible to use it on the uplink. For the uplink SC-FDMA will be used.
• these are main reasons how OFDM is different from other access.

Advantages over FDM:

OFDM has been used in many high data rate wireless systems because of the many advantages it provides.

• Immunity to selective fading:   One of the main advantages of OFDM is that is more resistant to frequency selective fading than single carrier systems because it divides the overall channel into multiple narrowband signals that are affected individually as flat fading sub-channels.
• Resilience to interference:   Interference appearing on a channel may be bandwidth limited and in this way will not affect all the sub-channels. This means that not all the data is lost.
• Spectrum efficiency:   Using close-spaced overlapping sub-carriers, a significant OFDM advantage is that it makes efficient use of the available spectrum.
• Resilient to ISI:   Another advantage of OFDM is that it is very resilient to inter-symbol and inter-frame interference. This results from the low data rate on each of the sub-channels.
• Resilient to narrow-band effects:   Using adequate channel coding and interleaving it is possible to recover symbols lost due to the frequency selectivity of the channel and narrow band interference. Not all the data is lost.
• Simpler channel equalisation:   One of the issues with CDMA systems was the complexity of the channel equalisation which had to be applied across the whole channel. An advantage of OFDM is that using multiple sub-channels, the channel equalization becomes much simpler.

FDM OVER OFDM:

FDM and OFDM are the multiplexing techniques used mainly in the analog system. These techniques are distinguished depending upon the spacing between the various subchannels (in the form of the composite signal) transmitted through a single channel. So, in FDM the message signals prevent noise by separating the signals with the help of the guard bands. On the contrary, the OFDM technique does not use guard band, in fact, it allows the overlapping of the signals. Thus, enabling the better utilization of the provided bandwidth.Multiplexing is the technique which permits the transmission of the numerous signals through a single channel. There are various types of multiplexing methods such as TDM, FDM, CDM, WDM, OFDM, etcetera.

S.NO	FDM	OFDM
1.	FDM stands for Frequency Division Multiplexing.	OFDM stands for Orthogonal Frequency Division Multiplexing.
2.	In FDM, Guard band is required.	While in OFDM, Guard band is not compulsory.
3.	The spectral efficiency of FDM is low.	While the spectral efficiency of OFDM is high.
4.	The effect of interference in FDM is face down.	The effect of interference in OFDM is trivial.
5.	There is no relationship exist between the carriers in FDM.	There is exist the relationship between the carriers in OFDM.
6.	In FDM, Bandwidth(B) is committed to the different sources.	In OFDM, Single data source attaches all the sub-channels.
7.	FDM supports low data rate.	While OFDM gives higher data rate.
8.	Carriers in FDM are loosely arranged and much aside from each other	Carriers in OFDM are densely arranged and close to each other.
9.	Applications of FDM are in radio and satellite communication.	Applications of OFDM are in LTE technologies and broadband internet.

above chart show the differences between fdm and ofdm .it will help us to understand how OFDM is better than Fdm.