Monday, December 21, 2009

OFDM - Accelerating data rates

Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data. Orthogonal FDM (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. It is identical to Coded FDM and the Discrete Multitone (DMT) modulation.

Orthogonality
In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver. A separate requirement for different filters is thus eliminated. This results in high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. But this also means high accuracies in synchronization between transmitter and receiver is required.

OFDM exhibits lower multi-path distortion (delay spread), since the sub-signals are sent at lower data rates. Because of the lower data rate transmissions, multi-path-based delays are not nearly as significant as they would be with a single-channel high-rate system. For example, a narrow band signal sent at a high rate over a single channel will likely experience greater negative effects from delay spread because the transmitted symbols are closer together. In fact, the information content of a narrow band signal can be completely lost at the receiver if the multi path distortion causes the frequency response to have a null at the transmission frequency. The use of the multi-carrier OFDM significantly reduces this problem.

Simple Implementation
The orthogonality allows for efficient modulator and demodulator implementation using the FFT algorithm on the receiver side, and inverse FFT on the sender side. Although the principles and some of the benefits have been known since the 1960s, OFDM is popular for wideband communications today by way of low-cost digital signal processing components that can efficiently calculate the FFT.

Elimination of intersymbol interference
One key principle of OFDM is that since low symbol rate modulation schemes i.e. where the symbols are relatively long compared to the channel time characteristics suffer less from inter symbol interference caused by multi path propagation, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. Since the duration of each symbol is long, it is feasible to insert a guard interval between the OFDM symbols, thus eliminating the inter symbol interference. The cyclic prefix, which is transmitted during the guard interval, consists of the end of the OFDM symbol copied into the guard interval, and the guard interval is transmitted followed by the OFDM symbol. The reason that the guard interval consists of a copy of the end of the OFDM symbol is so that the receiver will integrate over an integer number of sinusoid cycles for each of the multi paths when it performs OFDM demodulation with the FFT.

Simplified equalization
The effects of frequency-selective channel conditions, for example fading caused by multipath propagation, can be considered as constant (flat) over an OFDM sub-channel if the sub-channel is sufficiently narrow-banded, i.e. if the number of sub-channels is sufficiently large. This makes equalization far simpler at the receiver in OFDM in comparison to conventional single-carrier modulation. The equalizer only has to multiply each detected sub-carrier (each Fourier coefficient) by a constant complex number, or a rarely changed value.

Importance of channel coding
Channel coding is used in most cases of digital communication and especially in case of mobile communication. Channel coding implies that each bit of information to be transmitted is spread over several, often very many, code bits. If these coded bits are then, via modulation symbols, mapped to a set of OFDM subcarriers that are well distributed over the overall transmission bandwidth of the OFDM signal, each information bit will experience frequency diversity in case of transmission over a radio channel that is frequency selective over the transmission bandwidth, despite the fact that the subcarriers, and thus also the code bits, will not experience any frequency diversity. Thus, in contrast to the transmission of a single wideband carrier, channel coding (combined with frequency interleaving) is an essential component in order for OFDM transmission to be able to benefit from frequency diversity on a frequency-selective channel.

OFDM for Access control
OFDM can also be used as a user-multiplexing or multiple-access scheme, allowing for simultaneous frequency-separated transmissions to/from multiple mobile terminals. In the downlink direction, OFDM as a user-multiplexing scheme implies that, in each OFDM symbol interval, different subsets of the overall set of available subcarriers are used for transmission to different mobile terminals. Similarly, in the uplink direction, OFDM as a user-multiplexing or multiple access scheme implies that, in each OFDM symbol interval, different subsets of the overall set of subcarriers are used for data transmission from different mobile terminals.

Issues
A drawback of OFDM modulation, as well as any kind of multi-carrier transmission, is the large variations in the instantaneous power of the transmitted signal. Such power variations imply a
reduced power-amplifier efficiency and higher power-amplifier cost. This is especially critical for the uplink, due to the high importance of low mobile-terminal power consumption and cost. Several methods have been proposed on how to reducethe large power variations of an OFDM signal. However, most of these methods have limitations in terms of to what extent the power variations can be reduced. Furthermore, most of the methods also imply a significant computational complexity and/or a reduced link performance.

10 comments:

Anonymous said...

Good post and this mail helped me alot in my college assignement. Thank you for your information.

Anonymous said...

Well I agree but I think the brief should secure more info then it has.

Anonymous said...

Ah, great! This settled up some contradictions I've heard.

Anonymous said...

Genial brief and this mail helped me alot in my college assignement. Thanks you seeking your information.

Anonymous said...

Ah! I'm so glad this was poste| There is tons of different
information out there, this dispells, puts to rest
some of what I've read.

Anonymous said...

Nice fill someone in on and this mail helped me alot in my college assignement. Thanks you for your information.

Anonymous said...

Hello i am new on here. I came accross this site I have found It exceedingly accommodating & its helped me out a lot. I hope to give something back and aid other people like it has helped me.

Thanks Everyone, See You About.

Anonymous said...

Whats's Up im fresh here. I came accross this website I have found It absolutely useful and it's helped me out a great deal. I should be able to give something back and aid other people like it has helped me.

Thank You, Catch You Around.

Anonymous said...

Whats's Up i'm new to this, I stumbled upon this message board I have found It amply accessible & its helped me alot. I hope to give something back & guide others like its helped me.

Cheers, Catch You Later

Anonymous said...

Generally I do not post on blogs, but I would love to mention that this post extremely forced me to try and do therefore! really nice post. thanks, here's a similar subject you'll like [url=http://corina33.blogdrive.com/archive/1.html]Vintage stuff[/url]