How does ofdm work

To subscribe to our mailing list for our online platform where you can learn all this and more for example all the details on OFDM transmission visit GrandmetricWatch. Marcin Dryjanski received his Ph. You can reach Marcin at marcin. Your email address will not be published.

Sign up to our newsletter! Save my name, email, and website in this browser for the next time I comment. Toggle navigation. How does OFDM work? Author: Marcin Dryjanski, Ph. Inter-Symbol Interference Lets look at the example of the actual impairments see figure below.

It, simply speaking, combines all requirements so that we get: long symbols — decreasing ISI, narrow BWs — avoiding frequency selectivity, high throughput. Other materials To see other posts on network and wireless fundamentals — for example about pathloss , shadowing or MIMO — see our explained section. More Posts. Explained: Multiple Access Methods - Grandmetric.

So here it is. Therefore, it seems imperative to have a signal level understanding of how OFDM works. We start with a short introduction to a wireless channel. In Figure 1, the direct path to the hiker is shown by the bold line. We will use these numbers in our examples below. In the discussion that follows, we avoid technical terms such as delay spread, frequency-selective fading, frequency flat fading and so on. Also, we ignore the RF carrier in the subsequent discussion to highlight the important concepts relevant to the current discussion.

Finally, we will consider binary modulation to avoid using the term symbols and stick to bits instead. Digital modulation is concerned with mapping of the bits 1s and 0s to a property of the signal suitable for transmission.

For example, consider a rectangular pulse shape. That translates to a bit time also known as bit duration of. This is shown in Figure 4. Since nature adds the signals at the antenna, the Rx will have a summation of these three paths, effectively the same signal delayed by different amounts but with different attenuation and phase shifts of the carrier waves not drawn in the figure.

Those phase shifts depend on the path delay and the carrier frequency. That is to say that bit 1 through its last path will interfere slightly with bit 2 to a little extent but not with any other bit farther than that. This is a situation that can be handled without much effort in terms of computational resources. We claim that the wireless channel does not pose a significant problem to Rx processing time in this low data rate scenario.

It is not necessary to understand this in the context of this article but the disadvantage here is that in case of destructive interference, there is no other way to recover except providing diversity to the Tx signal. Diversity is a signal replica in some form whether in time, frequency, space, etc.

Fast forward to a decade, say early s. Escaping the fast pace of life, our hiker wants to sit on top of that peaceful mountain In fact, he considers having access to it anywhere as a basic necessity of life like water and electricity. The main point is that the environment is still the same and does not care about our data rates! This is illustrated in Figure 6 again ignoring the carrier. The different path lengths will translate into different attenuations and phase shifts resulting in constructive and destructive interference throughout the signal span.

What does this imply for the high rate transmission? Notice that in this case, the initial bits of the transmission are interfering with many tens of bits in the future through the late arriving paths, a phenomenon known as Inter-Symbol Interference ISI. In most cases of interest, this ISI could have been observed even extending to hundreds or even thousands of bits. We can conclude that the same harmless channel for low rate communication has become harsh for high rate communication!

It turns out that a solution for this kind of problem was devised by Robert Lucky at Bell Labs in an adaptive equalizer. Its input is the distorted waveform sum of the original signal and its multi-path components and the output is the clean desired bit stream as shown in Figure 7.

Figure 7: Equalizer input is a distorted waveform and its output is a clean bit stream. Don't think of it as a physical device. Just like everything physical got transformed into digital logic in the history of communications leading to software defined radios , an equalizer sits as mere lines of code in a microprocessor. An equalizer has its advantages and disadvantages. Needless to say, it considerably improves the bit error rate and consequently fundamental to making the system functional.

Technically, it exploits the frequency diversity available in a broad spectrum. On the other hand, for a high rate system, it is the most demanding and resource intensive component of a conventional wireless Rx.

In high speed communication, it is impractical to be busy processing a bit in a certain window of time while many future bits are arriving at the Rx. That will result in filling the buffer faster than being emptied.

We can conclude that a low data rate communication requires a relatively simple Rx processor while high rate communication requires a heavy duty Rx processor. Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another.

This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of demodulators, translating each carrier down to DC. The resulting signal is integrated over the symbol period to regenerate the data from that carrier. The same demodulator also demodulates the other carriers. As the carrier spacing equal to the reciprocal of the symbol period means that they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution.

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission. In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency.

In some systems the peaks are limited. Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them. The traditional format for sending data over a radio channel is to send it serially, one bit after another. This relies on a single channel and any interference on that single frequency can disrupt the whole transmission. OFDM adopts a different approach. The data is transmitted in parallel across the various carriers within the overall OFDM signal.

Being split into a number of parallel "substreams" the overall data rate is that of the original stream, but that of each of the substreams is much lower, and the symbols are spaced further apart in time. This reduces interference among symbols and makes it easier to receive each symbol accurately while maintaining the same throughput. The lower data rate in each stream means that the interference from reflections is much less critical.

This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal. This can be achieved far more effectively within a low data rate substream. The distribution of the data across a large number of carriers in the OFDM signal has some further advantages.