Modulation is a technique in which information is transmitted over a medium. For example, to send voice over long distance, rather than shouting over air, voice can be transmitted using a phone line or radio. So the process of converting your information (like voice) over the medium (like radio waves) is called Modulation.
A carrier or medium is usually represented by a sine wave which you have studied in your physics class. Information to be sent is information signal. Amplitude, phase and frequency are three parameters which can be varied in a sine wave. Once, information is mapped over the carrier, it is not any more sine wave, then it is called signal.
OFDM: Orthogonal Frequency Division Multiplexing is block transmission of N symbols (information) on N orthogonal sine time-limited waves. Each sine wave is called one sub carrier frequency since one modulation symbol is mapped to one waveform per symbol time. In OFDM, each subcarrier is modulated at very low symbol rate as this make symbol much longer than channel impulse response.
Orthogonal mean is perpendicular, at right angles. Here let’s consider a symbol on a particular time on one cycle. Every subcarrier must have an integer number of cycles within the useful symbol time. The interference between the symbols will be zero and also summing up over the entire interval, get zero for a total. The OFDM allows simultaneous transmission on a lot of sub-carriers in a tight frequency space without interference to each other.
Please note that OFDM and OFDMA are different variant of same technology. OFDMA is Orthogonal Frequency Division Multiple Access.
The difference between OFDM and OFDMA is that OFDMA has the ability to dynamically assign a subset of those subcarriers to individual users, attuning the technology to the particular demands of mobility. Thus, OFDM technologies occupy nomadic, fixed and one-way transmission standards, such as TV transmission, Wi-Fi, Fixed WiMAX, and Qualcomm’s Forward Link Only (FLO). OFDMA adds true mobility, forming the backbone of Mobile WiMAX and the 3GPP’s new standards for long-term evolution (LTE). Furthermore, S-OFDMA allows for an increase in range of channel bandwidths from 1.25 MHz up to 20 MHz.
In OFDM, all the channels over the period of time in the downlink are given to a single user. In OFDMA, number of channels can be assigned to different users in downlink to multiple users simultaneously.
OFDMA can give one user, who isn’t doing much, one or two subcarriers, and give another user, who is downloading something big, many more subcarriers. Multiple users can share the subcarriers. OFDM divides into subcarriers. OFDMA shares across multiple users.
Information sent on one subcarrier for a certain amount of time is called Useful Signal Time, or Tu. To calculate data rate, take the number of bits sent on one subcarrier times the number of subcarriers and
divide by the time to transmit one modulation symbol.
To avoid reception problems when receiving multi-path radio signals, each OFDM symbol is extended by a ’cyclic prefix.’ At the transmitter the last part of each symbol is inserted at the start of the same OFDM symbol. At the receiver the data contained in the cyclic prefix of the OFDM symbol is ignored after synchronization. If two signals are received due to multi-path reception then the switch between two consecutive symbols in the delayed signal should occur within the cyclic prefix and this does not cause a problem.
In an OFDM symbol the cyclic prefix is a repeat of the end of the symbol at the beginning. Remember, every subcarrier has an integer number of cycles. The cyclic prefix makes the receiver design easier. The receiver knows it can ignore the cyclic prefix and doesn’t get interference. (And the transmitter isn’t much harder since the transmitter is sending something all the time.)
By making the guard interval larger than the expected multipath delay spread, ISI can be completely eliminated. Adding a guard interval, however, implies power wastage and a decrease in bandwidth efficiency. The amount of power wasted depends on how large a fraction of the OFDM symbol duration the guard time is. Therefore, the larger the symbol period for a given data rate the smaller the loss of power and bandwidth efficiency.