LTE: What is OFDM transmission and how it differs from other mechanism.

Question

What is the duration of a sub-frame and a frame in LTE? How many OFDM symbols per sub-frame? What is the transmit time interval (TTI) over which an allocation happens in the DL or UL?

Answer 1

1.OFDM is a multi carrier system as signal representing each bit uses all of the available spectrum in case of single carrier but in case of OFDM the spectrum divided into many narrow bands anf data transferred in separate bands.
2.Here it uses orthogonal principle so that the bands in which data transferred are interference free as the chooses frequency are orthogonal to each other.
3.The main problem solved by OFDM is Inter Symbol interference,which was happening due to multipath propagation.
4.As ISI became less so less no of equalizers required at receiver side and to avoid also OFDM uses Cyclic prefix.
5.But one disadvantage of OFDM is high Peak to Average Power Ratio (PAPR) because the single OFDM symbol is the sum of many separate carriers.

Answer 2

Bandwidth requirement is proportional to symbol rate. That is if we need to transmit data at high rate, we need to have a very large bandwidth.

Now modulating a signal with high bandwidth over a carrier has certain undesirable property. The symbols have a very small time period (of course if you want to increase data rate, you need to send more symbols in a given time and hence each symbol would be shorter in length).

And one may ask how this will harm. The answer is due to presence of various paths between transmitter and receiver, there will be multipaths. Each of these multipath would be replica of original signal, attenuated by some factor and distorted by channel. When at receiver side you receive all these multipath components together , because of small symbol period, there will be interference between symbols of one multipath with other. The last multipath to be received determines the delay spread of the channel, i.e. how much delay the channel induces in the signal so that you receive the signal late (due to multipath effect). Number of symbols affected is proportional to delay spread and inversely proportional to symbol period. More the delay spread , more likely the interference between symbols and larger the symbol period less likely; and vice versa.

We can solve this problem by splitting our data and sending different portion of our data on different narrowband carriers (Narrowband means larger symbol period and hence less chances of delay spread causing inter symbol interference). Problem is we may sacrifice bandwidth as narrowband carriers should have some guard band between them in order to help in designing realistic bandpass filters which can filter out each of these carriers for demodulation purpose.

The most efficient way to pack the narrowband carriers was found out to be when we have all the carriers as integral multiples of a fundamental frequency f. The spectra overlap each other , but the sampling instant of one carrier coincide with nulls of all other carriers. Hence this arrangement is called orthogonal, as there is no component of other carriers at sampling instant of one carrier. Since data is again split over carriers , i.e, frequency division multiplexing happens this entire arrangement is called OFDM or Orthogonal Frequency division multiplexing. OFDM has the best efficiency in arrangement of multicarriers in frequency domain and also since each sub carrier is narrowband, we also have large symbol periods. Moreover when we have multiple sub carriers, each subcarrier can "carry" a symbol of its own. So one can increase the symbol rate as much one wants by just increasing the number of subcarriers, while enjoying narrowband properties.

Answer 3

OFDM (Orthogonal Frequency Division Multiplexing) uses multiple carriers that are mutually orthogonal; OFDM includes multiplexing in the phase domain as well as the frequency domain. OFDM uses multiple carriers that are mutually orthogonal; OFDM includes multiplexing in the phase domain as well as the frequency domain. Likewise, OFDM is a means to divide up a single user’s message for conveying it over a set of orthogonal frequencies. Its multiple access methods (OFDMA) are used in latest wireless technologies including LTE and LTE-A (fourth generation of mobile wireless networks), WiFi (IEEE 802.11), WiMAX (IEEE 802.16) as well as in in digital subscriber lines (DSL).

Since the subcarriers in OFDM do not interfere with each other due to their orthogonality, the spectrum of each tone is allowed to be overlapped, thus the amount of required spectrum for OFDM compared to conventional FDM. OFDM used in conjunction with PSK or QAM modulation techniques overcomes the limitations of FDM; particularly, OFDM does not require the use of expensive bandpass filters required in conventional FDM systems. Nonetheless, OFDM requires strict frequency synchronization.

Given that the total channel bandwidth W is divided up into K slots, each one having a subcarrier frequency of Fk with spacing given that the total channel bandwidth W is divided up into K slots, each one having a subcarrier frequency of Fk with spacing ∆F=F(k+1)+Fk (k should be read as a subscript here)
then Fk=k(W/K). Likewise, assume symbol rate to be r=1 / T = ∆F, where T is the symbol duration such that the modulated subcarriers will be mutually orthogonal, or

∫cos(2π.Fk.t).cos(2π.Fj.t).dt = 0 ∇j,k; j≠k (integrated from 0 to T, again, j and k should be read as subscripts).

Thus, with orthogonal frequency division multiplexing (OFDM) we have a method of sending K symbols concurrently without interference. In other words, we take a given message that consists of a set of frames where each frame consists of K symbols, so that, instead of sending the entire frame over the channel at a bandwidth and rate of Rs = 1 = Ts = K/T = W, each symbol is sent over a separate subcarrier frequency Fk and thus send K symbols in parallel over the channel at the slower rate of R = Rs/K Hz, so then, symbol duration becomes T=KTs (s is a subscript here as k and j before).

An important consequence of this lower data rate is the corresponding reduction in ISI (InterSymbol Interference), and multipath effects like it happens in 3GPP’s UMTS WCDMA (Wideband Code Division Multiple Access), where multipath interference is a big issue thus forcing implementation of complex use of multiple antenna configuration called “rake receivers” where the output of each of the incoming signal multipliers is then fed to a diversity combiner that consists of a set of adjustable delay and gain elements so that the outputs add constructively, since the signal from each finger will have identical delays. The signal from each path is scaled so as to increase the level of signals from the fingers with high signal-to-noise ratios and reduce the level of those with low signal-to-noise ratios. OFDMA in conjunction with a matrix an antennas named MIMO (Multiple Input Multiple Output) is much more efficient than WCDMA and Rake Receivers, thus used in LTE in favour of CDMA methods.