How data is transmitted ?
In LTE data is transmitted using OFDMA in the downlink and SCFDMA in the uplink .
High power is consumed in an OFDM signal generation which is feasible for the eNode B but communication from the user equipment has power constraints so SCFDMA is used for signal generation from UE to eNodeB.
Its a special case of FDMA where users are provided a set of sub carriers overlapping in frequency domain.These subcarriers are designed to be orthogonal to each other which allows them to occupy the same bandwidth without any interference ,this inturn negates the use of guard bands as a result the subcarriers can be closely packed to improve channel efficiency
How it works?
In OFDM high-speed data streams of large bandwidth are split into parallel sub streams of lower bandwidth called sub carriers these sub carriers are centered around frequencies in multiples of 15 kHz on both sides of DC as the lowest subcarrier is of 15 kHz
TS =1/ 15 kHz or 66.7 microseconds
for example LTE bandwidth of 20 megahertz which has 1200 sub carriers
Subcarrier bandwidth is of 18 MHz because approximately 2 MHz is used as guard band here we have 600 sub carriers on both side of the DC frequency all these carrier frequencies are harmonics of 15kilo Hertz varying from minus 9 megaHertz to 9 mega Hertz in time domain these sub carriers will be represented as everlasting sinusoids at these carrier frequencies as shown below
The symbol duration for each of these subcarriers is always equal to 66.7 microseconds which means that all these sub carriers have a whole number of cycles in one symbol duration
Figure below shows the subcarriers near the DC subcarrier
Figure above shows all these sub carriers have a whole number of cycles in one symbol duration
As we know that a rectangular function can be represented in frequency domain as a sinc function which is centered around DC when multiplying a signal to a carrier frequency and time domain signal will be shifted in frequency domain by the same amount of carrier frequency thus we can represent these modulated sub carriers in frequency domain as a series of sync waves centered around the carrier frequencies.
Data Transmission in the DL
Data is transmitted in OFDMA over parallel subcarriers of 15kHz.The subcarriers are further divided on the time axis as blocks of 1 symbol duration or 66.7 microseconds.This basic unit is known as resource element.Each resource element carries one symbol.Now in order to transmit data over these resource elements first data is modulated with specific modulation scheme this scheme depends on the physical channels mapped on the resource grid.
Suppose we have 8 bits of data to be transmitted using QPSK modulation the data will be divided into four parallel streams of 2 bits.Phase and amplitude assignment is done according to the QPSK constellation
Finally data is placed over the resource elements by adjusting the phase and amplitude of subcarrier to those derived for the data stream.Mathematically it means multiplying the complex modulation symbol to the corresponding subcarrier frequency
Thus in a 20 megahertz channel all the 1200 resource elements carrying 1200 symbols over 1200 sub carriers are modulated with appropriate modulation scheme.Now since the data is modulated over 1200 sub carriers a transmitter will require 1200 oscillators for its generation and another 1200 will be required by the receiver for proper demodulation The hardware complexity and sheer amount of power consumption would have left OFDM to theoretical idea and far from implementation
OFDM was made reality with the advent of digital signal processing techniques.So instead of using 1200 oscillators IFFT solves this problem by converting the parallel frequency domain signals into samples of a composite time domain signal which are much easier to generate at the transmitter side.
All we need to do is to send these time domain samples at radio frequencies.As the data is being sampled by IFFT samples,it must be taken above the Nyquist rate for faithful reproduction at the receiver .For an LTE spectrum of 20 megahertz the highest frequency component is 9 megahertz which means the sampling rate should be greater than 18mega samples per second.
The FFT size of the IFFT processor thus depends on the LTE bandwidth as shown above.
Guard Period
Abrupt transition in time domain generate high frequency components and disturb the orthogonality in frequency domain which introduces inter carrier interference
Figure above shows a Time Domain Representation
Figure above shows a Frequency Domain Representation
So in OFDMA we employ a more complex kind of guard period called cyclic prefix in which
end part of a symbol is transmitted in the preceding guard period. It also ensures orthogonality between the subcarriers by keeping the OFDM symbol periodic over the extended symbol duration and therefore avoiding intercarrier and inter channel interference simultaneously.Since OFDMA uses composite IFFT samples,cyclic prefix is added by taking some samples from the end of a symbol period and placing them at the beginning.
Now this time sampled signal is converted into analog wave by a digital to analog converter further composite waveform is modulated at the desired radio frequency for transmission
for an example an operator having license for 2320-2340MHz band the 18 MHz band will be mapped from 2321-2339 MHz using analog modulation and finally the EnodeB transmits the signal over air .Enode B informs the user about the allocated subcarriers and the corresponding modulation scheme so when the RF signal reaches at the user's terminal all these processes are reversed and finally user is able to receive the data intended for it
Data Transmission in the UL
Although there are many positives in OFDMA but IFFT summation of multiple
parallel self carrier results in high peak to average power ratio(PAPR).High PAPR results in high power consumption for signal generation but handheld devices have limited power capacity.This makes OFDMA unfavorable for uplink transmission.In order to overcome this LTE uses SCFDMA in uplink direction.In OFDMA we have one-to-one mapping between symbol and subcarrier but as SCFDMA allows a symbol to be transmitted in parts over multiple sub carriers.For example in OFDMA one symbol occupies one sub carrier of 15 kilo Hertz but in FDMA same symbol is distributed among multiple sub carriers of 15 kilo Hertz
In short SCFDMA behaves like a single carrier system with short symbol duration compared to OFDMA.To achieve this SCFDMA introduces an N point FFT block right after the serial to parallel converter in the OFDMA structure the FFT block converts parallel sequence of symbols in time domain to different frequency points
Now peak to average power ratio is proportional to the square of number of carriers involved SCFDMA reduces PAPR by reducing the number of carriers .The side effects are reduced by ensuring that the total bandwidth over which a symbol is transmitted is still not too high apart from this remaining block in the uplink direction is same as used in downlink
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