#10:The basic principle for TDD is to use the same frequency band for transmission and reception but to alternate the transmission direction time (UL or DL). Like FDD, TDD supports bandwidths from 1.4MHz up to 20 MHz but depending on the frequency band, the number of supported bandwidths may be less than the full range.
Since the bandwidth is shared between UL and DL and the maximum bandwidth is 20MHz the maximum data rates are lower in TDD than in FDD mode.
The TDD system could be implemented on an unpaired band while the FDD system always requires a pair of bands with some separation between UL and Dl for the duplex separation.
In FDD UE implementation requires a duplex filter for the separation of UL and DL. The filter is not required for the TDD mode. The complexity of the duplex filter is increasing when the UL and DL frequency bands are in close proximity.
In TDD mode since the UL and DL share the same frequency band the signals in these 2 transmission directions can interfere to each other. For uncoordinated deployment (not synchronized) on the same frequency band, the devices connected to cells with different timing and/or different UL/DL allocation may cause blocking of other users. In TDD Mode the base stations need to be synchronized to each other at frame level in the same coverage area to avoid this interference.
In FDD mode there is no need for base station synchronization.
#24:The guard period after each rectangular pulse carrying the modulated data symbol is a simple and efficient method to deal with multi-path reception.
The cyclic prefix (CP) simply consists of the last part of the following symbol. The size of the cyclic prefix field depends on the system and can even vary within one system. Cyclic prefixes are used by all modern OFDM systems and their sizes range from 1/4 to 1/32 of a symbol period. Most receiver structures use the cyclic prefix to make an initial estimation of time and frequency synchronization (pre-FFT synchronization, non-data assisted synchronization).
A receiver typically uses the high correlation between the cyclic prefix and the last part of the following symbol to locate the start of the symbol and begin then with decoding.
In multi-path propagation environments the delayed versions of the signal arrive with a time offset, so that the start of the symbol of the earliest path falls in the cyclic prefixes of the delayed symbols. As the CP is simply a repetition of the end of the symbol this is not an inter-symbol interference and can be easily compensated by the following decoding based on discrete Fourier transform.
Of course cyclic prefixes reduce the number of symbols one can transmit during a time interval. This method to deal with inter-symbol interference from multi-path propagation is theoretically sub-optimal. CDMA with RAKE receiver for instance provides a much better efficiency. On the other hand non-ideal implementations of RAKE receivers also degrade system performance drastically but still require a lot of hardware capacity for the basic implementation. The rectangular pulse with cyclic prefix requires far less hardware, so the free capacity can be used to implement other performance optimization techniques like MIMO.
#34:A typical OFDM transmitter is shown on the following figure. To reduce the amount of RF hardware required for OFDM the modulation process is split into two parts. A first part uses the inverse discrete Fourier transform (IDFT) or one of its more efficient but equivalent implementations known as Inverse Fast Fourier Transform to modulate all the OFDM subcarriers in the baseband around the center frequency 0. In the second step the signal is then modulated to higher frequencies for transmission over air.
The binary data sequence is put into the bit distribution where each bit is assigned to a subcarrier. This function is highly specific to the system using OFDM. In EUTRAN for instance the scheduler has great influence to this step. For each subcarrier a modulation mapper takes a number of bits from the assigned stream and maps them to a single complex valued data symbol. How many bits will be mapped in one symbol period depends on the selected modulation scheme (e.g. 1 bit of OOK, BPSK; 2 bits for QPSK, 4 bits for 16QAM and 6 bits for 64QAM). Note that each subcarrier can use a different modulation scheme at the same time.
Then the complex valued data symbols from the modulation mappers are interpreted as frequency domain signal for one symbol period. They are fed into the IFFT algorithm which transforms the frequency domain vector into the corresponding time sequence. The number of time symbols (also complex of course) is typically equal to number of carriers. Note also that some subcarriers before the IFFT step begins might be inserted without data symbol (so called virtual subcarriers). They are usually used as guard bands to protect from interference of adjacent radio systems.
The time sequence of complex valued samples is next brought to the OFDM symbol generator, which inserts cyclic prefix and if required cyclic suffix. This is simply done be taking some bits from the end of the symbol and placing them as cyclic prefix in front of the symbol. Similar is the mechanism for cyclic suffixes. This step is equivalent to the insertion of cyclic prefix and suffix for each subcarrier, but it requires lower number of arithmetical operations.
Optionally an up-conversion unit can increase the sampling rate now before we go to the DAC. The up-conversion can be used to reduce the amount of hardware required for the anti-aliasing filter after the DAC which translates the signal into an analog waveform such that the digital sampling values before corresponds to voltage or current afterwards. Because a DAC generates a signal that contains the original spectrum again in mirrored versions in higher bands, a low pass (anti-aliasing filter) filter is required to suppress the unwanted spectrum.
The last step is to modulate the signal onto the radio carrier. This is done using a classical I/Q modulator where the real part of the complex samples goes to the cosine and the imaginary part of the complex samples goes on the sine of the carrier frequency. Then we fed the signal to some spectral filter (to suppress out-of-band emissions) and to the RF amplifier.
#36:The receiver is like in any other radio system the more complicated part. In radio systems and of course also OFDM there are two special points a receiver has to pay attention to: time/phase and frequency synchronization. Both are crucial for the performance of the receiver.
A receiver gets its input from the antenna (or antennas) and the attached low noise amplifier. A band pass suppresses signals out of the spectrum. The demodulator converts the signal back into the baseband and with this recovers the complex valued data signal. At this step we have the time domain representation of the signal.
The time signal is now given to the “De-rotator” which applies to each time sample a phase offset to compensate frequency drifts and global phase offsets. A special unit in the receiver is responsible to determine and track the frequency and phase drifts and calculate the associated correction value for each sample. This is a quite critical task, as errors made here, apply as additional (receiver intrinsic) noise to all data symbols. The frequency and time synchronization unit uses typically as input the autocorrelation of the input time sequence (especially cyclic prefix) and reference (or pilot) symbol interleaved with the data at predefined positions.
The corrected signal is now fed into the Fast Fourier Transform (FFT) which implements a fast and efficient algorithm for the discrete Fourier transform to bring the signal back into the frequency domain representation. In other words the FFT decodes the complex valued data symbols for each subcarrier. Of course before the FFT is applied, the cyclic prefix has to be removed.
The recovered subcarrier data symbols are not useful yet, as there might be still distortion from phase offsets and from the channel propagation (multi-path propagation) on it. Thus the next step is to correct the data according to the known channel response. The channel estimation uses the pilot and reference signals that are interleaved with the normal data at predefined positions to estimate and permanently correct the channel state information. A nice thing of the frequency domain representation is, that a distortion coming from channel propagation and time offset are in first order simple correction factors to each subcarrier, so that no complex filtering is required here.
After we have corrected our data symbols for each subcarrier, the symbol de-mapping can take place. Here we recover the original bit sequence either as hard decided bits or as soft decided bits. (Soft bits have some advantages in the further processing, namely in the channel decoding.)
#40:4. Fast Fourier Transform Size – Nfft
The FFT/ IFFT (Inverse Fast Fourier Transform) it is used for the generation of the subcarriers.
Input for the FFT/ IFFT are the modulation symbols.
FFT/ IFFT could be seen as a kind of operation acting on a Nfft discrete points of the input signal
Therefore the terminology is naming the FFT/ IFFT sampling.
Nfft size:
→ The number of samples Nfft on which FFT/ IFFT is applied should be big enough to satisfy the sampling theorem (giving the minimum number of samples)
From this: Nfft > Nc number of the input subcarriers
→ FFT/IFFT operation requires that input length must be a power of 2. This is because in this way the operation is much faster than ordinary DFT (Discrete Fourier Transform).
Example:
For a bandwidth BW = 20 MHz there are 1200 subcarriers -> the length of the IFFT input is a signal with 1200 symbols
1200 is not a power of 2 so that the IFFT operation requires zero padding-> Next power of 2 is 2048
The rest of input: 2048 - 1200 = 848 will padded with zeros
#54:Normally there are 2 steps for the QPSK modulation:
Step 1:
-> map the input bits to the symbols in the complex space I + jQ (complex = inphase + quadrature). At the end we have +1 and -1 symbols to be transmitted
Step 2:
-> Inphase modulates a cos (2*pi*f0*t) and
quadrature modulates a sin(2*pi*f0*t)
where f0 is the carrier frequency
As a result we have the inphase and quadrature with a series of + 1 and -1
However:
Step 2 (modulation of sin and cos waves) is not happening in classical way
This is because the multiplication of sinus and cosinus of 1200 inphase and 1200 quadrature is time consuming
->Instead in LTE the phase 2 is implemented through an IFFT operation
#56:IFFT Input:
The input of the IFFT is the complex signal I+jQ which we get in the previous step.
The IFFT input should be in frequency domain -> so we look at the signal as it is having a frequency domain representation!
IFFT Parameters:
FFT/IFFT operation requires that input length must be a power of 2. This is because in this way the operation is much faster than ordinary DFT.
The length of the IFFT input is a signal with 1200 symbols.
But 1200 is not a power of 2 so that the IFFT operation requires zero padding: Next power of 2 is 2048.
Thus the padded data is 2048 - 1200 = 848 zeros. The padding zeros just in the middle of the data set. This is required because of the guard band subcarriers
What is actually IFFT doing? -> The actual modulation:
Inphase symbols are multipled with:
cos (2*pi*(f0+n)*t) where n=0,…,N-1 with N=length IFFT=2048. In LTE f0 = 1/Tu=1/66.67μs=15kHz
Quadrature symbols are multiplied with:
sin (2*pi*(f0+n)*t) where n=0,…,N-1.
-> IFFT provides orthogonal and harmonics functions which are modulated by each of the QPSK symbols
#59:The digital to analog conversion simulation is not shown in detail here. This is because the process is not LTE specific.
We note however that 2 steps are required for the digital to analog conversion:
Step 1:
Convolution of the IFFT result in time domain with one pulse shape filter
Step 2:
Low pass filtering – Why needed?
The frequency response of the filtering in the first step is periodic as required by the frequency response of a discrete-time system.
Rectangular pulses causes multiple harmonics above the Nyquist frequency.These harmonics have to be removed from the spectrum. This is done with the help of the low pass filtering.
#61:Target:
Send the signal on the high carrier
We choose for e.g. fc = 2150 MHz (refarming of carrier from UMTS)
There is actually a quadrature double-sideband amplitude modulation:
![25 © Nokia Siemens Networks
Cyclic Prefix
T [TS] 160 2048 144 2048 144 2048 144 2048 144 2048 144 2048 144 2048
T [µs] 5,2 66,7 4,7 66,7 4,7 66,7 4,7 66,7 4,7 66,7 4,7 66,7 4,7 66,7
max. delay [km] 1,6 1,4 1,4 1,4 1,4 1,4 1,4
T [TS] 512 2048 512 2048 512 2048 512 2048 512 2048 512 2048
T [µs] 16,7 66,7 16,7 66,7 16,7 66,7 16,7 66,7 16,7 66,7 16,7 66,7
max. delay [km] 5,0 5,0 5,0 5,0 5,0 5,0
In LTE the slot of 500 µs is subdivided in the (useful part of the) symbol (grey)
and CPs as follows:
For the extended CP slot structure the overall 500 µs is kept but the number
of symbols is reduced in order to extent the cyclic prefix durations:](https://image.slidesharecdn.com/03ofdma-250406110627-a67f6191/85/OFDMA-ppt-OFDMA-ppt-OFDMA-ppt-OFDMA-OFDMA-ppt-24-320.jpg)
![39 © Nokia Siemens Networks
OFDM Key Parameters
3. The number of Subcarriers Nc
→ Nc x Δf = BW
In LTE not all the available channel bandwidth (e.g. 20 MHz) will be used. For the transmission
bandwidth typically 10% guard band is considered (to avoid the out band emissions).
If BW = 20MHz → Transmission BW = 20MHz – 2MHz = 18 MHz
→ the number of subcarriers Nc = 18MHz/15KHz = 1200 subcarriers
Transmission
Bandwidth [RB]
Transmission Bandwidth Configuration [RB]
Channel Bandwidth [MHz]
Resource
block
Channel
edge
Channel
edge
DC carrier (downlink only)
Active Resource Blocks](https://image.slidesharecdn.com/03ofdma-250406110627-a67f6191/85/OFDMA-ppt-OFDMA-ppt-OFDMA-ppt-OFDMA-OFDMA-ppt-38-320.jpg)
