Carrier and Bandwidth
TDD = Time Division Duplex
-The Uplink and the Downlink transmissions are separated by the time.
-Only one bandwidth is used.
-Example: WiMAX
FDD = Frequency Division Duplex
-The Uplink and the Downlink transmissions are separated by the frequency.
-2 bandwidths are used.
-Example: WCDMA, CDMA2000
# The 3GPP specifies that the 3G LTE can be deployed in the existing IMT- 2000 frequency bands.
- IMT-2000 bands: from 450 MHz to 2.6 GHz
- Including WCDMA/HSPA, CDMA2000/EV-DO, and GSM bands
The bandwidth is more flexible than in the previous 3GPP standards.
-Scalable from 1.4, 3, 5, 10, 15, 20MHz
-The capacity of a cell depends strongly on its allocated bandwidth.
The FDD frequency bands are paired to allow simultaneous transmission on two frequencies. The bands also have a sufficient separation to enable the transmitted signals not to unduly impair the receiver performance.
If the signals are too close then the receiver may be "blocked" and the sensitivity impaired. The separation must be sufficient to enable the roll-off of the antenna filtering to give sufficient attenuation of the transmitted signal within the receive band.
With the interest in TDD LTE, there are several unpaired frequency allocations that are being prepared for LTR TDD use. The TDD LTE allocations are unpaired because the uplink and downlink share the same frequency, being time multiplexed.
# e-UTRAN is designed to operate in the frequency bands defined in the
following table:
Spectral efficiency is increased by up to four-fold compared with UTRA, and improvements in architecture and signalling reduce round-trip latency. Multiple Input / Multiple Output (MIMO) antenna technology should enable 10 times as many users per cell as 3GPP’s original W CDMA radio access technology. To suit as many frequency band allocation arrangements as possible, both paired (FDD) and unpaired (TDD) band operation is supported. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP’s previous radio access technologies
# The supported carrier depends on the eNodeB hardware
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OFDMA Principles
There are several ways to transmit over the frequency band and to share the resource between several devices.
Notion of Orthogonality
In FDM, the sub-carriers are separated in the frequency domain to avoid interference between the sub-channels
It results in a loss of spectrum efficiency because the frequency guard
band can not be used to send data.
-The OFDM allows one to remove the frequency guard band.
Benefit: There are more sub-carriers, so more symbols are sent at the same time. The orthogonality brings a better spectrum efficiency.
-In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other,
meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not
required. This greatly simplifies the design of both the transmitterand the receiver; unlike conventional FDM a separate filter for each sub-channel is not required.
-The orthogonality requires that the sub-carrier spacing is Vf = k/(TU) Hertz, where TU seconds is the useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1. Therefore, with N sub-carriers, the total passband bandwidth will be B ≈ N—Vf (Hz).
# The duration of the symbol depends on the width of the sub-carrier.
- It is inversely proportional. The shorter the symbol, the wider the sub-carrier and vice-versa.
- The frequency center of the sub-carrier is linked to the frequency of the carrier.
# The inter-channel (or inter sub-carrier) interferences are cancelled because they are located in a such way that when there is the peak for a given sub-carrier, the adjacent subcarriers are null.
#OFDM allows high density of carriers, without generating Inter-Channel
Interference (ICI).
BASIC IDEA : The channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-selective fading.
-A single wideband signal is transformed into multiple narrow band signals transmitted on orthogonal subcarriers
- One single stream at high rate
- Each symbol occupies the whole bandwidth
- Very short symbol duration to ensure high rate
#Due to the signal propagation phenomena, like reflection or diffraction, a
receiver can receive several delayed versions of the same signal.
-This creates Inter-Symbol Interference (ISI).
The multi-path impact is an overlapping of 2 symbols, called Inter-Symbol Interference (ISI).
The modulation is based on the amplitude and on the phase, so in case of overlapping there are 2 different amplitudes and phases.
The receiver is not able to decode the state of the symbol
The problem is fixed by adding a guard time between each symbol to avoid the ISI.
-The ISI is still present but is not disturbing for the receiver.
Principle : add a prefix to absorb channel effect and avoid ISI
-Cyclic prefix permits to facilitate demodulation
- The cyclic prefix transform the classical channel convolution into a cyclic convolution which permits easy demodulation after FFT
# The guard time is called the Cyclic Prefix (CP). It permits to facilitate demodulation.
The cyclic prefix transforms the classical channel convolution into a cyclic convolution which permits easy demodulation after FFT.
In FDM, the sub-carriers are separated in the frequency domain to avoid interference between the sub-channels
band can not be used to send data.
-The OFDM allows one to remove the frequency guard band.
Benefit: There are more sub-carriers, so more symbols are sent at the same time. The orthogonality brings a better spectrum efficiency.
-In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other,
meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not
required. This greatly simplifies the design of both the transmitterand the receiver; unlike conventional FDM a separate filter for each sub-channel is not required.
-The orthogonality requires that the sub-carrier spacing is Vf = k/(TU) Hertz, where TU seconds is the useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1. Therefore, with N sub-carriers, the total passband bandwidth will be B ≈ N—Vf (Hz).
- It is inversely proportional. The shorter the symbol, the wider the sub-carrier and vice-versa.
- The frequency center of the sub-carrier is linked to the frequency of the carrier.
# The inter-channel (or inter sub-carrier) interferences are cancelled because they are located in a such way that when there is the peak for a given sub-carrier, the adjacent subcarriers are null.
Interference (ICI).
BASIC IDEA : The channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-selective fading.
-A single wideband signal is transformed into multiple narrow band signals transmitted on orthogonal subcarriers
- One single stream at high rate
- Each symbol occupies the whole bandwidth
- Very short symbol duration to ensure high rate
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What is the multipath?#Due to the signal propagation phenomena, like reflection or diffraction, a
receiver can receive several delayed versions of the same signal.
-This creates Inter-Symbol Interference (ISI).
The modulation is based on the amplitude and on the phase, so in case of overlapping there are 2 different amplitudes and phases.
The receiver is not able to decode the state of the symbol
The problem is fixed by adding a guard time between each symbol to avoid the ISI.
-The ISI is still present but is not disturbing for the receiver.
-Cyclic prefix permits to facilitate demodulation
- The cyclic prefix transform the classical channel convolution into a cyclic convolution which permits easy demodulation after FFT
# The guard time is called the Cyclic Prefix (CP). It permits to facilitate demodulation.
The cyclic prefix transforms the classical channel convolution into a cyclic convolution which permits easy demodulation after FFT.
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OFDMA Transmitter
In the downlink, OFDM is selected to efficiently meet E-UTRA performance requirements. With OFDM, it is straightforward to exploit frequency selectivity of the multi-path channel with lowcomplexity receivers. This allows frequency selective in addition to frequency diverse scheduling and one cell reuse of available bandwidth.
Furthermore, due to its frequency domain nature, OFDM enables flexible bandwidth operation with low complexity. Smart antenna technologies are also easier to support with OFDM, since
each sub-carrier becomes flat faded and the antenna weights can be optimized on a per sub-carrier (or block of sub-carriers) basis. In addition, OFDM enables broadcast services on a synchronized single frequency network (SFN) with appropriate cyclic prefix design.
This allows broadcast signals from different cells to combine over the air, thus significantly increasing the received signal power and supportable data rates for broadcast services.
OFDMA Receiver
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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SC-FDMA in UL
Difference between DL and UL
OFDMA
#Advantages
- Robust against narrow-band co-channel interference
- Robust against Intersymbol interference (ISI) and fading
- High spectral efficiency
- Efficient implementation using FFT
# Drawbacks
- High Peak-to-Average Power Ratio
#LTE uses in UL a modified form of OFDMA process, called SC-FDMA
SC-FDMA = Single Carrier – Frequency Division Multiple Access
- SC-FDMA improves the peak-to-average power ratio (PAPR) compared to OFDM
- Reduced power amplifier cost for mobile
- Reduced power amplifier back-off improved coverage
DFT spreading of modulation symbols reduces PAPR
- In OFDM, each modulation symbols “sees” a single 15 kHz subcarrier (flat channel)
-In SC-FD-A, each modulation symbol “sees” a wider bandwidth (i.e. m x 180 KHz)
Equalization is required in the SC-FDMA receiver
OFDMA Parameter for LTE
The width of a Sub-carrier is 15 kHz whatever the bandwidth
-The bandwidths are: 1.4, 3, 5, 10, 15 and 20 MHz
- Note that in LA1.1, only 5, 10 MHz are implemented
-The symbol duration is always the same whatever the bandwidth
#There are 2 times more sub-carriers in 10 MHz than in 5 MHz
- 2 times more symbols can be sent or received at the same time.
-The capacity is multiplied by 2
For the 5 MHz, there are 512 sub-carriers of 15 kHz. The total band is
7.68 MHz. It is larger than the 5 MHz band!
- But only 301 sub-carriers are used (Pilot, DC, data), the other ones are guard sub-carriers:
- 301 Sub-ca * 15 kHz = 4.515 MHz
#Flexible bandwidth allocation supported by OFDM
- Still different RF filter will be required
- Frame structure always the same
- Sampling frequency is an transmitter and receiver implementation issue
- Sampling rate is multiple of 3.84 MHz Ł single clock for multi-mode UE with WCDMA
- Smallest bandwidth that is supported was modified recently and needs to be updated
The symbol duration depends on the sub-carrier width.
2 Cyclic Prefixes are defined by the 3GPP:
- Long CP: 16.67 micro seconds
- Normal CP: 4.69 micro seconds
- Only the normal CP is supported in LA1.x
#The total duration of a symbol is:
- Useful duration + CP = 66.6 + 4.69
- Total duration = 71.29 <s
- With normal CP