RE56 project
Documents to write
Introduction
LTE is like an UMTS upgrade. 4G network means LTE Advanced (next step).
Access methodology is OFDMA/SC-FDMA instead of CDMA.
LTE is an IP based network v4 & v6.
Voice over LTE: VoLTE
Modulation type | Max speed downlink | Max speed uplink |
SISO | 100 Mbit/s | |
QSPK (4QAM) | | 50 Mbit/s |
MIMO 2×2 | 172 Mbit/s | |
MIMO 4×4 | 326 Mbit/s | |
16QAM | | 57 Mbit/s |
64QAM | | 86 Mbit/s |
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QPSK = 4QAM, 2 bits per symbol ; 16 QAM, 4 bits per symbol, 64QAM, 6 bits per symbol
channel bandwidths: 1.4Mhz, 3Mhz, 5Mhz, 10Mhz, 15Mhz, 20Mhz
duplex schemes: FDD (Frequency Division Duplexing), TDD(Time Division Duplexing)
mobility: 0-15km/h optimized, 15-120km/h high perf
OFDMA used for downlink, SC-FDMA used for uplink
LTE is deployed since 2009 in Stockholm and Oslo, since 2010 in 38 USA cities
LTE technologies
OFDM: Orthogonal Frequency Division Multiplex
enables high data bandwidth
high degree of resilience to reflections and interference
different access schemes for up and downlink: OFDMA (Orthogonal FDM Access) for downlink, SC-FDMA (Single Carrier FDM Access) for uplink
FDMA = share in frequency (see LO53 “The radio link” p13)
OFDM is the signal bearer for LTE
tailored to meet the exact requirements of LTE
a large number of close spaced carriers modulated with low rate data
used in 802.11a (for 54Mbps data rate in 5ghz), 802.11g (in 2.4Ghz), Digital Terrestrial Television (DTT/fr:TNT)
OFDM consists in transmitting a lot of closed space carriers. Those carriers do not need to be separated from each other with a “security space” (to permit to the transmitter to filter each carrier) because each is orthogonal from the other.
lower data rate ⇒ less critical interferences from reflections
error coding techniques ⇒ corrupted data reconstructed (most of the time) by the receiver (error correction code is transmitted on a different part of the signal)
OFDMA : scheme to provide multiple access capability for cellular telecommunications when using OFDM
OFDM can be used in both FDD (Frequency Division Duplex) and TDD (Time Division Duplex)
equalization = correct the effects of distorsions on multi-path transmissions
constellation = taille d'un symbole/nombre de bits émis (french) : BPSK (1 bits), QPSK (2 bits), 16-QAM(4), 32-QAM(16)
augmenter la taille de la constellation pourrait permettre d'augmenter le débit si cela n'augmentait pas le risque d'erreurs ⇒ limite de débit = capacité du canal
bande de cohérence = bande de séparation entre les porteuses, ex: porteuses fréquentielles séparées de plusieurs fréquences
delay spread = lap of time between the reception of the first multipath component and the last one
OFDM divides the propagation channel in (frequency) sub-carriees separated by the inverse of the period symbol 1/T [bande de cohérence ?]. It uses the transmission of blocks of symbols. Those symbols form the OFDM symbol. The modulation of this block is made by a Inverted Fourier Transform
Equalization
Modulation
Transformation of data to be transmitted in bits (more generally: in what is adapted to the medium)
Coding
Insertion of error correction bits on the signal ⇒ less effective throughput output to the total output
Diffusion of those “control” bits on different carriers to prevent from total loss due to attenuation on a given frequency : “interweaving”
⇒ 3 ways:
frequency interweaving: control bits spread across frequency carriers separated by [bande de cohérence]
temporal interweaving: bits spread on the same frequency carrier but at different times !! increased latency !!
spatial interweaving: multiple antennas = multpiple carriers
BPSK Binary Phase Shift Keyring: (-1,1) ⇒ (0,1)
QPSK Quadrature Phase Shift Keyring: (1,i,-1,i) ⇒ (00,01,10,00)
Transmission steps
mapping data to a vector (Serial to Parallel)
attribution to sub carriers
IDFT or IFFT: inverted fourier transform
cyclic prefix
digital-to-analog conversion ⇒ radio frequency
Reception steps
analog-to-digital conversion
cyclic prefix
DFT or FFT: fourier transform
equalization + demapping
parallel to serial
Since FC-FDMA uses only one carrier there is additionnal steps to consider to serialize data. (it's out of the scope of our project)
Sub-carrier allocation
2 perspectives:
NP-hard problem in downlink (1), dowlink (2), uplink (1) and uplink (2) scenarios
Vectors
Matrices
channel gain for each subcarrier and user (optionnal)
connectivity: maximum number of packets that a given subcarrier can serve for a given queue in a given timeslot
allocation: boolean matrix subcarrier/queue/timeslot 1 if the subcarrier is assigned to eht queue at the given timeslot
mapping between matrices
Algorithms
Proportional fair
Round robin
Max CQI
Elements
SAE: System Architecture Evolution
a part of functions handled by the core network are moved to the periphery
latency time can be reduced
data can be routed more directly
Bibliography
Sub-carrier allocation