LTE (Long Term Evolution) - 4G

• Specifications document
• Round Robin scheduler
• Max-CQI scheduler
• Proportional Faire scheduler
• Round robin example

LTE is like an UMTS upgrade. 4G network means LTE Advanced (next step).

LTE is an IP based network v4 & v6.

Voice over LTE: VoLTE

• QAM: Quadrature Amplitude Modulation, QPSK: Quadrature Phase Shift Keyring http://turboblogsite.com/quadrature-phase-shift-keying-qpsk-modulation.html
• 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

• 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

Depends on:

• the number of paths in the propagation channel
• the kind of symbol = the size of the symbol/the number of bits transmitted

Transformation of data to be transmitted in bits (more generally: in what is adapted to the medium)

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:

1. frequency interweaving: control bits spread across frequency carriers separated by [bande de cohérence]
2. temporal interweaving: bits spread on the same frequency carrier but at different times !! increased latency !!
3. 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)

1. mapping data to a vector (Serial to Parallel)
2. attribution to sub carriers
3. IDFT or IFFT: inverted fourier transform
4. cyclic prefix
5. digital-to-analog conversion ⇒ radio frequency

1. analog-to-digital conversion
2. cyclic prefix
3. DFT or FFT: fourier transform
4. equalization + demapping
5. 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)

2 perspectives:

• minimize the amount of transmit power (1)
• achieve maximum throughput (2)

NP-hard problem in downlink (1), dowlink (2), uplink (1) and uplink (2) scenarios

Vectors

• number of packets arrivals on each queue during the nth time slot
• number of packets occupying a queue (on each queue) at a given timeslot

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

• number of users
• buffer capacity
• number of subcarriers
• packets size #KB size
• error probability
• are users allowed to use the same subcarrier at a given time ?
• throughput
• throughput per subcarrier
• coherence bandwidth

Sub-carrier mapping allocation block diagram

• additionnal signal paths used to increase the throughput
• multiple antennas 2×2 4×2 4×4

• 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

• Tutorial about LTE
• French Wikipedia with some interesting formula
• French blog about 4G and OFDMA
• OFDM tutorial
• French course talking about OFDM
• Fading and multipath article
• Frame structure and more
• LTE: the evolution of mobile broadband

• An Investigation of Dynamic Sub-carrier Allocation in OFDMA Systems
• Subcarrier Allocation In OFDMA With Time Varying Channel And Packet Arrivals
• Sub-carrier allocation in OFDM systems: complexity, approximability and algorithms
• OFDMA basics
• LTE Downlinki scheduling
• Physical layer parameter