====== RE56 project ====== [[https://github.com/bpetit/lte_multiplexing_simulator|Produced code]] //**LTE (Long Term Evolution) - 4G**// ===== Documents to write ===== * [[re56_specs|Specifications document]] * [[re56_scheduler_round_robin|Round Robin scheduler]] * [[re56_scheduler_max_cqi|Max-CQI scheduler]] * [[re56_scheduler_proportional_fair|Proportional Faire scheduler]] * [[re56_implementation_example|Round robin example]] ===== 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 2x2 | 172 Mbit/s | | | MIMO 4x4 | 326 Mbit/s | | | 16QAM | | 57 Mbit/s | | 64QAM | | 86 Mbit/s | * 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 ===== 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 * **[[http://en.wikipedia.org/wiki/Delay_spread|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 == Depends on: * the number of paths in the propagation channel * the kind of symbol = the size of the symbol/the number of bits transmitted == 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) {{http://upload.wikimedia.org/wikipedia/en/thumb/b/bc/SC-FDMA.svg/750px-SC-FDMA.svg.png}} == 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: * 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 {{http://ubuntuone.com/4jw9bOJ4TSHm7KSbM99L0V|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 {{http://ubuntuone.com/6SdiO67kdn4vG7wrsS3NWz|Sub-carrier mapping allocation block diagram}} === MIMO: Multiple Input Multiple Output === * additionnal signal paths used to increase the throughput * multiple antennas 2x2 4x2 4x4 === 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 ====== * [[http://www.radio-electronics.com/info/cellulartelecomms/lte-long-term-evolution/3g-lte-basics.php|Tutorial about LTE]] * [[http://fr.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing|French Wikipedia]] with some interesting formula * [[http://4glte.over-blog.com/article-ofdma-103962236.html|French blog]] about 4G and OFDMA * [[http://www.radio-electronics.com/info/rf-technology-design/ofdm/ofdm-basics-tutorial.php|OFDM tutorial]] * [[http://www.supelec.fr/d2ri/flexibleradio/cours/cours-ofdm-french.pdf|French course]] talking about OFDM * [[http://www.mike-willis.com/Tutorial/PF15.htm|Fading and multipath]] article * [[http://3gpplte-longtermevolution.blogspot.fr/2010/06/introduction-to-lte.html|Frame structure and more]] * [[http://www.comsoc.org/files/Publications/Tech%20Focus/pdf/2010/jan-sample/6.pdf|LTE: the evolution of mobile broadband]] ===== Sub-carrier allocation ===== * [[https://rose.bris.ac.uk/bitstream/handle/1983/325/Peng_IEEE_VTC_Spring2005.pdf?sequence=1|An Investigation of Dynamic Sub-carrier Allocation in OFDMA Systems]] * [[http://www.eee.metu.edu.tr/~elif/Publications/B15_TUY_euroW2009.pdf|Subcarrier Allocation In OFDMA With Time Varying Channel And Packet Arrivals]] * [[http://ceng.usc.edu/assets/002/64813.pdf|Sub-carrier allocation in OFDM systems: complexity, approximability and algorithms]] * [[http://www.slideshare.net/deepakecrbs/ofdma-basics|OFDMA basics]] * [[http://www.nt.tuwien.ac.at/forum/?tx_mmforum_pi1%5Baction%5D=get_attachment&tx_mmforum_pi1%5Bfid%5D=att196|LTE Downlinki scheduling]] * [[http://www.lteportal.com/Files/MarketSpace/Download/325_5-23commsdesign.pdf|Physical layer parameter]]