Telecommunication Networks Lab.DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 1 A Finite State Modeling for Adaptive Modulation in Wireless OFDMA Systems Dania Marabissi, Daniele Tarchi, Federico Genovese, and Romano Fantacci University of Florence, Italy Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 2 Outline Standard IEEE S-OFDMA (Scalable-OFDMA) Adaptive modulation Proposed adaptation techniques Numerical results Conclusions Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 3 WiMAX system, that is based on the IEEE specifications, offers broadband wireless accces in a wide are achieving about 50 Km in LoS connections and 10 Km in NLoS connections The standard aims to have until 70Mbit/s per cell Almost recently has been finalized the IEEE802.16e version that adds the user mobility functionalities by exploiting the OFDMA technique This allows to offers mutimedia services in mobility such as Internet, Voice over IP (VoIP) and video streaming Standard IEEE802.16 Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 4 PHY characteristics It transmits in the 2 11GHz band Recently has been almost defined four operational bands at 2.3, 2.5, 3.5, 5 GHz It allows to use QPSK, 16QAM and 64QAM, and variable coding rates Both TDD and FDD (half and full duplex) are allowed The channel width can varies from 1.75 MHz to 20 MHz and in TDD the frame time can vary from 2 ms to 20 ms The IEEE e has improved the OFDMA support Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 5 S-OFDMA The IEEE e standard has introduced a particolar OFDMA called S-OFDMA (Scalable-OFDMA) It mainly differs from the classical OFDMA because it can support several channel width by varying the FFT size during the subchannel mapping Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 6 Channel state adaptativity Adaptive Modulation It selects automatically the modulation order for: minimize the error probability; maximize the transmitting rate; The AM (Adaptive Modulation) module manage the adaptivity based one: channel state information and received power receiver quality in terms of error rate Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 7 TDD Channel information The proposed system is to be used in the TDD scheme It aims to estimate the channel behaviour in the uplink and use the estimation for selecting the best modulation order to be used in the downlink Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 8 Channel state adaptivity Three state Moore machine Three state Moore machine Adaptive management Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 9 QPSK 16QAM 64QAM Throughput: Thr(SNR, , M) = log 2 (M)[1-SER(SNR, , M)] Throughput: Thr(SNR, , M) = log 2 (M)[1-SER(SNR, , M)] For each fixed SNR we select two thresholds [ A, B ] by solving : For each fixed SNR we select two thresholds [ A, B ] by solving : Thr(SNR, A, M=16) = Thr(SNR, A, M= 4) Thr(SNR, A, M=16) = Thr(SNR, A, M= 4) Thr(SNR, B, M=64) = Thr(SNR, B, M=16) Thr(SNR, B, M=64) = Thr(SNR, B, M=16) Maximum Throughput Technique Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 10 For each fixed SNR at the BS two thresholds are selected [ A, B ]: For each fixed SNR at the BS two thresholds are selected [ A, B ]: SER(SNR, A, M=16) = SER(SNR, M=4) SER(SNR, A, M=16) = SER(SNR, M=4) SER(SNR, B, M=64) = SER(SNR, M=4) SER(SNR, B, M=64) = SER(SNR, M=4) where, supposing, we have: where, supposing, we have: SER(SNR, M=4) Minimum SER Technique Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 11 Example with SNR=15dB We can define a target SER value (TSER) based on the Quality of Service requested We can define a target SER value (TSER) based on the Quality of Service requested For each SNR we search two thresholds [ A, B ]: For each SNR we search two thresholds [ A, B ]: SER(SNR, A, M=16) = TSER SER(SNR, A, M=16) = TSER SER(SNR, B, M=64) = TSER SER(SNR, B, M=64) = TSER TSER = constant SNR Target SER Technique Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 12 Decision thresholds Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 13 Channel model ITU-R M.1225 (vehicular A) with 6 Rayleigh distributed main paths and Jake Doppler Bandwidth equal to 10MHz N FFT = 1024 carriers Carrier at 3.5 GHz Maximum speed equal to 125Km/h Frame duration TDD: T F = 8ms; T DL =T UL =3,995ms; (TTG=RTG=5s) OFDMA symbols in each frame: 40 in DL + 39 in UL Distributed subcarriers allocations System Parameters Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 14 Minimum SER and Maximum Throughput For SNR=16 dB 16QAM is the most used modulation order For SNR=8 dB 16QAM is the most used modulation order Maximum throughput allows higher rate but higher SER For SNR=16 dB 64QAM is the most used modulation order 2dB Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 15 TSER is imposed based on the requested QoS level Target SER Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 16 Conclusions IEEE e standard has been designed to perform a broadband wireless access by allowing multimedia services to mobile devices; this can be done thanks to the high flexibility in several parts of the system (like modulation, coding, subcarrier allocation). We propose a state model for adapt the modulation order to the channel behavior; three different techniques has been presented, that differs from the threshold calculation The Maximum throughput method is foreseen for real-time applications where the throughput has more importance the error rate. The Minimum SER aims to achieve the best performance in terms of error probability for a certain SNR The Target SER aims to respect a target in terms of error probability in order to be considered to be used in specific applications. What we are working on: Error rate as a performance indication; Extension of the model for considering variable coding rate jointly with modulation; Joint modulation/coding adaptation with subcarrier allocation, also by taking into account the upper layers queue size (opportunistic scheduling techniques). Telecommunication Networks Lab. DET Department of Electronics and Telecommunications 11/04/2007COST289 4th Workshop - Gothenburg, Sweden 17 AA BB Thr = n (1 SER), dove: n=(numero di bit trasmessi)/(numero di simboli trasmessi) SER=(numero di simboli errati)/(numero di simboli trasmessi) Prob. di stato per adattativit basata sul canale: