PHY-MAC Dialogue with Multi-Packet Reception Workshop on Broadband Wireless Ad-Hoc Networks and Services 12 th -13 th September 2002 ETSI, Sophia Antipolis, France Marc Realp-CTTC/Ana I. Pérez-Neira-UPC www-tsc.upc.es IST ANWIRE TIC C02-02 GIRAFA

Contents Motivation Cross-Layer Design MPR matrix. PHY-MAC dialogue. Parameters Exchange. PHY level Matched Filter. Activity User Detection. MAC level Dynamic Queue Protocol-DQP. Modified Dynamic Queue Protocol-MDQP. Simulations Conclusions & Further Work

Motivation In wireless systems a common channel is shared by many users. Traditionally, information is lost when a collision occurs, i.e., when two or more packets are sent trough the channel. Diversity at physical level allows more than one packet to be transmitted simultaneously. Conventional MAC algorithms do not consider Multi-Packet Reception (MPR) capability. CROSS-LAYER DESIGN MAC fully exploits PHY reception capabilities. Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

MPR matrix The probability of a packet to be correctly received is: Hence, Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

MPR matrix The MPR matrix is defined as: Expected number of correctly received packets when n packets have been transmitted: Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work PHY-MAC Dialogue Cross-Layer design reduces PHY-MAC dialogue to a BER exchange. Should other parameters be considered in order to improve system performance? Scheduling Fairness Traffic Modelling Throughput Delay PERBER Modulation Scheme Power Tx/Rx Transceiver Architecture Diversity Channel & Signal Estimation Battery Life Bit Rate MAC Layer PHY Layer Error Correcting Code (Binomial) Number of Users QoS ? ? ? ?

Parameters Exchange Information flows between PHY and MAC levels: BER is used for MPR computation. Active Users used for MAC efficiency. Access Set used for PHY efficiency. MAC PHY BER ACT. US. ACC. SET Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

PHY Level CDMA System Model Receiver Structure Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work e (t) Detection of Active Users Set of Active Users

PHY Level Data Demodulator Matched Filter Active Users Detector Motivation Cross-Layer Design PHY level MAC level Simulation Conclusions & Further Work MF |||| 2 State Estimation e Power detection Decision based on Traffic Information : Indicator function that takes value 1 if the kth user is active and 0 otherwise

Third TP L3=5 Second TP L2=7 First TP L1=4 Dynamic Queue Protocol-DQP System with M users to transmit data to a central controller. Time axis is divided into transmission periods (TP). A TP ends when all packets generated in the previous TP are successfully transmitted. The basic structure is a waiting queue where all users in the system are processed in groups of Access Set size. Based on packet user probability in one TP (q i ) and the MPR matrix, the size of the access set which contains users who can access the channel in the ith TP is chosen optimally. Transmit packets generated before 0 Transmit packets generated in (0,4] Transmit packets generated in (4,11] Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

Dynamic Queue Protocol-DQP The probability that a user has a packet to transmit in the ith TP is: The basic structure is a waiting queue where all users in the system are processed in groups of Access Set size. Based on q i and the MPR matrix, the size of the access set which contains users who can access the channel in the ith TP is chosen optimally. Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

DQP Vs MDQP MDQP stands for Modified Dynamic Queue Protocol. Central controller in DQP is capable to distinguish between: Empty slots. Successfully received packets in non-empty slots. Central controller in MDQP is capable to distinguish between: Empty slots. Successfully received packets in non-empty slots. Packets lost due to collision in non-empty slots. Nodes with empty buffers in non-empty slots. Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work Non-empty slot. Node 3 packet successfully received. Node 2 packet lost. Empty slot Non-empty slot. Nodes 2 and 4 packets successfully received. Access Set= Non-empty slot. Node 2 packet lost. Node 3 packet successfully received. Node 1 empty Successfully received packet Packet waiting for transmission Empty buffer Packet Lost Non-empty slot. Nodes 2 and 4 packets successfully received. Node 5 empty. Central controller do not know whether packets from nodes 1 and 5 have collided or the buffers of these nodes were empty. Central controller determines that nodes 1 and 5 have empty buffers. DQPMDQP

MDQP Optimal Access Set N i is chosen in order to minimise the absorbing time of a finite state discrete Markov chain. Each state (j,k) defines: j=number of unprocessed users in one slot k=number of packets sent in one slot 2,2 2,1 2,0 1,1 1,0 0,0 C 2,2 C 2,1 C 1,1 C 1,0 1 C 2,0 C 1,0 C 1,1 1 1 Number of Users(M)=2 Access Set (N)=2 Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

MDQP Probability Transition Matrix In general, the transition probability from state (j,k) to state (l,m) is given by: Then, Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

Access Set Vs User Packet Probability Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work Access Set User Packet Probability (q i ) Number of Users (M)=15 SNR=10 Spreading Gain (SG)=6 Packet Length (pl)=200bits Number of Correcting Errors(e)=2 Receiver type: Matched Filter MDQP DQP

Throughput Vs User Packet Probability Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work SNR=10 Spreading Gain (SG)=6 Packet Length (pl)=200bits Number of Correcting Errors(e)=2 Receiver type: Matched Filter User Packet Probability (q i ) Throughput MDQP DQP M=15 M=10 M=5

Packet Delay Vs User Packet Probability Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work User Packet Probability (q i ) SNR=10 Spreading Gain (SG)=6 Packet Length (pl)=200bits Number of Correcting Errors(e)=2 Receiver type: Matched Filter Packet Delay MDQP DQP M=15 M=10 M=5

Conclusions Cross-Layer design concept. New idea of PHY-MAC dialogue. Number of active users used as an additional parameter exchange between layers. Proposal of a centralised system PHY layer with active users detector. MAC layer with MDQP. System improvements in terms of throughput and packet delay. Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

MPR matrix for Ad-Hoc Networks MPR must be modified considering communications in Ad-Hoc scenarios. Communications are Half-Duplex. A node can not receive a packet while is transmitting. A node might successfully receive a packet not intended for it. Packet might be lost due to collision of many packets. Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work Packet intended for that node Packet not intended for that node

PHY-MAC Dialogue in the IEEE802.11b CSMA/CA is used. Medium is sensed by means of active user detection mechanism. Medium is determined IDLE when Number of users sensed < Nopt for a period longer than DIFS. After deferral, Back-off procedure adjusted depending on Number of users sensed. Busy Medium (N. Users>=Nopt) Back-Off procedure Contention Period (CP) SIFS PIFS DIFS Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work

PHY-MAC Dialogue in the IEEE802.11b Analytical throughput expression Nopt for throughput maximisation Modifications on the current standard. Additional information flows: PHY->MAC: Number of current active users. MAC->PHY: Number of users (Nopt) to consider busy medium. Additional field in Beacon frames or broadcast message to transmit Nopt. Carrier sense mechanism modifications User activity detection and MUD at PHY level. Possible change in back-off procedure for better performance. Simulations Motivation Cross-Layer Design PHY level MAC level Simulations Conclusions & Further Work