DOPPLER SHIFT. Thus, the frequency of the received signal is

Transcription

1 DOPPLER SHIFT Radio Propagation Doppler Effect: When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source. When they are moving toward each other, the frequency of the received signal is higher than the source. When they are opposing each other, the frequency decreases. Thus, the frequency of the received signal is f R = f C f D where f C is the frequency of source carrier, f D is the Doppler frequency. Doppler Shift in frequency: f = D v θ λ cosθ where v is the moving speed, λ is the wavelength of carrier. Signal MS Moving speed v 14

2 Radio Propagation FAST FADING Fast fading The signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles. Many signals from 360 directions at the receiver Different frequencies by Doppler shift Sum of signals with different frequencies Distribution No LOS (line-of-sight): Rayleigh distribution LOS: Rician distribution 15

3 Radio Propagation DELAY SPREAD When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections. This forces signal to follow different paths. Each path has different path length, so the time of arrival for each path is different. This effect which spreads out the signal is called Delay Spread. 16

4 Radio Propagation DELAY SPREAD The signals from close by reflectors Sig gnal Stre ength The signals from it intermediate dit reflectors The signals from far away reflectors Delay 17

5 Radio Propagation INTERSYMBOL INTERFERENCE (ISI) Transmission signal Time Received signal (short delay) Time Propagation time Received signal (long delay) Delayed signals Time 18

6 Radio Propagation COHERENCE BW/TIME Coherence bandwidth Nearly flat bandwidth Related to the inverse of delay spread Coherence time Nearly constant time interval Related to the inverse of the maximum Doppler shift 19

7 MULTIPLE ACCESS

8 INTRODUCTION Multiple access control channels Each node is attached to a transmitter/receiver which communicates via a medium shared by other nodes Node 3 Node 2 Shared Multiple Access Medium Node 4 Node 1 Node N 2

9 INTRODUCTION (cont d) Multiple li l Access Issues If more than one node transmit at a time on the broadcast channel, a collision occurs. How to determine which node can transmit? Multiple Access Protocols Solving multiple access issues Different types: Contention-based protocols : a collision resolution protocol after each collision Collision-free (conflict-free) )protocols : no collision Multiple Access 3

10 CONTENTION-BASED PROTOCOLS ALOHA Slotted ALOHA CSMA (Carrier Sense Multiple Access) CSMA/CD (CSMA with Collision Detection) CSMA/CA (CSMA with Collision Avoidance) CSMA/CA with ACK CSMA/CA with RTS/CTS 4

11 ALOHA Developed in the 1970s for a packet radio network by Hawaii University. Whenever a station has a data, it transmits. Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. Sender retransmits after some random time if there is a collision. Node 1 Packet Waiting a random time Node 2 Packet Retransmission Retransmission Collision Time Node 3 Packet 5

12 SLOTTED ALOHA Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration. Node 1 Packet Nodes 2 & 3 Packets Retransmission Retransmission 1 2&3 2 3 Time Slot Collision 6

13 THROUGHPUT Throug ghput, S Slotted ALOHA 0.1 ALOHA G Offered load 6 8 7

14 CSMA Carrier Sense Multiple Access Max throughput achievable by slotted ALOHA is CSMA gives improved throughput compared to Aloha protocols. Listens to the channel before transmitting a packet (avoid avoidable collisions). Node 5 sense Node 1 Packet Node 2 Packet Node 3 Packet Dl Delay Delay 4 5 Collision Time Node 4 sense 8

15 KINDS OF CSMA CSMA Nonpersistent CSMA Persistent CSMA Unslotted Nonpersistent CSMA Slotted Nonpersistent CSMA Unslotted persistent CSMA Slotted persistent CSMA 1-persistent CSMA p-persistent CSMA 9

16 THROUGHPUT OF CSMA Multiple Access S persistent 0.01-persistent CSMA 0.9 Nonpersistent CSMA persistent CSMA persistent CSMA persistent CSMA Slotted Aloha 0.2 Aloha G 10

17 CSMA/CD CSMA with Collision o Detection In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place). Wasting medium for an entire packet time. CSMA/CD Step 1: If the medium is idle, transmit Step 2: If the medium is busy, continue to listen until the channel is idle then transmit Step 3: If a collision is detected during transmission, cease transmitting Step 4: Wait a random amount of time and repeats the same algorithm 11

18 COLLISION MECHANISM IN CSMA/CD T 0 A begins transmission A B T 0 +α-ε B begins transmission A B T 0 +α B detects collision A B T 0 +2α -εε A detects collision just before end of transmission Time (α is the propagation time) A collision detection time = 2 x end-to-end propagation delay B 12