Overview PROF. MICHAEL TSAI 2011/09/15 1 The oldest form of Smoke wireless communications (and multihop!) Flag Voice propagation is actually quite similar to RF wireless communications in lots of aspects. Let’s talk about 3 examples: 1. Received power 2. Interference 3. A simple MAC protocol 2 Key events in the wireless history • Marconi’s experiments: • Cross-strait experiment: • 13 May 1897 • Transversed the Bristol Channel from Lavernock Point (South Wales) to Flat Holm Island, a distance of 6 km. • Later relocated to Brean Down Fort on the Somerset coast, stretching the range to 16 km. • "Are you ready“. • Transatlantic experiment: • 12 December 1901 • 152.4-metre (500 ft) kite-supported antenna • 3500 km • TX: Poldhu, Cornwall • RX: Signal Hill in St John's, Newfoundland 3 Key events in the wireless history • Maxwell: theory • Hertz: fundamental experiments confirming Maxwell’s theory • Tesla, Bose, Marconi: First experiments for wireless information transmission • 1980s: Analog cellular systems • 1990s: GSM & 802.11 (WiFi, WLAN) • 2000s: 3G and beyond Guglielmo Marconi 4 Types of Services: broadcast • One-way • Same content for all receivers • Continuous • Multiple transmitters sending the same content 5 Types of Services: paging • Receive only • The information is for only 1 user • Very small amount of data • 150 MHz • 1980s and early 1990s 6 Types of Services: cellular telephony • Bidirectional • Can be anywhere in the network • A call can originate from the network or the user • A call is intended only for a user • High mobility 7 Types of Services: cordless telephony & Wireless Local Area Network • Simpler BS • No central system • Free BS MS 8 Types of Services: ad hoc network • Same type of devices • No infrastructure or centralized controller • Organize themselves into a network • Low cost 9 Other Types of Services • Fixed wireless systems • Long distances between BS and MS • No mobility requirements • Typically high data rates, but can also be used for voice systems • WiMax standard (IEEE 802.16) • Satellite systems • Cover very large area • No high density • Iridium system with LEO systems tried to get large user density, but went broke 10 Requirements for Small question: Services: data rate When we say that the data rate is ㄅ kbit/s, what does • Sensor networks: that really mean? • <1kbit/s; • central nodes need up to 10 Mbit/s • Speech communications: • 5-64 kbit/s, depending in speech coder (vocoder) • Elementary data services: • 10-100 kbit/s • Communications between computer peripherals: • 1 Mbit/s • Wireless LANs: • broadband internet speeds, 1-100 Mbit/s • Personal Area Networks: Note: 1 Mb/s and 1 MB/s are different! • >100 Mbit/s 8 b/s = 1 B/s 11 Requirements for Services: range and number of users • A few examples (you can name the others) • WLANs: up to 100m, 10 users • Cellular: macrocells: 10-30km, microcells: 500m, 5-50 active users Trade-off between range & data rate KEY MESSAGE: there’s always trade-offs. no free lunch! 12 Types of Energy Supply Passive Active Energy harvester Energy from One-way or the base Convert heat, Rechargeable vibration, station Batteries (transmitted pressure to wirelessly) electricity RFIDs or, power mains: 13 Requirements for Services: Energy consumption • Cellphone: 2 hrs talk time & 48 hrs standby time • Laptops: 2 hrs – 10 hrs (but wireless radio is not the only factor) • Wireless sensors: months – years of operation time • Base station: no energy consideration! • How do we minimize the energy consumption? 1980s Heavy! 2kg 2000s 14 Requirements for Services: Spectrum Usage http://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf 15 Requirements for Services: Spectrum Usage • Spectrum dedicated to service and operator • Spectrum allowing multiple operators • Dedicated to a service • Free spectrum: ISM bands (2.4-2.5 GHz, 5.725-5.875GHz) • New systems: • Ultra Wide Bandwidth (UWB) • Adaptive spectral usage 16 Requirements for Services: Service Quality (or QoS) • General: • Packet reception rate • Delay • Different for various applications: • Voice Quality: Mean Opinion Score (MOS) • Service quality: fraction of blocked calls + 10 * fraction of dropped calls • Time to finish a file transmission 17 Technical challenges of wireless communications • Multipath Propagation • Fading • Inter-symbol Interference • Spectral Limitation • Limited Energy • User Mobility 18 Multipath Propagation 19 Small-scale Fading 20 Large-scale Fading 21 Consequences of Fading • Error probability is dominated by probability of being in a fading dip • Error probability decreases only linearly with increasing SNR (will see this later in the semester) • Fighting the effects of fading becomes essential for wireless transceiver design • Deterministic modeling of channel at each point very difficult • Statistical modeling of propagation and system behavior 22 Inter-symbol Interference 23 Inter-symbol Interference 24 Recap • Designing a wireless device or technology is always about trade-offs between various performance metrics • Wireless technology has to address the unique challenge of multipath propagations • We will continue with the rest of the technical challenges next week 25.