History of Wireless Communications

History of Wireless Communications

The first wireless networks were developed in the Pre-industrial age. These systems transmitted infor-
mation over line-of-sight distances (later extended by telescopes) using smoke signals, torch signaling,
flashing mirrors, signal flares, or semaphore flags. An elaborate set of signal combinations was developed
to convey complex messages with these rudimentary signals. Observation stations were built on hilltops
and along roads to relay these messages over large distances. These early communication networks were
replaced first by the telegraph network (invented by Samuel Morse in 1838) and later by the telephone.
In 1895, a few decades after the telephone was invented, Marconi demonstrated the first radio trans-
mission from the Isle of Wight to a tugboat 18 miles away, and radio communications was born. Radio
technology advanced rapidly to enable transmissions over larger distances with better quality, less power,
and smaller, cheaper devices, thereby enabling public and private radio communications, television, and wireless networking.
Early radio systems transmitted analog signals. Today most radio systems transmit digital signal
composed of binary bits, where the bits are obtained directly from a data signal or by digitizing an
analog voice or music signal. A digital radio can transmit a continuous bit stream or it can group
the bits into packets. The latter type of radio is called a packet radio and is characterized by bursty
transmissions: the radio is idle except when it transmits a packet. The first network based on packe
radio, ALOHANET, was developed at the University of Hawaii in 1971. This network enabled compute
sites at seven campuses spread out over four islands to communicate with a central computer on Oahu
via radio transmission. The network architecture used a star topology with the central computer at its
hub. Any two computers could establish a bi-directional communications link between them by going
through the central hub. ALOHANET incorporated the first set of protocols for channel access and
routing in packet radio systems, and many of the underlying principles in these protocols are still in
use today. The U.S. military was extremely interested in the combination of packet data and broadcas
radio inherent to ALOHANET. Throughout the 70’s and early 80’s the Defense Advanced Research
Projects Agency (DARPA) invested significant resources to develop networks using packet radios for
tactical communications in the battlefield. The nodes in these ad hoc wireless networks had the ability to
self-configure (or reconfigure) into a network without the aid of any established infrastructure. DARPA’
investment in ad hoc networks peaked in the mid 1980’s, but the resulting networks fell far short o
expectations in terms of speed and performance. DARPA has continued work on ad hoc wireless network
research for military use, but many technical challenges in terms of performance and robustness remain
Packet radio networks have also found commercial application in supporting wide-area wireless data
services. These services, first introduced in the early 1990’s, enable wireless data access (including email
file transfer, and web browsing) at fairly low speeds, on the order of 20 Kbps. The market for these
wide-area wireless data services is relatively flat, due mainly to their low data rates, high cost, and lack
of “killer applications”. Next-generation cellular services are slated to provide wireless data in addition
to voice, which will provide stiff competition to these data-only services.
The introduction of wired Ethernet technology in the 1970’s steered many commercial companie
away from radio-based networking. Ethernet’s 10 Mbps data rate far exceeded anything available using
radio, and companies did not mind running cables within and between their facilities to take advantage
of these high rates. In 1985 the Federal Communications Commission (FCC) enabled the commercia
development of wireless LANs by authorizing the public use of the Industrial, Scientific, and Medica
(ISM) frequency bands for wireless LAN products. The ISM band was very attractive to wireless LAN
vendors since they did not need to obtain an FCC license to operate in this band. However, the wireless
LAN systems could not interfere with the primary ISM band users, which forced them to use a low powe
profile and an inefficient signaling scheme. Moreover, the interference from primary users within thi
frequency band was quite high. As a result these initial LAN systems had very poor performance in
terms of data rates and coverage. This poor performance, coupled with concerns about security, lack
of standardization, and high cost (the first network adaptors listed for $1,400 as compared to a few
hundred dollars for a wired Ethernet card) resulted in weak sales for these initial LAN systems. Few o
these systems were actually used for data networking: they were relegated to low-tech applications like
inventory control. The current generation of wireless LANS, based on the IEEE 802.11b and 802.11a
standards, have better performance, although the data rates are still relatively low (effective data rate
on the order of 2 Mbps for 802.11b and around 10 Mbps for 802.11a) and the coverage area is still smal
(100-500 feet). Wired Ethernets today offer data rates of 100 Mbps, and the performance gap between
wired and wireless LANs is likely to increase over time without additional spectrum allocation. Despite
the big data rate differences, wireless LANs are becoming the prefered Internet access method in many homes, offices, and campus environments due to their convenience and freedom from wires. However,
most wireless LANs support applications that are not bandwidth-intensive (email, file transfer, web
browsing) and typically have only one user at a time accessing the system. The challenge for widespread
wireless LAN acceptance and use will be for the wireless technology to support many users simultaneously,
especially if bandwidth-intensive applications become more prevalent.
By far the most successful application of wireless networking has been the cellular telephone system.
Cellular telephones are projected to have a billion subscribers worldwide within the next few years. The
convergence of radio and telephony began in 1915, when wireless voice transmission between New York
and San Francisco was first established. In 1946 public mobile telephone service was introduced in 25 cities
across the United States. These initial systems used a central transmitter to cover an entire metropolitan
area. This inefficient use of the radio spectrum coupled with the state of radio technology at that time
severely limited the system capacity: thirty years after the introduction of mobile telephone service the
New York system could only support 543 users.
A solution to this capacity problem emerged during the 50’s and 60’s when researchers at AT&T
Bell Laboratories developed the cellular concept [1]. Cellular systems exploit the fact that the power
of a transmitted signal falls off with distance. Thus, the same frequency channel can be allocated to
users at spatially-separate locations with minimal interference between the users. Using this premise, a
cellular system divides a geographical area into adjacent, non-overlapping, “cells”. Different channel sets
are assigned to each cell, and cells that are assigned the same channel set are spaced far enough apart so
that interference between the mobiles in these cells is small. Each cell has a centralized transmitter and
receiver (called a base station) that communicates with the mobile units in that cell, both for control
purposes and as a call relay. All base stations have high-bandwidth connections to a mobile telephone
switching office (MTSO), which is itself connected to the public-switched telephone network (PSTN). The
handoff of mobile units crossing cell boundaries is typically handled by the MTSO, although in current
systems some of this functionality is handled by the base stations and/or mobile units.
The original cellular system design was finalized in the late 60’s. However, due to regulatory delays
from the FCC, the system was not deployed until the early 80’s, by which time much of the original
technology was out-of-date. The explosive growth of the cellular industry took most everyone by surprise,
especially the original inventors at AT&T, since AT&T basically abandoned the cellular business by the
early 80’s to focus on fiber optic networks. The first analog cellular system deployed in Chicago in 1983
was already saturated by 1984, at which point the FCC increased the cellular spectral allocation from 40
MHz to 50 MHz. As more and more cities became saturated with demand, the development of digital
cellular technology for increased capacity and better performance became essential.
The second generation of cellular systems are digital. In addition to voice communication, these
systems provide email, voice mail, and paging services. Unfortunately, the great market potential for
cellular phones led to a proliferation of digital cellular standards. Today there are three different digital
cellular phone standards in the U.S. alone, and other standards in Europe and Japan, none of which are
compatible. The fact that different cities have different incompatible standards makes roaming throughout
the U.S. using one digital cellular phone impossible. Most cellular phones today are dual-mode: they
incorporate one of the digital standards along with the old analog standard, since only the analog standard
provides universal coverage throughout the U.S. More details on today’s digital cellular systems will be
given in Section 15.
Radio paging systems are another example of an extremely successful wireless data network, with 50
million subscribers in the U.S. alone. However, their popularity is starting to wane with the widespread
penetration and competitive cost of cellular telephone systems. Paging systems allow coverage over very
wide areas by simultaneously broadcasting the pager message at high power from multiple base stations or satellites. These systems have been around for many years. Early radio paging systems were analog 1 bit
messages signaling a user that someone was trying to reach him or her. These systems required callback
over the regular telephone system to obtain the phone number of the paging party. Recent advances
now allow a short digital message, including a phone number and brief text, to be sent to the pagee as
well. In paging systems most of the complexity is built into the transmitters, so that pager receivers
are small, lightweight, and have a long battery life. The network protocols are also very simple since
broadcasting a message over all base stations requires no routing or handoff. The spectral inefficiency
of these simultaneous broadcasts is compensated by limiting each message to be very short. Paging
systems continue to evolve to expand their capabilities beyond very low-rate one-way communication.
Current systems are attempting to implement “answer-back” capability, i.e. two-way communication.
This requires a major change in the pager design, since it must now transmit signals in addition to
receiving them, and the transmission distances can be quite large. Recently many of the major paging
companies have teamed up with the palmtop computer makers to incorporate paging functions into these
devices [2]. This development indicates that short messaging without additional functionality is no longer
competitive given other wireless communication options.
Commercial satellite communication systems are now emerging as another major component of the
wireless communications infrastructure. Satellite systems can provide broadcast services over very wide
areas, and are also necessary to fill the coverage gap between high-density user locations. Satellite mobile
communication systems follow the same basic principle as cellular systems, except that the cell base
stations are now satellites orbiting the earth. Satellite systems are typically characterized by the height
of the satellite orbit, low-earth orbit (LEOs at roughly 2000 Km. altitude), medium-earth orbit (MEOs
at roughly 9000 Km. altitude), or geosynchronous orbit (GEOs at roughly 40,000 Km. altitude). The
geosynchronous orbits are seen as stationary from the earth, whereas the satellites with other orbits have
their coverage area change over time. The disadvantage of high altitude orbits is that it takes a great
deal of power to reach the satellite, and the propagation delay is typically too large for delay-constrained
applications like voice. However, satellites at these orbits tend to have larger coverage areas, so fewer
satellites (and dollars) are necessary to provide wide-area or global coverage.
The concept of using geosynchronous satellites for communications was first suggested by the science
fiction writer Arthur C. Clarke in 1945. However, the first deployed satellites, the Soviet Union’s Sputnik
in 1957 and the Nasa/Bell Laboratories’ Echo-1 in 1960, were not geosynchronous due to the difficulty
of lifting a satellite into such a high orbit. The first GEO satellite was launched by Hughes and Nasa in
1963 and from then until recently GEOs dominated both commercial and government satellite systems.
The trend in current satellite systems is to use lower orbits so that lightweight handheld devices can
communicate with the satellite [3]. Inmarsat is the most well-known GEO satellite system today, but
most new systems use LEO orbits. These LEOs provide global coverage but the link rates remain low
due to power and bandwidth constraints. These systems allow calls any time and anywhere using a single
communications device. The services provided by satellite systems include voice, paging, and messaging
services, all at fairly low data rates [3, 4]. The LEO satellite systems that have been deployed are not
experiencing the growth they had anticipated, and one of the first systems (Iridium) was forced into
bankruptcy and went out of business.
A natural area for satellite systems is broadcast entertainment. Direct broadcast satellites operate in
the 12 GHz frequency band. These systems offer hundreds of TV channels and are major competitors to
cable. Satellite-delivered digital radio is an emerging application in the 2.3 GHz frequency band. These
systems offer digital audio broadcasts nationwide at near-CD quality. Digital audio broadcasting is also
quite popular in Europe.