This lecture is divided into hyperlinked sections
Introduction
Radio Technology
Linking frequency to
Wavelength
Amplitude Modulation
Frequency Modulation
Phase Modulation
Mobile Telephony
Operators
and Systems
1G, 2G and 3G
Structure
of Cells
Cell
size
Base
Station Aerials
Connection
of the Base Stations
Location Operation
Frequency
Hopping
Paging
Location
Updating
WAP
Wireless Ethernet
Bluetooth
Bluetooth
Specifications
Advantages
of Bluetooth
Bluetooth
Networks
Security
Cambridge
Silicon
Radio
Red-M
Uses
of Bluetooth
Possible
problems
Conclusion
Traditional technologies have always used cables to
connect
our
data production and processing devices together. There was a time when
the telephone was hard-wired into our houses. Advances in hardware
and radio technology
have made wireless telephony (mobile phones) and other non-wired
devices a reality. There are now more mobile telephones in use in the
UK then traditional wired 'land line' telephones - figures relaeased in
2007 indicate that there are over 70 million mobile handsets in the UK
alone.
This lecture will examine the fundamentals behind wireless
operation and will specifically examine modulation techniques of
Amplitude, Frequency and Phase modulation.
Following on the heels of the wireless telephony
technology,
other technologies have sought to free us from data cables. This
lecture
will examine some of these technologies and give an insight into their
operation
and possibilities.
In the early days of computer networking during the late 1960s, the computers were linked using standard telephone lines which were twisted pair with perhaps some coaxial sections on the trunk routes. This technology gave the lowly data transfer rate of 56 kbits/ second. However, this was a connection between fairly low specification machines (compared to today) and the data that was to be transferred would not have been as large as that which we send today.
As time passed, the number of computers worldwide
increased and the requirement for higher bandwidth connections
increased too.
| EHF = Extremely high frequency (Microwaves) | 1mm to 1cm |
| SHF = Super high frequency
(Microwaves) |
1cm to 10cm |
| UHF = Ultrahigh frequency | 10cm to 1m |
| VHF = Very high frequency | 1m to 10m |
| HF = High frequency |
10m to 100m |
| MF = Medium frequency | 100m to 1km |
| LF = Low frequency | 1km to 10km |
| VLF = Very low frequency | 10km to 100km |
| VF = Voice frequency | 100km to 1000km |
| ELF = Extremely low frequency | 1000km to 10000km |
To convert between these values we need to use a
simple formula
With amplitude modulation, the frequency of the
carrier
wave stays constant, it is just the amplitude that is varied according
to the input signal.
Amplitude Shift Keying
ASK is susceptible to sudden gain changes and is a rather inefficient modulation technique for metallic media, however this is the technique that is used to modulate digital data onto optical fibres and signals onto a radio carrier. This sort of modulation method is unsuitable for high-quality audio. For digital transmission, the presence of a signal can constitute a 1 and the absence a zero.
In FSK, binary 1 is represented by one frequency and
binary
0 is represented by another frequency. These frequencies are chosen to
be near the carrier frequency. This scheme is much less error prone
than
ASK. It is commonly used in the frequency range 4 to 30 MHz. Note that
the amplitude of the signal remains constant.
Frequency Division
Multiplexing
This technique occupies a bandwidth defined by the frequency spread of each of the frequencies. Full duplex operation (i.e. simultaneous transmission of signals in both directions) may be accomplished using this technique.
One bandwidth can be used for signal transmission in
one
direction and one bandwidth can be used for the opposite direction.
Because
the range of frequencies that the majority of the power contained by
the
separate signals does not overlap, the signals do not interfere with
each
other. This can be seen below, where two signals are sent
in opposite directions along a voice-grade line.
FSK used to implement Full Duplex Operation over a voice grade line
PSK varies the phase of the carrier signal to encode the data. In the figure below, a binary 0 is represented by a signal of the same phase as the previous signal sent and a binary 1 is represented by sending a signal of the opposite phase to the previous signal that was sent. This is a differential signalling scheme where absolute values are not uses, rather comparisons with the previous state define the value represented by the PSK signal.
PSK can use more than two phase shifts. A four phase
system
would encode two bits with each signal burst. PSK is more
noise-resistant
and efficient than FSK.
In recent years, there has been an explosion in the number of
mobile telephony handsets sold worldwide. Despite the high prices for
network time, their popularity has not waned. The original services
were analogue in nature, but these services have been phased out in the
UK, although some areas of the world, notably amongst these the USA,
the
analogue systems still exist.
The digital handsets sold today offer a number of services
apart from voice communication including digital photography, text
messaging, web access amongst others. Operators of these networks come
and go regularly and their tariff and services very widely. Today's
technology is based on a set of frequencies whose licences were sold
several years ago for billions of pounds sterling to the mobile
operators and these operators are still trying to recover the original
high cost of these licences.
In the UK there were originally 4 operators of mobile cellular radio systems. These were Cellnet, Vodafone, Orange and One2one. There were a total of three types of service offered by the companies, TACS (Total Access Communication System) [now turned off] and GSM (Global System for Mobiles). Both of theses systems operate in the 900 MHz band. A third type of service known as GSM1800 (formerly known as PCN Personal Communication Network) is offered by Orange and One2one and operates at in the 1800 MHz band.
The first operational cellular communication system was
deployed in
the Norway in 1981 and was followed by similar systems in the US and
UK. These first generation systems provided voice transmissions
only
using frequencies around 900 MHz. These 1G systems used analogue
modulation and provide only for voice transmission. Second
generation
(2G) GSM (Global System for Mobile Communications) was first used in
Europe in the early 1990s. GSM provides voice and limited data
services and uses digital modulation with improved audio quality.
So-called ‘2.5G’ systems recently introduced enhance the data capacity
of GSM and mitigate some of its limitations. The new third
generation
(3G) cellular services known as Universal Mobile Telecommunications
System (UMTS) or IMT-2000 will sustain higher data rates still and
opens the door to many internet style applications.1
3G Technology will eventually replace all
earlier technologies and the technology rollout is currently well under
way.
Glossary of mobile telephony
terms.
Regardless of the radio frequency in use, all mobile telephony works on the cellular principle. An area that is served by cellular radio is divided into small areas known as macrocells. Each macrocell has a range of channels that it can use. The adjacent cells to this macrocell will use a different set of channels. This avoids interference from adjacent macrocells.
At the centre of each cell is a base station whose coverage is limited to that particular cell. The advantage of this is to allow re-use of channels in more distant macrocells where co-channel interference will be so low as to be within acceptable limits.
This allows frequencies (channels) to be re-used in other macrocells around the entire area of coverage and is known as frequency re-use.
Illustrating frequency re-use
In the above figure it can be seen that the cells are grouped into sets of 7. It can be seen that it is possible to re-use frequencies as there is a large enough distance between similar cells to avoid interference. The 7-cell grouping is the most commonly used configuration. Other cell groupings are possible using 4, 7, 12 or 21 cells.
Within each cell there are two sets of frequencies in use. One is from the base station to the mobile and the other is from transmissions from the mobile to the base station. This is illustrated for GSM below.
The use of GSM frequencies from mobile to base
The use of GSM frequencies from base to mobile
The carrier frequencies used within each cell are assigned to allow re-use of the same carrier frequency only a few cells distant. The distance between cells using the same frequencies is known as the repeat or re-use distance.
Cells can vary in size from about 1 km radius to about 8 km radius. In certain busy areas, cells may be subdivided into microcells having a radius of 2 - 300 metres. This could be used in a railway station or a shopping centre. In more busy areas, microcells may be divided into picocells.
The aerials for macrocells are mounted on the top of towers or high buildings. For microcells and picocells, the aerials will be located at a lower height and will have a lower transmission power than that of a macrocell. The base station is in the centre of the hexagon.
To help reduce co-channel interference, sectored aerials are used at each base station. A three-sectored aerial gives 3 X 120 degree coverage. This divides each cell into 3 parts known as sectors.
A 3-sectored base station aerial
Connection of the Base Stations
The base stations are themselves connected by high bandwidth links to a mobile switching centre (MSC). The MSCs are themselves interconnected and also connected to the Public Switched Telephone Network (PSTN).
As mobile telephones move around within the area of coverage, the precise location of each mobile set is monitored by the system to allow communication at all times. The location of each mobile set is achieved by a combination of paging and location update procedures.
When a mobile moves from one cell to another, there must be a transparent hand-over. This means that the user must not realise that a change-over is taking place. The procedure is known as hand-over. This takes place when the signal level from one cell begins to drop. The mobile set constantly monitors the received power from its current base station and adjacent base stations from other cells. When the signal power from the current base station falls to a level below the power from the cell adjacent for a period of time, the mobile informs the current base station that another cell has higher power level. Hand-over then takes place.
GSM has a main function as a telephony service but also offers SMS, a text messaging service, data services, FAX and access to the packet data system. This means that information from any site on the Internet may be accessed providing that it is in the correct format.
While a mobile call is in progress, the frequency that is being used to carry the signal is constantly changed. This improves the performance of the system when the signal power drops in bad reception areas. It also shares bad frequencies between a number of users so that the bad frequency will hopefully not be noticed.
To page a mobile telephone, a message is broadcast by the base station to inform the mobile set that a call is waiting to be received. If the system has knowledge of the location of the phone, only one cell needs to be paged. If the location were not known, many cells would have to be paged and this is a waste of bandwidth. To keep track of a mobile, a technique known as location updating is used.
The cellular system is divided into a number of location areas and this is typically a larger area than one cell. When a mobile moves from one location area to another, it informs the network of its new location. To keep these records up to date, mobiles are requested periodically to perform a location update.
WAP (Wireless Application Protocol) is a specification for a set of communication protocols to standardise the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access, including e-mail, the World Wide Web, newsgroups, and Internet Relay Chat. While Internet access has been possible in the past, different manufacturers have used different technologies. In the future, devices and service systems that use WAP will be able to interoperate.
The WAP layers are:
· Wireless Application Environment (WAE)
· Wireless Session Layer (WSL)
· Wireless Transport Layer Security (WTLS)
· Wireless Transport Layer (WTP)
The WAP was conceived by four companies: Ericsson, Motorola, Nokia, and Unwired Planet (which is now Phone.com). 1
The problem with WAP services is the low bandwidth that is currently available to users. This means that pages of information take considerably longer to download than on even a dial-up Internet connection.
Bluetooth is a short-range communication technology introduced by Ericsson. Bluetooth is an Open technology, so that the specifications are available to anybody who wishes to build a Bluetooth product. This will hopefully allow Bluetooth to become a single worldwide communication system.
It is a radio system that uses packets of data to allow Bluetooth-enabled devices to communicate. It is envisaged that Bluetooth will be embedded into all digital devices so that inter-device communication can take place transparently.
If you have a Bluetooth-enabled digital camera, it can transmit the pictures to a computer without having to plug any cables in.
The hardware (microchip) is cheap and available now. This cuts out the need for cables to carry data at a reasonably high rate over short distances (up to 10 metres and possibly further).
The Bluetooth devices have many radio channels and a few Bluetooth devices in a close area can be networked together to form a piconet or sometimes named Personal Area Network (PAN).
Bluetooth enabled devices are now available in the
shops.
Some of the first applications were wireless headsets for mobile
telephones.
This will hopefully reduce the perceived worry about the signal power
from the
mobile
phone’s aerial damaging the user’s brain.
Today Bluetooth is embedded in most new mobile
handsets and allows users to exchange contacts, photographs and other
data without the need to use the provider's (expensive) network.
Transmission
range
up to 10 metres
Frequency
band
2.4 GHz
Gross data
rate
1 Mbit/ second
Maximum power consumption 30 mWatts
Packet switching is based on a protocol that is based on a frequency hop scheme with 1600 hops per second.
The Bluetooth technology limits the radio output power to exactly what is required. If the transmission distance is short, the transmitting power is automatically lowered to suit the exact range. When the traffic volume drops, the radio goes into low-power mode and transmits only periodically to verify any established connections. A Bluetooth radio only consumes 3% of the power of a typical mobile telephone.
The technology has the capability to remove wires that carry signals from controller to device. Cars and aeroplanes and buildings have much wiring between devices and the technology will enable this wiring to be reduced. All that should be wired is the power cables for the devices.
The Bluetooth technology supports both point-to-point and point-to-multipoint connections. By placing several devices near to each other, a piconet can be established instantly. All of the devices, when turned on, will be synchronised.
This will form the primary unit of a multiple piconet structure.
The full-duplex data rate within a multiple piconet structure with 10 fully loaded, independent piconets is more than 6 Mbit/ second.
All data are protected by advanced error-correction techniques and encryption and authentication routines exist to promote user security. Despite this, most mobile users are unaware of the security implications of wireless networks and often do not enable the security that is provided and leave themselves open to attack.
This is a UK company that has integrated a microprocessor with a Bluetooth chip and produced a Bluetooth-enabled portable digital device that will have many applications.
Red-M (subsidiary of Madge Networks)
Another company Red-M has produced a server for Bluetooth to receive and transmit signals from other Bluetooth devices to and from the PSTN and the Internet. This server may be placed in airport lounges to allow travellers to access their email as soon as they come into range of the system.
The server allows up to 7 active connections at any one time per device. The Red-M 3000AS access server provides reliable mobile access to the Internet and local Intranet from a wide range of Bluetooth-enabled devices.
The 3000AS, combined with the 1000AP access point, enables a Bluetooth network to be provided inside a building or concourse for the delivery of next generation mobile services across a wide range of applications. The 3000AS incorporates Wide Area Networking (WAN) and Local Area Networking (LAN) interfaces, plus web page caching, secure firewall and Virtual Private Networking (VPN) functions as standard. In addition, the 3000AS hosts an email and web server for delivery of local email and web content to the Bluetooth devices.
The Red-M 1000AP access points can be connected to the 3000AS access server over a standard LAN connection. Both the 3000AS and 1000AP incorporate a high power, high sensitivity radio design that provides coverage of up to 100m per access server or access point. Red-M’s 3000AS access server retail price is £1800 and the 1000AP access point retail price is £250. Red-M expects to start shipping first product to initial customers in November 2000.
Currently companies worldwide are working on ideas for Bluetooth. Microsoft are working to put Bluetooth hardware and software into their devices.
Perhaps in the near future your refrigerator and waste bin will communicate with your network to automatically re-order any items that are running low or have been disposed of. With a higher range radio network, cars could be networked together to allow congested areas to be avoided and to provide instant feedback to drivers on a motorway if an accident has happened ahead and allow evasive action to be taken well in advance.
The radio frequency of Bluetooth is the same as that used by microwave ovens and of IEEE802.11b and g. This may be a problem if you are at the limits of the transmitting range and you turn a microwave oven on or are in an Ethernet wireless hotspot. The signal may suffer from interference or even loss in this situation.
The radio frequency used by the Japanese military is the same as Bluetooth and this will need to be resolved if Bluetooth is to be adopted globally.
Wireless technologies rely on the modulation of a carrier wave by the signal that we intend to carry. Three main techniques exist, Amplitude, Frequency and Phase modulation. These technologies may be integrated to provide better services.
Mobile devices will eventually replace our cable-bound devices. Problems that still need to be overcome are the power supply/ consumption of mobile devices and broadband access to the Internet at a reasonable price.
Until these problems have been overcome, mobile devices will remain slow and expensive to operate.