In principle, everything can deduced from Maxwell's equations, but the medium (and its boundaries) are too complex for this analysis. In practice, one has to proceed empirically.
As electromagnetic waves propagate at velocity of light:
for example for the center of the GSM frequency spectrum (in meters and seconds):
Basic principle
Emit by antennas
Effects
The higher the frequency, the lower the range.
Attenuation in free space
in the atmosphere (C), at rain (A), fog (B)
Note the effect on the satellite link (e.g. 14 GHz = 2cm)
Multi-path propagation
Waves (and the signals they carry) may propagate along more the one path
==> copies may reach the receiver at different points in time
, applies to any waves)
More power does not solve the problem since the interfering signals also have more power!
One may make good use of interference, see e.g. BLAST
caused by movement of transmitter or receiver (or both):
Seemingly, wavelengths become shorter when they approach each other
(and longer otherwise).
Maximal Doppler displacement (axial movement)
velocity, 
wavelength
5 GHz (0,06 m) at 2 m/sec ==> 33 Hz
may be different for send/receive
results in channels
partition the available bandwidth
More strict observation of boundaries is necessary.
partition time into time slots
allocate channels to time slots
More exact synchronization is necessary.
Allocation of frequency varies over time
optimal for GSM: permutations
protects against eavesdroppers and troublemakers
partition space into cells
Transmitter power must suit to the size of the cell. E.g. mobile terminals
Finer granularity by directed antennas.
The smaller the cells, the more participants per space can operate.
Scientists of the Bell Labs see good chances to use the capacity of radio channels 10 to 20 times better than up to now. You cannot by-pass the 50 year old transmission theory of Shannon. But note that it is based on a point-to-point connections which the developers of BLAST (Bell Labs Layered Space Time) replace by volume-to-volume connections. To the dimensions time and frequency, they add, according to the theory, a spatial one which can be used for the transmission of more information per channel.
In practice BLAST uses fields of antennas on the transmitter and on the receiver side. As all transmitting antennas use the same frequency, a spatial interference pattern arises, which the receiver traces back into one single signal after having spent a lot of time for calculation. In the laboratory the Bell Labs, researchers could already achieve "at least ten times the capacity" of todays usual wireless radio links.
Owing to the complicated construction of antennas and the considerably high cost of calculation, which arises at a moving effort, the BLAST developers think that the system is less suitable for mobile apparatus like for example telephones. But wireless fixed connections are approaching an increase in capacity. Lucent, the parent company of Bell Labs, which was splintered from AT&T, was the first which profited of it. (un/iX, 9.9.98)
as an example of a combined modulation technique
here 16-QAM, shown as a constellation graph:
Each point defines a state, which in turn encodes a symbol.
Height of amplitude = Distance of point from origin
Phase displacement = Angle between its radial ray and the x-axis
Side Bands (for voice transmission)
arise (for amplitude modulation) from mixing high frequency of the carrier with the low "payload" frequency.
For more precise calculation, see Fourier analysis.
Analogously, superposition of the carrier HF with a stream of data
distributes the transmitted energy over the frequency spectrum.
The higher the bit rate, the larger the interval.
Or stated otherwise:
The larger the available frequency range is, the higher are the bit rates (Bit/s):
Shannon's Theorem:
By the techniques
the bit rate can increase though spectrum efficiency remains constant.
Socialization of the error risk, see diversification.
Increase the data rate artificially, in order to spread the spectrum.
The transmitter "modulates" (by XORing) the fast "carrier" data stream (code sequence) with the payload data stream.
The receiver generates the same code sequence and synchronizes it with that of the transmitter by stepwise shifting until the payload data stream ressults.
The code sequence is pseudo-random for better synchronization and to be easily identifiable.
Sequences resulting from multi-path propagation from one and the same transmitter can be identified and combined to improve transmission quality(RAKE receiver).
More than one DSSS channel may use the same frequency range if their code sequences are orthogonal to one another ("different enough").
Orthogonal sequences can be created by Walsh functions (based on Hadamard matrices).
However:
A combination with FDMA and/or TDMA?
Not clear whether this is useful and manageable.
In UMTS (a GSM-successor), there is a choice of
For this controversy, see
Self-induced errors are caused by the pecularities of Wave Propagation:
Better avoid by appropriate transmission techniques.
Errors induced by outer sources:
(Only peculiarities of wireless communication are given in this lecture)
Errors are cumulative in space, time and frequency (the theorem of Shannon is also applies to the sources of errors!), but unpredictable.
Mitigation by diversification
As resources have to be shared with other users, diversification often means a distribution of the error risk among many users (just like insurance companies socialize the risk of damage).
For a single station, the risk is bearable: Short-term collapses of signal strength can be easier controlled than long-term ones.
All users can be granted roughly the same quality of service.
Most of the error correction methods can easily handle equally distributed (not correlated) errors, but not bursty ones.
Aim of Interleaving: Make equally distributed errors out of bursty errors.
Often used: Bit-Interleaving within a block
With m rows and n columns, a delay of m x (n-1) bits results.
Bearable for voice transmission: 40 msec.
Also: Block-Interleaving
by block codes: (n,k)-Bose-Chadhauri-Hocquenhem-Code (BCH-Code)
Length of block before coding
Length of block after coding
Redundancy
Hamming Distance
For example in the Radio Link Protocol (RLP) of GSM:
(240,216)-BCH-Code with 
and an error residue
by convolutional codes ...
Different signal strength and delay
==> Collision Detection a la CSMA/CD is not useful
==> Before starting the transmission, negotiate a reservation of the
mediums.
I.e. instead of
better
to reduce the probability of collision. However: The frames have to be so long that the transmitter is able to unambiguously assign collisions to frames.
Exposed Terminal
Therefore the problem of CSMA/CD is:
C receives RTS (but no CTS), must not send during CTS, only when a long frame is transmitted.
D receives (not RTS, but:) CTS, has to wait with the transmission until the end of the data is reached.
also called: cellular networks
Components
basic setup
First Generation (analog):
Second Generation (digital):
Voice:
Data:
Third Generation (specified as International Mobile Telecommunication (IMT-2000), alias PCS, PCN, FPLMTS,...):
CEPT Resolution 1982
Foundation of the study group: Groupe Speciale Mobile (GSM)
Passing of Standard and Timetable 1987
Memorandum of Understanding on the Introduction of the Pan-European Digital Mobile Communication Service (MoU)
GSM-study group is integrated into the European Telecommunications Standards Institute (ETSI)
Start of test mode in 5 countries 1.7.1991
Today, the standard includes ca. 8000 pages
MS Mobile Station
BTS Base Station Transceiver System
BSC Base Station Controller
MSC Mobile Switching Center
OMC Operation and Maintenance Center
HLR Home Location Register
VLR Visitor Location Register
EIR Equipment Identity Register
AuC Authentication Center
|
Parameter |
Value |
|---|---|
|
Uplink-Frequency |
890-915 MHz |
|
Downlink-Frequency |
935-960 MHz |
|
Channel Numbers |
0..124 and 975..1023 |
|
Tx/Rx Frequency Distance |
45 MHz |
|
Tx/Rx Time Slot Distance |
3 |
|
Data Rate (Traffic+Control) |
270,833333 kb/s |
|
Time Frame Duration |
4,615 ms |
|
Time Slot per Time Frame |
8 |
|
Time Slot Duration |
576,9 micro-sec |
|
Bit Duration |
3,692 micro-sec |
|
Modulation |
0,3 GMSK |
|
Channel Distance |
200 kHz |
|
Max. Delay of Interleaving |
40 ms |
|
Max. Traffic Data Rate |
24,7 kb/s |
|
Voice Data Rate (Full Rate) |
13,4 kb/s |
In Germany, the two frequency ranges are assigned to the D1-Network (T-Mobil) and D2-Network (Mannesmann), one half each. So each operator has 2*12,5 MHz of spectrum available.
Remember: Frequency, Time und Space Multiplex
GSM-Band
)
)
Extension Band (from 2001 on)
)
)
n = Absolute Radio Frequency Channel Number (ARFCN)
A physical channel consists of
Results in about 1000 physical channels.
E.g.: Voice multiframe and regular burst
A control multiframe consists of 51 frames
Burst-types (of different format):
For the shape of a burst, an upper limit is given:
In order to avoid that the MSs transmit and receive at the same time, the time frame of the uplink is 3 slots (TS) away from the downlink:
Example: (A regular) voice multiframe consists of 26 time frames
Logical Channels (Rates in bit/s)
A different coding was used partly.
are special circuit-switched data services
OSI layer 1 only
OSI layers 2 and 3 also
Transparent carrier service and additional
bit error probability < 
data service using GSM
use 2..7 time slots in parallel
up to 76,8 kbit/s
requires a costly transmitter and receiver equipment
Hence, the present standard has only
Packet-switched Data Service
realized by GSM with as few changes/extentions as possible
voice service is possible
interesting as an access to the internet
SGSN = Serving GPRS Support Node
GGSN = Gateway GPRS Support Node
GR = GPRS Register (~ HLR)
PDN = Packet Data Network
User Data : solid lines
Control data : dashed lines
U.S. Telecommunications Industry, in particular Qualcomm
compatible with IS-54, e.g. AMPS, dual mode devices
Direct Sequence Spread Spectrum
repectively 
bits
Code Division Multiple Access (CDMA)
Power control for solving the Near-Far problem at the basis station
robust against a multi-path propagation
| Multiple Access | CDMA/FDM |
| Chip-Rate | 1,2288 MHz = 128*9600 bit/s |
| Carrier Bandwidth | 1,25 MHz (,Narrow Band") |
| Frame Duration | 20 ms |
| Modulation | QPSK/OQPSK |
| Synchronization | BS : GPS; MH : Pilot+Synch |
| Capacity | theoretically up to 120 channelsper 1,25 MHz |
| Frequency Band | 925-960 MHz Uplink 880-915 MHz Downlink |
| Coding | FEC r=1/2 , Convolutional Coding |
| Interleaving | 20 ms |
| Channels Downlink | Sync 1,2 kbit/s, 1024 chip/bit
Paging 4,8 .. 9,6 bit/s, 256 .. 128 chip/bit Traffic 1,2 .. 9,6 kbit/s, 1024 .. 128 chip/bit Pilot |
| Channels Uplink | Access 4,8 kbit/s Traffic as above |
Pilot Channel: Phase and Frame Synchronization
Synchronization Channel: Network Synchronization (Channel Assignment, Efficiency Control)
BS reduces transmitter power in small steps (0,5 dB every 20 ms), until MH notices an increase of the frame error rate and transmits a signal to increase power (Closed Loop).
MH measures the power of the pilot signal and estimates its own efficiency. For a fast improvement of the state of the channel, an open loop (no feedback) at the MH reacts and regulates its own sending power analogly (up to 85dB).
For the compensation of the power at a multi-path fading, a closed loop is used: At the BS, the input power of the MH is measured and compared with a rated value. If necessary, the MH has to change its transmitter power. Step length is 0,5 dB each 1,25 ms.
(An open loop does not suffice to cope with multi-path fading, as the up- and the downlink work in different frequency bands and thus have an asymmetrical fading behaviour.)
(according to [Walke] I 4.3.3)
dissemination (1997)
range (~ number of base stations)
users per space
field experience
costs of the infrastructure
personal assessments
Universal Mobile Telecommunications System, Mobile Radio System of the 3. Generation, supported by R&D programs of the European Community
at ETSI the corresponding technical committee SMG 5
UMTS-Forum (= Signatory of the "MoU of the Introduction of UMTS")
Features
Time Schedule
Air interface(s)
Bearer Services
Tele Services
Additional Services (looking up the telephone number, forwarding of a telephone call, ...)
Added-Value Services (Virtual Home Environment, ...)
include
structured like wireless wide area networks
but in comparison to them
Standards
pushed by the CEPT in the middles 80s
since 1988 ETSI is responsible
1992 formal european standard ETS 300-175
voice and data
voice coding according to ITU-T G.721
| Parameter | Value |
|---|---|
| Frequency (up/down) | 1,88 - 1,9 GHz |
| Time Slots per Time Frame | 12 |
| Modulation | GMSK |
| Channel Distance | 1,728 MHz |
| max. Delay of Interleaving | 40 ms |
| Control Data Rate | 6,4 kbit/s |
| Voice Data Rate | 32 kbit/s |
1990 - IEEE802 Standards Committee: 802.11 Wireless Local Area Networks Standards Working Group
1997 - ready
Type of Networks
contains 3 transmitting techniques
ETSI expert groups RES 10 (Radio Equipment & Systems)
Interface to IEEE 802.11
5 channels 5,15..5,30 GHz
Gross Data Rate for each channel 23,5294 Mbit/s
Net Data Rate 10-20 Mbit/s
Range 50 m at 1 W transmitter power
0,3 GMSK
up to a speed of 36 km/h
PCMCIA devices
Network Types
Channel Access (MAC)
(taken from http://www.wcai.com/lmds.htm )
LMDS is a broadband wireless point-to-multipoint communication system operating above 20 GHz (depending on country of licensing) that can be used to provide digital two-way voice, data, Internet, and video services. The acronym LMDS is derived from the following:
L (local)—denotes that propagation characteristics of signals in this frequency range limit the potential coverage area of a single cell site; ongoing field trials conducted in metropolitan centers place the range of an LMDS transmitter at up to 5 miles
M (multipoint)—indicates that signals are transmitted in a point-to-multipoint or broadcast method; the wireless return path, from subscriber to the base station, is a point-to-point transmission
D (distribution)—refers to the distribution of signals, which may consist of simultaneous voice, data, Internet, and video traffic
S (service)—implies the subscriber nature of the relationship between the operator and the customer; the services offered through an LMDS network are entirely dependent on the operator's choice of business
Standardization
international (within ITU)
european
Companies
(from Heise Online News Archive)
The American Secret Service intercepts emails Europe-wide (09.01.1998)
Espionage in US-High-Tech firms - The Federal Republic of Germany is also accused (13.01.1998)
The European Commission officially does not know anything about ECHELON (02.10.1998)
SORM 2: The Secret Service of Russia and the internet (21.02.1999)
Bugging Operation for Industrial Espionage (11.05.1999)
As the Londoner newspaper "Independent" reports, the manufacturers of mobile phones have indirectly admitted that the user may suffer some risks from their products. Six leading manufacturers, among them Ericsson and Alcatel, had explained on corresponding patent applications that they are intented to reduce health hazards.
(http://www.ix.de/newsticker/data/dz-02.11.98-000/)
[Black] Uyless Black, Mobile and Wireless Networks, Prentice Hall,
Upper Saddle River 1996
especially for the cellular networks, also interpretation for the expert
[Bluetooth] http://www.bluetooth.com/v2/document/default.asp
[BG] Egon Bohländer, Walter Gora, Mobilkommunikation. Technologien
und Einsatzmöglichkeiten, DATACOM-Verlag, Bergheim 1992
Simple introduction, especially GSM
[Glas] Jack Glas (?), The Principles of Spread Spectrum Communications,
http://cas.et.tudelft.nl/~glas/ssc/techn/techniques.html
especially for spread spectrum techniques
[Jones] Derek E. Jones, An Introduction to Wireless Technology, IBM
ITSO Redbook, http://www.redbooks.ibm.com/abstracts/sg244465.html (abstract),
http://www.redbooks.ibm.com/pubs/pdfs/redbooks/sg244465.pdf (whole document)
Introduction for the HF layman
[Rappaport] Theodore S. Rappaport, Wireless Communications. Principles
and Practice, Prentice Hall, Upper Saddle River 1996
Introduction for the HF expert
[Walke] B. Walke, mobile radionetze und ihre Protokolle (2 volumes),
Teubner, Stuttgart 1998
German introduction for the HF expert
[Wesel] E. K. Wesel, Wireless Multimedia Communications. Networking
Video, Voice, and Data, Addison Wesley, Reading MA 1998
Very good introduction into (local)wide-band networks for the HF layman