Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

Eng. Maths.mobile unit
cell site
Radio & Telecommunications Systems
antenna system
Cell site provides interface between the MTSO and the mobile units, it has
control unit
radio cabinets
*
MTSO is the switching office, coordinating element for all cell sites, contains
cellular processor
cellular switch
controls call processing
handles billing activities
each mobile unit can only use one channel at a time for its communication link; channel is not fixed, can lie any one in the entire band assigned by the serving area
Propagation
incident angle of the direct wave is q1 and the incident angle of the reflected wave is q2, q1 is also called the elevation angle (Fig. 2)
C is inversely proportional to R4
C µ R -4 = a R -4
where
a = is a constant
Fading
antenna of the mobile unit is lower than its typical surroundings
multi-path waves are generated
at the mobile unit
the sum of the multi-path waves causes a signal-fading phenomenon (Fig. 3)
*
signal fluctuates about 40 dB (10 dB above and 30 dB below the average signal)
if the mobile unit moves fast, the rate of fluctuation is fast
Rayleigh fading is also called multi-path fading in the mobile radio environment (Fig. 4)
multi-path waves bounce back and forth due to the buildings and houses
summed together and become an irregular way fading structure
radio channel consists of a pair of frequencies
one for each direction of transmission
for full-duplex operation
used in one geographic zone to call a cell C1
coverage radius R
can be used in another cell
with the same coverage radius at a distance D away (Fig. 1)
simultaneously use the same frequency channel
increase the spectrum efficiency
serious interference may occur
interference due to the common use of the same channel
called co-channel interference
Frequency reuse distance
minimum distance which allows the same frequency to be reused depend on
number of co-channel cells in the vicinity of the center cell
type of geographic terrain contour
antenna height
D = (3K)1/2R
R is the radius of cell
assume all the cell sites transmit the same power
if K increases
increases D => reduces cochannel interference
the total number of allocated channels is fixed
when K is too large
number of channels assigned to each of K cells becomes small
smallest value of K is 3, obtained by setting
i = 1, j = 1 then
K = i2 + ij +j2
Cochannel Interference Reduction Factor
to find the minimum frequency reuse distance in order to reduce cochannel interference
co-channel interference is a function of a parameter q defined as:
q = D / R
the ratio q increases, cochannel interference decreases
user activates the receiver of the mobile unit
the receiver scans set-up channels
selects the strongest and locks on for a certain time
each site is assigned a different set-up channel
Mobile originated call
user places the called number into an originating register in the mobile unit
request for service is sent on a selected set-up channel obtained from a self-location scheme
*
cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use
cell site sends a request to the mobile telephone switching office (MTSO)
MTSO selects an appropriate voice channel for the call
the cell site acts on it through the best directive antenna to link the mobile unit
the MTSO also connects the wire-line party through the telephone company zone office
telephone company forwards the call to the MTSO
MTSO sends a paging message to certain cell sites based on the mobile unit number
each cell site transmits the page on its own set-up channel
mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site
mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert
a particular signal transmits to the cell site
both sides free the voice channel
mobile unit resumes monitoring pages through the strongest set-up channel
during the call, two parties are on a voice channel
mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak (Fig. 1)
present cell site requests a handoff
the system switches the call to a new channel in a new cell site without either interrupting the call or alerting the user
call continues as long as the user is talking
CS: Cell Site
Mobile Assisted Handoff (MAHO)
the mobile receiver is capable of monitoring the signal strength of the setup channels of the neighboring cells while serving a call
Soft Handoff
applied to CDMA systems
all cells use the same radio carrier, change from one code to another code
Most portable phones are divided into two parts: (Fig. 1)
RF part
digital part
takes care of the data processing, control, and signaling functions
1157.bin
antenna combiner
couples the receiving and the transmitting paths onto the single antenna connector or a fixed antenna
receiver
contains the front end, a receiving filter network, and a mixer to down-convert the input signal onto an IF that is eventually converted into the data domain by the ADC
equalizer
due to multipath propagation and other reflections, the signals arriving at the receiver are distorted
the equalizer compensate distortions
demultiplexer
sorts the received information from the different time slots and frames
channel codec
compresses digitized speech coming from the ADC before being encoded
minimizes bandwidth requirement
multiplexer
assigns each individual burst to a time slot within a numbered frame
modulator
transmitter
an amplifier increases the level of the signal
output filters limit the bandwidth of the output to its assigned channel
synthesizer
provides the internal timing references for the bit and frame clock as well as for the RF sources in the transmitter and the receiver
voltage controlled oscillator (VCO) provides a stable operating frequency
The general structure of a base station consists of
base station control function (BCF)
one to sixteen transceivers (TRX)
receiver
contains the receiving filter which blocks frequencies other than the desired receiving band
the signals are down-converted to an IF or directly to the baseband frequency, where the signals are sampled and quantized with an ADC
equalizer
demodulator
extracts the bit stream from the equalized signal and passes it to the demultiplexer
channel codec
detect errors that have been introduced into the RF path and correct them
speech codec
signaling unit
the logical interface for the control messages between the network and the mobile stations
performs all the internal control tasks of the base station
multiplexer
maps the single bursts onto the single time slots bound for the individual mobile stations
modulator
contains the output filters to band-limit the signals
controls the output level depending on the base station's power class
synthesizer
provides the necessary frequencies for the different entities in the BTS (Base Transceiver Station)
usually synchronized with the clock from the BSC (Base Station Controller)
alternatively, it is possible to have a local clock reference in each base station
poor system performance (high hand-off failure rate) & low reliability due to sync problem
Frequency Division Multiple Access
multiple access scheme for land mobile communication systems (Fig. 1 - 3)
analog cellular systems use FDMA
assigned system bandwidth is divided into bands with its bandwidth of Wch
guard space to prevent spectrum overlapping
each user sends a call request to the BS
BS assigns one of the unused channels to the user
channel is used exclusively by that user during a call
when the call is terminated, the channel is reassigned to a different user
Time Division Multiple Access
enables users to access the assigned bandwidth on a time basis (Fig. 4-6)
each channel occupies the whole system bandwidth
occupies only a fraction of the time, called slot, on a periodic basis
one frame consists of Nch slots
frame length is T second
Uplink
each terminal transmits information using an assigned slot in each frame
each terminal has to transmit its slot exactly in the assigned slot timing to prevent signal collisions
Downlink
Code Division Multiple Access
all the transmitted signals other than the desired signal are regarded as cochannel interference (CCI) signals (Fig. 7 - 9)
at the CDMA receiver
the desired signa1 can be picked up by
taking correlation between the received signal and a code used at the transmitter (code#1)
*
spreading code sequence for the receiver and that used at the transmitter are synchronized
the resultant signal spectrum becomes the same as that for the source signal
signal bandwidth of the interference signals still remains the same bandwidth even after taking correlation
when each user sends a call request to the BS
BS assigns one of the spreading codes to the user
Seiichi Sampei, “Applications of Digital Wireless Technologies to Global Wireless Communications,” Prentice-Hall, 1997
Siegmund M. Redl, et. Al., “An Introduction to GSM,” Artech House, 1995
Lee, William C. Y., “Mobile cellular telecommunications : analog and digital systems,” McGraw-Hill, 1995
http://www.radiodesign.com/cellwrks.htm

Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle.

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