Mobile RF Communication
Mobile RF Communication
• Cellular System
• Co-Channel Interference
• Handoff
• Path loss and Multipath fading
• Diversity
• Delay spread
• Interleaving
Mobile RF Communication
Introduction
• A mobile system is one of the which users can physically move while communicating with
one another.
– Pagers, cellular phones and cardless phones
• The transceiver carried by the user is called the “mobile unit” or “terminal” or “handheld
unit”
• The complexity of the wireless infrastructure often demands that the mobiles communicate
only through a fixed, relatively expensive unit called the “base station”
• Each mobile receives and transmits information from and to the base station via two RF
channels called the “forward channel ” or “downlink” and the “reverse channel ” or uplink”
respectively.
Cellular System
• With the limited available spectrum (25MHz –
900MHz) hundreds to thousands of people
communicate, how this is possible?
• Thousands of FM Radio broadcasting stations
may operate in a country in the 88MHz – 108MHz
band, this is possible because stations that are
physically far enough from each other can use the
same carrier frequency.
• This is called “frequency reuse”
• The minimum distance between two stations
that can employ equal carrier frequencies
depends on the signal power produced by
each.
• In mobile communications, the concept of
frequency reuse is implemented in a “cellular”
structure, where each cell is configured as a
hexagon and surrounded by six other cells.
• The center cell uses a frequency f1 for
communication, the six neighboring cells
cannot utilize this frequency but the cells
beyond the immediate neighbors may use.
• The mobile units in each cell are served by a
base station, and all of the base stations are
controlled by a “mobile telephone switching
office” (MTSO)
Co-Channel Interference
• An important issue in a cellular system is how
much two cells that use the same frequency
interfere with each other called co-channel
interference (CCI).
• This effect depends on the ratio of the
distance between two co-channel cells to the
cell radius and is dependent of the
transmitted power.
Handoff
• What happens when a mobile unit “roams” from
cell A to cell B?
• The power received from the base station in cell
A is insufficient to maintain communication, the
mobile must change its server to the base station
cell B.
• Since adjacent cells do not use the same group of
frequencies, the channel must also change this is
called handoff.
• This process is performed by the MTSO.
• To improve the handoff process, second
generation cellular systems allow the mobile
unit to measure the received signal level from
different base stations.
Path loss and Multipath fading
• Signals propagating through free space
experience a power loss proportional to the
square of the distance, d, form the source.
• In reality, signal travels through both a direct
path and an indirect (reflective) path.
• The actual loss profile may be proportional to
the 𝑑 2 for some distance and 𝑑 4 for another.
Diversity
• The effect of fading can be lowered by adding
redundancy to the transmission or reception
of the signal.
• Space diversity or antenna diversity
• Frequency diversity
• Time diversity
Delay Spread
• Suppose two signals in a multipath
environment experience roughly equal
attenuation but different delays.
Interleaving
• Interleaving is a process or methodology to make a
system more efficient, fast and reliable by arranging
data in a noncontiguous manner. There are many uses
for interleaving at the system level, including:
– Storage: As hard disks and other storage devices are used
to store user and system data, there is always a need to
arrange the stored data in an appropriate way.
– Error Correction: Errors in data communication and
memory can be corrected through interleaving.
– Multi-Dimensional Data Structures
Multiple Access Techniques
• Time and Frequency Division Duplexing
(TDD and FDD)
• Frequency Division Multiple Access
• Time Division Multiple Access
• Code Division Multiple Access
– Direct Sequence CDMA
– Frequency Hopping
Time Division Duplexing (TDD)
• Example Walkie-Talkies
• The user would press the “talk” button to
transmit while disabling the receive path and
release the button to listen while disabling the
transmit path.
• This is a simple form of “time division duplexing”
(TDD).
• Same frequency band is utilized for both transmit
and receive paths, but the system transmit for
half of the time and receives for other half.
Time Division Duplexing (TDD)
Frequency Division Duplexing (FDD)
• FDD employ two different frequency bands for
the transmit and receive paths.
• This technique incorporates bandpass filters
to isolate the two paths, allowing
simultaneous transmission and reception.
• Two transceivers cannot communicate
directly, TX band must be translated to the RX
band at some point and this translation is
achieved by Base station.
Frequency Division Duplexing (FDD)
• FDD is an older scheme that was best suited for applications - such as voice - that
generate symmetric traffic, while TDD is best suited for bursty, asymmetric traffic,
such as internet or other data- centric services.
• In TDD, both the transmitter and receiver operate on the same frequency but at
different times. Therefore, TDD systems reuse the filters, mixers, frequency
sources and synthesizers, thereby eliminating the complexity and costs associated
with isolating the transmit antenna and the receive antenna.
• An FDD system uses a duplexer and/or two antennas that require spatial
separation and, therefore, cannot reuse the resources. The result is more costly
hardware.
• TDD utilizes the spectrum more efficiently than FDD. FDD cannot be used in
environments where the service provider does not have enough bandwidth to
provide the required guard band between transmit and receive channels
Frequency Division Multiple Access
(FDMA)
• Frequency band is partitioned into many
channels, each of the channels assigned to one
user.
• Example radio and television broad casting
(where channel assignment does not changes
with time).
• In Mobile communication two channel will be
assigned to user one for TX and one for RX.
• After user hangs up the phone, the channel
becomes available to others.
Frequency Division Multiple Access
(FDMA)
Time Division Multiple Access (TDMA)
• Same band is available to each user, but at
different time.
• TDMA periodically enables each of the
transceivers for a time slot (Ts1).
• The overall period consisting of all of the time
slot is called a frame (Tf).
Time Division Multiple Access (TDMA)
Time Division Multiple Access (TDMA)
• What happens to the data produced (voice) by all
other users while only one user is allowed to
transmit?
• To avoid loss of information, the data is stored
(buffered) for Tf-Ts1 seconds and transmitted as a
burst during one time slot.
• Since buffering requires the data to be in digital
form, TDMA TX perform A/D conversion on the
analog input signals, allows speech compression
and coding.
Code Division Multiple Access
(CDMA)
• In CDMA different languages are created by
means of “orthogonal digital codes”.
• At the beginning of communication, a certain
code is assigned to each TX/RX pair, and each
bit of the baseband data is “translated” to that
code before modulation.
Code Division Multiple Access
(CDMA)
Direct Sequence CDMA
• A method of generating orthogonal codes is
based on Walsh’s recursive equation.
• Where /Wn is derived from wn.
Code Division Multiple Access
(CDMA)
Frequency Hopping CDMA
Wireless Standards
• Advanced Mobile Phone Services (AMPS)
• North American Digital Standard (NADC)
• Global System for Mobile Communication
(GSM)
• Qualcomm CDMA
• Digital European Cordless Telephone
Advanced Mobile Phone Service
North American Digital Standard
North American Digital Standard
Global System for Mobile
Communication (GSM)
Global System for Mobile
Communication (GSM)
Qualcomm CDMA
Digital European Cordless Telephone
