EELE 5490, Fall, 2009EELE 5490, Fall, 2009
Wireless CommunicationsWireless Communications
Ali S. Afana
Department of Electrical Engineering
Class 10
Dec. 20h, 2009
Duplexing
It is sharing the media between two parties.
If the communication between two parties is one way, the it is called simplex communication.
If the communication between two parties is two- way, then it is called duplex communication.
Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver.
Duplex communication achieved by: – Time Division (TDD)
– Frequency Division (FDD)
– Some other method like a random access method
Duplexing
Usually the two parties that want to communication in a duplex manner (both send and receive) are: – A mobile station
– A base station
Two famous methods for duplexing in cellular systems are: – TDD: Time Division Duplex
– FDD: Frequency Division Duplex
Duplexing - FDD
A duplex channel consists of two simplex channel with different carrier frequencies– Forward band: carries traffic
from base to mobile– Reverse band: carries traffic
from mobile to base
Reverse Channel
Forward Channel
frequencyfc,Rfc,,F
Frequency separation
Frequency separation should be carefully decidedFrequency separation is constant
B MF
RBase Station
MobileStation
Duplexing - TDD A single radio channel (carrier frequency) is
shared in time in a deterministic manner. – The time is slotted with fixed slot length
(sec)– Some slots are used for forward channel
(traffic from base to mobile)– Some slots are used for reverse channel
(traffic from mobile to base)
B M
Base Station
MobileStation
F R R R R
0 1 2 3 4 5 6 7 …
….
Reverse Channel
Forward Channel
timeTi
Time separation
Ti+1
channel
Slot number
F F F
Duplexing – TDD versus FDD
FDD– FDD is used in radio systems that can allocate individual radio
frequencies for each user. For example analog systems: AMPS
– In FDD channels are allocated by a base station. – A channel for a mobile is allocated dynamically– All channels that a base station will use are allocated usually statically. – More suitable for wide-area cellular networks: GSM, AMPS all use FDD
TDD – Can only be used in digital wireless systems (digital modulation). – Requires rigid timing and synchronization– Mostly used in short-range and fixed wireless systems so that
propagation delay between base and mobile do not change much with respect to location of the mobile.
Such as cordless phones…
Multiple Access - Coordinated
We will look now sharing the media by more than two users.
Three major multiple access schemes– Time Division Multiple Access (TDMA)
Could be used in narrowband or wideband systems– Frequency Division Multiple Access (FDMA)
Usually used narrowband systems– Code Division Multiple Access
Used in wideband systems.
Narrow- and Wideband Systems
Narrowband System– The channel bandwidth (frequency band allocated for the
channel is small) More precisely, the channel bandwidth is large
compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel)
AMPS is a narrowband system (channel bandwidth is 30kHz in one-way)
Wideband Systems– The channel bandwidth is large
More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel.
A large number of users can access the same channel (frequency band) at the same time.
Narrow- and Wideband Systems
Narrowband Systems– Could be employing one of the following multiple access
and duplexing schems FDMA/FDD TDMA/FDD TDMA/TDD
Wideband systems– Could be employing of the following multiple access and
duplexing schemes TDMA/FDD TDMA/TDD CDMA/FDD
CDMA/TDD
Cellular Systems and MAC
Cellular System Multiple Access Technique
AMPS FDMA/FDD
GSM TDMA/FDD
USDC (IS-54 and IS-136) TDMA/FDD
PDC TDMA/FDD
CT2 Cordless Phone FDMA/TDD
DECT Cordless Phone FDMA/TDD
US IS-95 CDMA/FDD
W-CDMA CDMA/FDD
CDMA/TDD
cdma2000 CDMA/FDD
CDMA/TDD
Frequency Division Multiple Access
Individual radio channels are assigned to individual users
Each user is allocated a frequency band (channel)– During this time, no
other user can share the channel
Base station allocates channels to the users
B
M M M…
f1,F f2,F fN,F
fN,Rf2,Rf1,R
Features of FDMA
An FDMA channel carries one phone circuit at a time
If channel allocated to a user is idle, then it is not used by someone else: waste of resource.
Mobile and base can transmit and receive simultaneously
Bandwidth of FDMA channels are relatively low.
Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low)
Lower complexity systems that TDMA systems.
Capacity of FDMA Systems
…
Frequency spectrum allocated for FDMA system
Guard Band
Guard Band
channel
c
guardt
B
BBN
2
Bt : Total spectrum allocationBguard: Guard band allocated at the edge of the spectrum bandBc : Bandwidth of a channel
AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz channel bandwidth (simplex): Number of channels is 416.
Time Division Multiple Access
The allocated radio spectrum for the system is divided into time slots– In each slot a user can transmit or receive
– A user occupiess a cyclically repeating slots.
– A channel is logically defined as a particular time slot that repeats with some period.
TDMA systems buffer the data, until its turn (time slot) comes to transmit. – This is called buffer-and-burst method.
– Leaky bucket
Requires digital modulation
TDMA Concept
1 2 3 … N 1 2 3 …. N …
1 2 3 … N 1 2 3 …. N …
Downstream Traffic: Forward Channels: (from base to mobiles)
Upstream Traffic: Reverse Channels: (from mobile to base)
Logical forward channel to a mobile
Base station broadcasts to mobiles on each slot
A mobile transmits to the base station in its allocated slotLogical reverse channel from a mobile
Upstream and downstream traffic uses of the two different carrier frequencies.
TDMA Frames
Multiple, fixed number of slots are put together into a frame.
A frame repeats.
In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels.
In TDMA/FDD: a different carrier frequency is used for a reverse or forward
– Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa).
– Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame.
General Frame and Time Slot Structure in TDMA Systems
Preamble Information Trail Bits
Slot 1 Slot 2 …Slot 3 Slot N
Guard Bits
Sync Bits
CRCInformation
One TDMA Frame
One TDMA Slot
A Frame repeats in time
Control Bits
A TDMA Frame
Preamble contains address and synchronization info to identify base station and mobiles to each other
Guard times are used to allow synchronization of the receivers between different slots and frames– Different mobiles may have different propagation delays to a base
station because of different distances.
Efficiency of a Frame/TDMA-System
Each frame contains overhead bits and data bits.– Efficiency of frame is defined as the percentage of data
(information) bits to the total frame size in bits.
xRTb
xb
befficiency
fT
T
OHf
%100)1(
bT: total number of bits in a frameTf: frame duration (seconds)bOH: number of overhead bits
Number of channels in a TDMA cell:
c
guardtot
B
BBmN
)2(
m: maximum number of TDMA users supported in a radio channel
TDMA
TDMA Efficiency
– GSM: 30% overhead
– DECT: 30% overhead
– IS-54: 20% overhead.
TDMA is usually combined with FDMA
– Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other.
– There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels)
For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA.
Contemporary TDMA SystemsGSM
(Europa)
IS-54
(USA)
PDC
(Japan)
DECT
(European Cordless)
Bit Rate 270.8 Kbps 48.6 Kbps 42 Kbps 1.152 Mbps
Bandwidth 200 KHz 30 KHz 25 KHz 1.728 MHz
Time Slot 0.577 ms 6.7 ms 6.7 ms 0.417 ms
Upstream slots
per frame
8 3 3 12
Duplexing FDD FDD FDD TDD
Efficiency of Time Slots 73 % 80 % 80 % 67 %
Modulation GMSK /4 DQPSK /4 DQPSK GMSK
Adaptive Equalized Mandatory Mandatory Optional None
Features of TDMA
Enables the sharing of a single radio channel among N users
Requires high data-rate per radio channel to support N users simultaneously.
– High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread parameter)
Transmission occurs in bursts (not continues)– Enables power saving by going to sleep modes in unrelated slots
– Discontinues transmission also enables mobile assisted handoff
Requires synchronization of the receivers. – Need guard bits, sync bits. large overhead per slot.
Allocation of slots to mobile users should not be uniform. – It may depend on the traffic requirement of mobiles.
– This brings extra flexibility and efficiency compared to FDMA systems.
Questions?Questions?
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