ECE 4331, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 26 Nov. 19 th , 2009. Outline. Term paper, only journal. For those who did not give me the title on time, I think you would not work for the term project, right? General Wireless System Architecture
Department of Electrical and Computer Engineering
Nov. 19th, 2009
Term paper, only journal. For those who did not give me the title on time, I think you would not work for the term project, right?
General Wireless System Architecture
Media Access Control
Classes of MAC protocols
Simplex and Duplex Channels
Coordinated MAC Schemes
Capacity of TDMA systems and which factors affect the capacity.
Spread Spectrum Access Methods
Case study: Bluetooth
Hybrid Spread Spectrum Schemes.
How can we share a wireless channel:
Results in Wireless Media Access Control Protocols
How we can change base stations: Results in Handoff algorithms and protocols
How can we seamlessly support mobile applications over wireless links:
Results in mobility protocols like Mobile IP, Cellular IP, etc.
How can we design efficient transport protocols over wireless links:
Results in solutions like SNOOP, I-TCP, M-TCP, etc.
How different wireless networks/systems are designed?
Bluetooth, IEEE 802.11, GSM, etc.
Neighbor Discovery and Registration,
Multicasting, Power Saving Modes, Address
Translation (IP-MAC), Routing, Quality of Services,
Medium Access Control, MAC level Scheduling,
Link Layer Queueing, Link Layer Reliability – ACKs,
(Layers 1 and 2 in ISO/OSI Network Reference Model)
Framing and frame synchronization, error control,
CRC, bit scrambling, widening, ….
Carrier frequency, channel bandwidth, carrier detect,
Captude detect, channel data rate, modulation,
Received signal strength (RSSI), transmit power,
Power control, …
Wireless spectrum (frequency band) is a very precious and limited resource.
We need to use this resource very efficiently
We also want our wireless system to have high user capacity
A lot of (multiple) users should be able to use the system at the same time.
For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system.
The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…)
Random Schemes (Less-Coordinated)
Examples: MACA, MACAW, Aloha, 802.11 MAC,…
More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs
Examples: TDMA, FDMA, CDMA
More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,…
Polling based Schemes
Examples: Bluetooth, BlueSky,…
Access is coordinated by a central node
Suitable for Systems that wants low-power, aims to carry voice and data at the same time.
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
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
Frequency separation should be carefully decided
Frequency separation is constant
0 1 2 3 4 5 6 7 …
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
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…
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.
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)
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.
Could be employing one of the following multiple access and duplexing schems
Could be employing of the following multiple access and duplexing schemes
An FDMA channel carriesone 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.
Frequency spectrum allocated for FDMA system
Bt : Total spectrum allocation
Bguard: Guard band allocated at the edge of the spectrum band
Bc : Bandwidth of a channel
AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz
channel bandwidth (simplex): Number of channels is 416.
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.
Requires digital modulation
Downstream Traffic: Forward Channels: (from base to mobiles)
Logical forward channel to a mobile
Base station broadcasts to mobiles on each slot
Upstream Traffic: Reverse Channels: (from mobile to base)
Logical reverse channel from a mobile
A mobile transmits to the base station in its allocated slot
Upstream and downstream traffic uses of the two different carrier frequencies.
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.
One TDMA Frame
One TDMA Slot
A Frame repeats in time
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.
bT: total number of bits in a frame
Tf: frame duration (seconds)
bOH: number of overhead bits
Number of channels in a TDMA cell:
m: maximum number of TDMA users supported in a radio channel
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.
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 s 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.
Capacity can be expressed as
System Capacity (the capacity of the overall system covering a region)
Range of cells
Whether the system can support macro-cells, micro-cells or pico-cells.
Depends on the radio link performance between a base-station and mobiles:
The lowest C/I (carrier-to-interence) ratio the system can operate for example quality of transmission. This in turn depends on the speech coding technique, desired speech quality, etc.
Data-rate over the channel which dependsmodulation efficiency (bits_per_second/Hz) and channel bandwidth.
The frequency re-use factor
SSMA uses signals that have transmission bandwidth that is several orders of magnitued larger than minimum required RF bandwidth.
Immunity to multipath interference
Robust multiple access.
Frequency Hopped Multiple Access (FHMA)
Direct Sequence Multiple Access (DSMA)
Also called Code Division Multiple Access – CDMA
Digital muliple access technique
A wideband radio channel is used.
Same wideband spectrum is used
The carrier frequency of users are varied in a pseudo-random fashion.
Each user is using a narrowband channel (spectrum) at a specific instance of time.
The random change in frequency make the change of using the same narrowband channel very low.
The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized.
Rate of hopping versus Symbol rate
If hopping rate is greather: Called Fast Frequency Hopping
One bit transmitted in multiple hops.
If symbol rate is greater: Called Slow Frequency Hopping
Multiple bits are transmitted in a hopping period
GSM and Bluetooth are example systems
Coarse estimate of the reverse link (uplink) capacity
The interference caused by other users in the cell can be modeled as AWGN.
Perfect power control is used, i.e. the received power of each user at the base station is the same.
If the received power of each user is Ps watts, and the background noise can be ignored (ex: microcells), then the total interference power (MAI) at the output of the desired user’s detector is
where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as:
Uses Frequency Hopping in cell (piconet) over a 79 MHz wideband radio channel.
Uses 79 narrowband channels (carrier frequencies) to hop through.
Freq (f) = 2402+k MHz, k = 0,...,78
Channel spacing is 1 MHz (narrowband channel bandwidth)
Wideband spectrum width = 79 MHz.
Hopping Rate = 1600 Hops/Second
Hopping sequence is determined by Bluetooth Hardware address and Clocks that are syncrozied between sender and receiver
A hop sequence could be: 7,1,78,67,0, 56,39,.......
Each node is classified as master or slave.
Master defines a piconet (a cell). Maximum 7 slaves can be connected to
a master. Master coordinates access to the the media.
All traffic has to go over master.
Slaves can not talk to each-other
Range = 10m
Raw Data-rate: 1 Mbps/piconet
Radio channel used by devices in
a piconet is 79MHz channel, which
Is frequency hopped: hopping
though 789 channels.
Hoprate = 1600 hops/sec
All slaves and the master hops according to the same hopping sequence.
The hopping sequence is determined by the clock and BT_address of the master.
Piconet can be combined
Red slave acts as a
bridge between two
Each piconet uses FHSS with different
hopping sequences (masters are different).
This prevents interference between piconets.
Inside a piconet, access to the
frequency hopped radio channel
is coordinated using time
division multiple access: TDMA/TDD.
Slot duration = 1/1600 sec = 625ms
In an even slot, master transmits to a
In an odd slot, the slave that is addressed
in the previous master-to-slave slot transmits.
0 1 2 3 4 5 6 7 …..
Logical Link Control
Medium Access (MAC)