Ονοματεπώνυμα φοιτητών: Άγγελοσ Αρχοντοβαςίλησ & Αντώνησ Πρελορέντζοσ Διδάσκων: Γ. Πολύζοσ Επιβλέπων: Β. Δούροσ Αςύρματα Δίκτυα και Κινητέσ
Spectrum (Licensed - Unlicensed) The Problem
White Spaces
Cognitive Radio Networks (Definitions) Spectrum Management
Spectrum Sensing Spectrum Access
CRN MAC Protocols (Infrastructure used – Ad Hoc) Conclusion
Future Directions References
Licensed Spectrum
◦ Operation of a wireless transmitter over particular frequencies
according to an authorization
◦ Spectrum licenses come with a frequency assignment
◦ Applications: Mobile telephony, GPRS
Unlicensed Spectrum
◦ Operation of a wireless transmitter at particular frequencies without
authorization
◦ Predefined rules to mitigate interference
Spectrum is assigned to users with a license on a long term basis
normally for huge regions like whole countries
◦ Primary users = rightful owners of a spectrum portion (LU - licensed)
◦ Secondary users = users who access the spectrum opportunistically
Advantages
◦ Exclusive access to spectrum in well-defined areas
◦ Allow transmission in high power levels
◦ Protection from interference
Disadvantages
◦ Long payback time on infrastructure
◦ High Prices
Advantages
◦ Low or no cost for spectrum access
◦ Allowing multiple users operate at the same frequencies
◦ Makes easier the participation of new or small businesses
Disadvantages
◦ no guarantee performance
◦ limited QoS
Increasing demand for radio spectrum
Much of the spectrum is idle for a period of time and at
large numbers of locations
White space (or spectrum hole)
◦ Is a band wider than 1Mhz that remains unoccupied for 10 minutes
or longer
◦ CR technology enables their identification and use
Definition
◦ Cognitive radio is an intelligent wireless communication system
periodically monitors the radio spectrum
detects occupancy in the different parts of the spectrum opportunistically communicates over white spaces
◦ The idea of CR was first presented officially in an article by Joseph Mitola and Gerald Maguire in 1999
Objective
◦ sense the spectral environment over a wide bandwidth
◦ find the best available spectrum
◦ If a band is used by a licensed user, CR
moves to another white space or
stays in the same band changing one or more of communication parameters to avoid interference
multihop architecture distributed operation
no infrastructure support dynamic network topology diverse QoS requirements CR users are mobile
Spectrum sensing
◦ Determine which portions of the spectrum is available and detect the presence of licensed users when a user operates in a licensed band
Spectrum decision
◦ Select the best available channel Spectrum sharing
◦ Coordinate access to this channel with other users Spectrum mobility
Cooperation
◦ CR users determine their actions based on observed information exchanged with their neighbors
improve accuracy, fair sharing, PU interference
Common control channel (CCC)
◦ Spectrum management functions rely on exchanging information between CR users over a common control channel
◦ In-Band CCC
local coverage
◦ Out-of-Band CCC
global coverage
PU Detection
◦ energy detection
sense presence/absence based on received signals energy
(+) easy to implement
(–) cannot differentiate signal types
◦ feature detection
sense presence/absence by extracting specific features
(+) most effective scheme for CRAHNs
(–) computationally complex, long sensing time
Sensing Control
◦ controller coordinates PU detection
How quickly a CR user can find the available spectrum band
How long and how frequently a CR user should sense the spectrum
Challenges [4]
◦ Support of Asynchronous Sensing
Objective
◦ choose the best spectrum band according to the QoS requirements & spectrum characteristics
Functionalities
◦ Spectrum Characterization
received signal strength, interference, user number
◦ Spectrum Selection
allocate the best spectrum band (QoS) available
◦ Routing Protocol
switch the spectrum or not?
◦ Reconfiguration
Objective
◦ Maintain QoS for SU without interfering to the PU
Functionalities
◦ Resource Allocation
channel selection and power allocation without interference
◦ Spectrum Access
coordination of access in order to avoid collisions random access, time slotted, hybrid
◦ Spectrum Sensing Support
PU transmission distinguished from other CR users
Challenges
◦ Topology Discovery
Objective
◦ change the SU frequency if a PU requires the specific portion
Functionalities
◦ Spectrum Handoff
starts with link failure: PU activity or quality degradation
users transfer their connections to an unused band
◦ Connection Management
sustain the QoS during spectrum switching
avoid temporary disconnection
Challenges
◦ Switching Delay Minimization
Sensing Periodicity
◦ Periodically sense the band in case a PU transmits
◦ Sensing has to be interleaved with data transmission
◦ Sensing period: maximum time of SU unawareness delay
Detection Sensitivity
◦ The minimum SNR at which the primary signal may still be
accurately detected by the CR
◦ Interference causes SIR to fall
harmful: If SIR falls below a certain threshold
◦ Strong dependency between the detection sensitivity and the
maximum power it is allowed to transmit in a licensed band
◦ We should be able to manage the total interference according to the
Channel Uncertainty
◦ Channel fading and shadowing
◦ CRs have to distinguish a faded or shadowed primary signal from a white space
Noise Uncertainty
◦ Limited accuracy on noise power estimation
calibration errors
thermal noise changes
◦ calculation of detection sensitivity with the worst case noise assumption
Aggregate-Interference Uncertainty
◦ multiple cognitive radio networks operating over the same licensed band
◦ energy detection: nearby CR Networks sense each other and avoid simultaneous transmission
◦ system-level coordination among CR networks overcomes uncertainty at increased implementation cost
Pros and Cons
(+) higher detection sensitivity
users employ less sensitive detectors
overcomes channel uncertainty
(–) additional communication overhead
band manager collects measurements
broadcast decision to all SU
control channel needed
Multiple CR users share the spectrum resource by
determining who will access the channel, or when a user accesses the channel
There are 3 types of access protocols:
Random access protocols
- No need time synchronization
- Based on the carrier sense multiple access with collision avoidance (CSMA/CA) principle
Time slotted protocols
- Need of network-wide synchronization, where time is divided into slots for both the control channel and the data transmission
Hybrid protocols
Interaction between the network and transport layers
with the link layer
Cooperation among the different users Research challenges:
i) Control channel design ii) Adaptation to PU
Random access protocols (CSMA)
-
Ensures coexistence among the CR users and the PUs by adapting the transmission power and rate of the CRnetwork
- CR users and PUs establish direct single-hop connections with their respective base stations
- Simultaneous transmission of the CR users are allowed even when the PUs are detected
- CR users have a longer carrier sensing time (τs, where τs >> τp)
Time slotted protocols (802.22)
- Centralized standard that uses base stations for spectrum access and sharing
- TDM in the downstream (DS) direction - TDMA in the upstream (US) direction
- Each of DS and US is composed of multiple MAC frames preceded by the frame preamble
- Superframe control header (SCH) is used to inform the CR users of the current available channels, different
bandwidths supported and future spectrum access time, among others
Spectrum sensing support
- Two stage sensing (TSS) mechanism:
i) Fast sensing
ii) Fine sensing
Spectrum Recovery
- IDRP (Incumbent Detection Recovery protocol)
used
Coexistence with users
Random access protocols (DOSS MAC)
- Dynamic combination of available bands for better
performance
- Three radios are assigned distinctly to the control, data and busy tone band
- It consists of the following steps:
i) PU detection (continuously vicinity monitoring) ii) Set-up of three operational frequency bands
(traffic limiting, BW radio setting, control channel migration) iii) Spectrum Mapping
Time slotted protocols (C-MAC)
- Two key concepts:
The rendezvous channel (RC), and the backup channel (BC) - RC can be used for the longest time throughout the network,
without interruption among all other available choices - BC is used to immediately provide a choice of alternate
spectrum bands in case of the appearance of a PU
- Each spectrum band has recurring superframes composed of a beacon period (BP) and a data transfer period (DTP)
-
RC is used on a network-wide communication, neighbor discovery, and sharing of load information for each band Distributed Beaconing (re-broadcasting received beacon
information)
Inter-channel coordination (periodical beacon transmission) Coexistence (quiet periods QP detect PUs from CR users)
Infrastructure-based networks
Dynamic Spectrum Access (DSA) is proposed - Game theoretic protocol
Ad-hoc networks
Opportunistic Spectrum MAC (OS-MAC)
- Pre-determined window periods use for coordinating the choice of spectrum among the CR users and exchanging control information to separate the latter into groups - Random Spectrum Access within window
Cognitive radio networks
◦ may solve current wireless network problems resulting from the
limited available spectrum
◦ create a new class of users who intelligently share spectrum when it
it’s idle
Spectrum Sensing
◦ has performance limitations by the uncertainties at various levels
◦ is a multifaceted problem demanding coordinated efforts of the
regulatory and technical sides
e.g. Cooperative Sensing which requires flexible policy
Spectrum Management
◦ management functions help with interaction among CR users
◦ cooperation among multiple users ensures protection to PU and
An overview of the state of the art for medium access
protocols in cognitive radio networks
Spectrum management, spectrum sensing and spectrum
access were discussed
There is further work needed in devising accurate models
that account for false alarm and missed detection probabilities in one framework
The simplified ON/OFF PU traffic model may not be
suitable in a practical environment where the licensed users may be cellular, contention-based, or have other possible access technologies
1. The information from multiple layers must be seamlessly
integrated in the working of the MAC protocol
2. Completely integration of the sensing function with more
accuracy
3. Significant scope for devising protocols that adapt the CR
transmissions based on the type of the interferer
4. Newer performance metrics that capture the CR specific
improvements should be devised and used for evaluating the different MAC protocols
CRs are an open area of research with industrial
and academic interest for the next few years
[1] Spectrum Management in Cognitive Radio Ad Hoc Networks,
Akyildiz, I.F. and Lee, W.Y. and Chowdhury, K.R., IEEE Network, 2009
[2] Spectrum sensing in cognitive radio networks: requirements,
challenges and design trade-offs, Amir Ghasemi, Elvino S. Sousa, IEEE Communications Magazine, 2008
[3] A survey on MAC protocols for cognitive radio networks,
Claudia Cormio, Kaushik R. Chowdhury, School of Electrical and Computer Engineering, Georgia Institute of Technology, 2009
[4] Sliding-Window Algorithm for Asynchronous Cooperative
Sensing in Wireless Cognitive Networks, Chengqi Song, Qian
