Cognitive Radio Networks - Startseite TU Ilmenau · PDF fileIntegrated HW&SW Systems Group Cognitive Radio Networks 2 Outline • Introduction • Cognitive Radio Technology − Spectrum

Integrated HW&SW System Group

Cognitive Radio Networks

Prof. Dr.-Ing. habil. A. Mitschele-ThielDipl.-Ing. Ali Diab

Integrated HW&SW Systems Group

Cognitive Radio Networks 2

Outline

• Introduction

• Cognitive Radio Technology− Spectrum Sensing− Spectrum Management− Spectrum Mobility− Spectrum Sharing

• Cognitive MAC Protocols

• IEEE Standards Activities

• Conclusions

• Control Questions

• References



Introduction



Introduction

• Limited spectrum• Inefficiency in spectrum usage

– Utilization of the assigned licensed spectrum varies between15% and85%

• A new technology is required to enable better utilization of unusedlicensed spectrum

Cognitive radio technology



Cognitive Radio Technology



What is a Cognitive Radio

• Definitions– Federal Communications Commission Definition: a cognitive

radio is a radio that can change its transmitter parameters based oninteraction with the environment in which it operates

– Mitola Definition: the cognitive radio identifies the point at whichwireless PDAs and the related networks are sufficientlycomputationally intelligent on the subject of radio resources andrelated computer-to-computer communications− to detect user communications needs as a function of use context, and− to provide radio resources and wireless services most appropriate to

those needs



What is a Cognitive Radio

• Characteristics– Cognitive capability: the capability of the radio technology of

capturing or sensing information from its radio environment(monitoring the power in some frequency band)

– Reconfigurability: the ability of the radio to be dynamicallyprogrammable according to the radio environment (transmission andreceipt of data on a variety of frequencies and using various radioaccess technologies)



What Should Cognitive Radio Enable to Do?

• Determination of unused portion ofspectrum (spectrum holes or whitespace)

• Selection of the best availablechannel

• Coordination of the access with otherusers

• Detection of the appearance oflicensed users vacate the channel



Cognitive Radio Cycle

Spectrum Decision

Spectrum Sharing

Spectrum Sensing

Spectrum Mobility

Radio environment

Spectrum characterizations

RFstimuli

Spectrum holeChannel

capacity

Transmittedsignal

Decisionrequest

Primary userdetection



Cognitive Radio Network Architecture



Cognitive Radio Network Architecture

• Primary networks– Networks with access right to certain spectrum bands, e.g. common

cellular systems and TV broadcast networks– Users of these networks are referred to as primary users. They have

the right to operate in licensed spectrum– Users of certain primary network do not care of other primary or

secondary networks users

• Secondary networks– Do not have license to operate in the spectrum band they currently

use or aim at using– Opportunistic spectrum access– Users of these networks are referred to as secondary users. They

have no right to access licensed bands currently used– Additional functionalities are required to share licensed spectrum

bands with other secondary or primary networks



Spectrum Sensing



Spectrum Sensing

• Spectrum sensing is determined as the capability of detection of spectrum holes

Spectrum Sensing

TransmitterDetection

Cooperative Detection

Interference- Based

Detection

Matched FilterDetection

EnergyDetection

Cyclostationary Feature Detection



Transmitter Detection

• Location of primary receiver is not known (no signaling betweenprimary and secondary users) detection of the weak signalfrom a primary transmitter through local observations ofsecondary users

• Mechanisms− Matched filter detection− Energy detection− Cyclostationary feature detection




− Matched filter detection− Optimal detector when all information of the primary user signal is

known (it maximizes the received signal-to-noise ratio)− Produces poor performance if primary users information not known

− Energy detection− Used if primary users information not known− Output of bandpass filter with bandwidth W is squared and

integrated over observation interval T. After that, a comparison witha threshold Y is achieved to decide if primary users exist

− Cyclostationary feature detection− Mean and autocorrelation of modulated signals exhibit periodicity

(modulated signals are coupled normally with sine wave carriers,pulse trains, repeating spreading, etc.)

− Differentiates noise from modulated signal (noise is a wide-sensestationary signal with no correlation)

− Computationally complex and requires long observation time




• Primary and secondary networks are physically separated transmitterdetection can not avoid interference due to the lack of the primaryreceiver‘s information

• Even if the secondary user has a line of sight to the transmitter, thesecondary user may not be able to detect the transmitter, e.g. due toshadowing, etc.

• Can not prevent hidden terminal problem

Primary user

Secondary user

Transmitter

Primary transmitter range

Can not detect transmitter

Interference

Secondary transmitter range

Interference



Cooperative Detection

• Incorporating information from multiple secondary users to detectprimary users

• Clasified into– Centralized cooperative detection

− Controlled mostly by cognitive BSs− Cognitive BS collects sensing information from all secondary users it

serves and detects spectrum holes− Distributed cooperative detection

− No centralized infrastructure− Observations are exchanged among secondary users

• More accurate detection than that based on single secondaryuser observations, e.g. transmitter detection. Moreover, multi-path fading and shadowing effects are mitigated



Interference-Based Detection

• Interference takes place at the receiver side (i.e. the receiver isdisrupted). However, it is controlled at the transmitter side, e.g.using power control, etc.

• Interference temperature is well-known example (receiver-detection model)

Orginal noise floor

Secondary users are allowed to communicate as

long as they do not produce more interference

than this limit

Licensed signal

New opportunities for spectrum access

Minimum service range with

interference cap

Service range at orginal noise floorPower at receiver



Challenges

• Interference temperature measurement– Secondary users are aware of their locations and transmission

power. They are not a ware of primary users locations– Currently, no practical way for a cognitive radio to measure or

estimate the interference temperature at neighbor primary users• Spectrum sensing in multi-user networks

– Multi-user environment makes it more difficult to sense primaryusers (secondary networks coexist with each other as well as withprimary networks)

– Cooperative detection can be considered as possible solution forsuch environments

• Detection capability– Detection of primary users in very short time is essential– OFDM-based secondary users are best adequate (multi-carrier

sensing can be exploited)



Spectrum Management



Spectrum Management

• Spectrum bands are spread over wide frequency range includinglicensed and unlicensed bands

• Radio environment characteristics show fast and mostly notpredictable variation over time

• Secondary users have to select the best spectrum band meetingtheir QoS requirements spectrum management functionsare required

• Spectrum management include following steps1. Spectrum sensing2. Spectrum analysis3. Spectrum decision



Spectrum Analysis

• Characterizes sensed spectrum holes to obtain the bandappropriate for user’s requirements

• Characteristics of spectrum holes– Interference

− Some spectrum bands are more crowded than others− Based on the interference at primary receivers, the allowed sending

power of secondary user can be derived channel capacity is estimated− Path loss

− Path loss increases as frequency increases. To retain the capacity whenswitching to higher frequency, sending power should be increasesmore interference produced

− Wireless link errors− Modulation scheme and interference affect strongly the error rate

− Link layer delay− Affected by the interference, path loss, etc.

− Holding time− Expected time duration the secondary user can occupy the channel



Spectrum Decision

• Once spectrum bands are characterized, the band best meetingQoS requirements should be selected spectrum decisionfunction should be aware of QoS requirements of current ongoingapplications

• Spectrum decision rules are required

• QoS requirements for secondary user– Data rate– Acceptable error rate– Delay– ...



Challenges

• Decision Model– Development of suitable decision rules that consider spectrum bands

characters is until now an open issue

• Multiple spectrum band decision– In case secondary users are capable of using multiple channels for

transmission simultaneously, it is important to determine the numberof spectrum bands available and select the bands appropriate

• Spectrum decision over heterogeneous spectrum bands– Support spectrum decision operations on both licensed and

unlicensed bands is challenging



Spectrum Mobility



Spectrum Mobility

• The process when a secondary user changes its frequency ofoperation, also called spectrum handoff as well

• Reasons– Operating channel becomes worse– Primary user wants to communicate on the channel– User movements (available spectrum bands change)

• Requirements- Low latency- Transparence to upper layers protocols if possible- No impairments on ongoing applications (ideal case)

• Multi-layer mobility management with which protocols of manylayers cooperate to support mobility is required



Challenges

• Smooth spectrum mobility schemes

• Synchronization between protocols of many layers and possiblywith applications to support smooth spectrum handoffs (e.g.applications or protocols switch from operation mode to anotherupon prediction of a spectrum handoff, etc.)

• Support of horizontal (changing channels while staying in thesame secondary network) and vertical handoffs (betweensecondary networks)

• Performing spectrum handoffs to maintain QoS requirementssatisfied



Spectrum Sharing



Spectrum Sharing

• Considered similar to Medium Access Control (MAC) issue inexisting systems. However, different challenges arise due to– Coexistence with licensed users– Wide range of available spectrum

• Spectrum sharing steps– Spectrum sensing: detect unused spectrum holes– Spectrum allocation: allocation of possible target channels based

on spectrum sensing results and allocation policies– Spectrum access: coordination of access to the allocated channel

to avoid collisions– Transmitter-receiver handshake: negotiation of communication

channel between sender and receiver– Spectrum mobility: enable continuous communication between

sender and receiver in spite of primary user appearance on the usedchannel



Spectrum Sharing Techniques

Spectrum sharing techniques are classified according to

• Architecture– Centralized

- Centralized entity controls the spectrum allocation and access- Secondary users do observations and report to the centralized entity,

which creates spectrum allocation map– Distributed

- Applied when construction of infrastructure is not possible or notpreferable

- Each node is responsible for the spectrum allocation



Spectrum Sharing Techniques

• Spectrum allocation behavior– Cooperative

- Observations results of each node are shared with other nodes spectrumallocation is done based on these measurements

- These techniques result in better spectrum utilization at the cost of considerablesignaling between nodes

– Non-cooperative (selfish)- Each node does its observations and allocates its spectrum band- These techniques result in reduced spectrum utilization. However, they may be

practical for certain applications or situations

• Spectrum access technology– Overlay spectrum sharing

- Secondary nodes access spectrum holes not used by primary networksInterference to primary users is minimized

– Underlay spectrum sharing- Based on spread spectrum techniques developed for cellular networks- After acquiring spectrum allocation map, secondary users begin sending, so that

their transmission power is regarded as noise by licensed users



Intra/Inter-Network Spectrum Sharing

• Inter-network spectrum sharing– Centralized inter-network spectrum sharing: secondary networks

organize cooperatively the spectrum allowed to be accessed by users ofeach secondary network, e.g. by means of central spectrum policy server,etc.

– Distributed inter-network spectrum sharing: BSs of secondary networks compete to allocate spectrum holes

Classified according to• Architecture• Spectrum allocation behavior• Spectrum access technology

(see the previous two slides)

Inter-network spectrum sharing

Intra-network spectrum sharing

Secondary user(operator1) Secondary user

(operator2)



Challenges

• Common control channel (CCC)– Tasks

− Transmitter-receiver handshake− Communication with a central entity organizing the spectrum allocation− Sensing information exchange

– Problems− Fixed CCC is infeasible (CCC must be vacated when a primary user appears on it)− CCC for all users seems to be topology-dependent, thus CCC varies over time− If no CCC is allocated, transmitter-receiver handshake becomes a challenge

• Dynamic radio range– Radio range and characteristics change with operating frequency CCC

must be selected carefully (better to select CCC in lower spectrum bands anddata channels in higher ones)

• Spectrum unit– Existing techniques consider channel as the basic spectrum unit. As known,

channel may be time slot, frequency, code, etc.



Cognitive MAC Protocols



Classification

Cognitive Radio (CR) MAC Protocols

Random Access MAC Protocols

Time Slotted MAC Protocols Hybrid MAC Protocols

(Partially time slotted andpartially random access)

(CSMA/CA-like random access for control packets and data)

(Time synchronized control and data slots)

Sin

gle

Rad

ioM

ulti

Rad

io

SCA-MACHC-MAC

DOSSDSA-MAC

C-MAC OS-MAC

SYN-MAC

Sin

gle

Rad

io

CR

Cen

traliz

edM

AC

CR

Ad

Hoc

M

AC

CSMA-MAC IEEE 802.22 DSA Driven MAC



Classification

• Classified according to the access method into– Random access MAC protocols

− No need for time synchronization− Based on Carrier Sense Multiple Access (CSMA)

– Time slotted MAC protocols− Need for network-wide synchronization− Time is divided into slots for both control and data channels

– Hybrid MAC protocols− Partially slotted transmission, in which

– Signaling generally occurs over synchronized time slots– Data transmission may have random channel access schemes without time

synchronization

• Classified according to the architecture into– CR centralized MAC

− Central entity manages, synchronizes and coordinates operations amongsecondary users

– CR Ad Hoc MAC− No central entity, neighbors cooperate to gain access to available channels



CSMA-MAC Protocol

Busy

Busy

RTS RTS CTS Data

RTS CTS DataCRN

PS

Carrier sense (ts)

Primary System (PS) Cognitive Radio Network (CRN)

(a)

Busy

Busy

RTS RTS CTS Data

RTSCollision

CRN

PS

Carrier sense(ts)(b)

Busy

Busy

RTS RTS

RTS CTS DataCRN

PS

Carrier sense(ts)(c)

Busy

Busy RTS CTS DataCRN

PS

Carrier sense(ts)(d)

Channel is free

Still no successfull transmisssion



CSMA-MAC Protocol

• Case (a)– Primary user gains the access to the channel and sends its data– Secondary user senses the channel for a period ts

– The secondary user finds the channel vacant (assuming the primary andsecondary users are separated by a large distance)

– The secondary user gains access to the channel and sends its data• Case (b)

– RTS packet sent by the secondary user experiences collision– The secondary user waits for the next transmission opportunity after

repeating the previous sensing process• Case (c)

– Primary user sends repeated RTS packets. However, a collision incurs eachtime

– Secondary user can start transmission• Case (d)

– Primary user has no packets to send, thus the channel is free– Secondary user can start transmission



SYN-MAC Protocol

A B C D E F

2 2

4 4

112

5

3

54 43 3

1 1

Ch1

Ch2

Ch3

Ch4

Ch5

Slot for Ch1 Slot for Ch2 Slot for Ch3 Slot for Ch5Slot for Ch4

RTS1 CTS1 Data

IE

Primary user active

IE

Available channels

RTS CTS DATA Information Event (IE) Backoff



SYN-MAC Protocol

• Time is divided into fixed-time intervals (slots)• Each time slot is dedicated to one channel• Each node has two radios, one for listening to control messages and one

for sending data• C wants to send data to D

– Each node knows the available channel sets of their neighbors– Channels 1 and 5 are common– C chooses Ch1 for transmission and starts negotiation over it using RTS/CTS– Once negotiation is successful, data transmission takes place on Ch1

• B observes that primary user of Ch 4 has returned– B knows that it can reach its neighbors (A and C) through Ch2– B waits for the time slot which represents Ch2– B sends Information Events (IE) control message with its new channel list– Nodes A and C, on receiving this information learn that node B will not be

available on Ch 4• E applies the same procedure as B to notify D and F that Ch 4 is

removed from its channels list



IEEE Standards Activities




• IEEE Standards Coordinating Committee 41 (SCC41) (formerly knownas P1900)

• IEEE 802.22 is standardized for cognitive radio networks




Standard Scope

IEEE 802.16.2-2001• Initiation: 9/1999• Completion: 11/2001

• One of the first coexistence standards• Minimizes interference in fixed broadband wireless access (BWA)

systems• Covers frequencies of 10 to 66 GHz in general (focus on 23.5 to

43.5 GHz)


• Coexistence with other selected wireless devices operating inunlicensed frequency bands


• Defines the protocol and interconnection of devices via radiocommunication in a Personal Area Network (PAN)

• Uses carrier sense multiple access with a collision avoidancemedium access mechanism

IEEE 802.11h-2003• Initiation: 12/2000• Completion: 9/2003

• Amendment to IEEE std. 802.11-1999• Provides mechanisms for Dynamic Frequency Selection (DFS) and

Transmit Power Control (TPC) that may be used to satisfyregulatory requirements for operation in the 5 GHz band in Europe

IEEE 802.16a-2003• Initiation: 2/2002• Completion: 4/2003

• Amendment to the 802.16-2001 standard• Expands the scope of 802.16-2001 standard to address operational

frequencies from 2–11 GHz


• Revision of the 802.16.2-2001• Added treatment of coexistence in the 2–11 GHz bands to the

802.16.2-2001 standard




Standard Scope

IEEE P1900.1: Terminology and Conceptsfor Next Generation Radio Systems andSpectrum Management

• Initiation: 3/2005• Completion: -

• Definitions and explanations of key concepts in the fields ofspectrum management, cognitive radio, policy-defined radio,adaptive radio, software-defined radio, and related technologies

• Describes how these technologies interrelate, create newcapabilities and new spectrum management paradigms such asdynamic spectrum access

IEEE P1900.2: Recommended Practice forInterference and Coexistence Analysis

• Initiation: 3/2005• Completion: -

• Analyzing the potential for coexistence or, in contrast, interferencebetween radio systems operating in the same frequency band orbetween frequency bands

IEEE P1900.3: Dependability andEvaluation of Regulatory Compliance forRadio Systems with Dynamic SpectrumAccess

• Initiation: 5/2005• Completion: Est. 2/2011

• Techniques for testing and analysis to be used during complianceand evaluation of radio systems with Dynamic Spectrum Access(DSA) capability

IEEE P1900.4: Architectural BuildingBlocks Enabling Network-DeviceDistributed Decision Making for OptimizedRadio Resource Usage in HeterogeneousWireless Access Networks

• Initiation: 12/2006• Completion: 2009

• Defines the building blocks comprising network resourcemanagers, device resource managers, and the information to beexchanged between the building blocks enabling coordinatednetwork-device distributed decision-making




Standard Scope

IEEE 802.22• Initiation: 9/2004• Completion: Est. 9/2009

• Specifies the air interface, including MAC and PHY layers, of fixedpoint-to-multipoint wireless regional area networks operating in theVHF/UHF TV broadcast bands between 54 and 862 MHz

IEEE 802.19• Initiation: 3/2006• Completion: Est. 2010

• Wireless Coexistence Technical Advisory Group (TAG) deals withcoexistence between unlicensed wireless networks

• Defines recommended coexistence metrics and methods ofcomputing these coexistence metrics

IEEE 802.16h• Initiation: 12/2004• Completion: 12/2009

• Amendment to the 802.16 standard• Specifies improved mechanisms (as policies and medium access

control enhancements) to enable coexistence among license-exemptsystems based on IEEE standard 802.16

IEEE 802.16m• Initiation: 12/2006• Completion: Est. 12/2009

• Amendment to the 802.16 standard• Provides an advanced air interface for operation in licensed bands. It

meets the cellular layer requirements of IMT-advanced next-generation mobile networks while providing continuing support forlegacy WirelessMAN-OFDMA equipment

IEEE 802.11y• Initiation: 3/2006• Completion: Est. 12/2009

• Amendment to the 802.11 standard• Allows application of 802.11-based systems to the 3650–3700 MHz

band in the U.S. It standardizes the mechanisms required to allowshared 802.11 operations with other users



IEEE 802.22

• Task– Standard for Wireless Regional Area Network (WRAN) that utilizes

white spaces in the TV frequency spectrum on a non-interferencebasis

• Topology– Point to multipoint basis (P2MP)– The system consists of Base Stations (BSs) and Customer-Premises

Equipments (CPEs)– BSs control the medium access for all CPEs attached to it

• Physical layer– OFDMA for transmission on uplink and downlink

• MAC layer– Dynamically adapts to changes in the environment by sensing the

spectrum



Deployment Scenario

WRANRepeater

TV TransmitterWRAN BS

WirelessMIC

WirelessMIC

WRAN BS

Typical ~33kmMax. 100km

• Controls the medium access for all CPEs attached to it• Gathers periodic sensing reports from CPEs makes decisions

based on the gathered information

Customer-PremisesEquipment (CPEs)



System Parameters

Parameter Specification Remark

Frequency range 54~862 MHz

Service coverage Typical range: 33 km

Bandwidth • Mandatory: 6, 7, 8 MHz with channel bonding• Optional: fraction BW

Allows the fractional use of TV channel and channel bonding up to 3 TV channels

Data rate • Maximum: 70 Mbps• Minimum: 4.5 Mbps

Maximum of 23 Mbps for 6 MHz

Spectral efficiency • Maximum: 3.94 bits/s/Hz• Minimum: 0.75 bits/s/Hz Single TV channel BW of 6 MHz

Modulation QPSK, 16QAM, 64QAM

Transmit power Default 4W EIRP

Multiple access Adaptive OFDMA Partial bandwidth allocation

FFT Mode 1024, 2048, 4096, 6144

Cyclic prefix mode 1/4, 1/8, 1/16, 1/32

Duplex TDD or FDD

Network topology Point-to-multipoint network



Conclusions

• Cognitive radio technology enables better utilization of unusedlicensed spectrum– Basic idea

− Allocation of an unused spectrum band− Utilization of the allocated band for communication as long as no primary

user appears on this band or no interference produced for primary usersworking around

– Requirements− Fast and accurate spectrum sensing equipments as well as mechanisms− Adequate spectrum decision approaches− Seamless spectrum mobility techniques− Efficient spectrum sharing methods

– However− Lots of new open issues and challenges to solve

• IEEE 802.22 is the standard for cognitive radio networks



Control Questions

• List different definitions of cognitive radio technology? What does theterm “white space” mean?

• Explain the following sentence: “spectrum sharing in cognitive networksis similar to MAC protocols in existing wireless networks. However,some issues are different”? Which issues are meant?

• Describe different steps in cognitive radio cycle? What are the differentsteps required for spectrum sharing in cognitive radio networks?

• Why spectrum handoff is different from the traditional handoff?

• Explain the operation of CSMA-MAC?

• What are the system parameters for IEEE 802.22?



References

Cognitive Radio Technology, Spectrum Sensing, Management, Mobility and Sharing• I.F. Akyildiz, W.Y. Lee, M.C. Vuran, S. Mohanty, “NeXt Generation/Dynamic Spectrum Access/Cognitive Radio

Wireless Networks: A Survey”, Computer Networks Journal, 2006• S. Haykin, “Cognitive radio: brain-empowered wireless communications”, IEEE Journal on Selected Areas in

Communications, 2005• H. Arslan “Cognitive radio, software defined radio, and adaptive wireless systems”, Springer, 2007• J.J. Mitola, “Cognitive Radio - An Integrated Agent Architecture for Software Defined Radio”, Doctoral thesis,

Royal Institute of Technology (KTH), Teleinformatics, ISSN 1403 ISSN 1403 – 5286. 5286, Stockholm, 2000Cognitive MAC Protocols• A. Chia-Chun Hsu, D. S. L. Weit, C. C. J. Kuo, “A cognitive mac protocol using statistical channel allocation for

wireless ad-hoc networks,” in Proceeding of IEEE Wireless Communications and Networking Conference(WCNC 2007), March 2007

• J. Jia, Q. Zhang, X. Shen, “Hc-mac: A hardware-constrained cognitive mac for efficient spectrum management,”IEEE Journal on Selected Areas In Communications, vol. 26, no. 1, January 2008

• L. Ma, X. Han, C. C. Shen, “Dynamic open spectrum sharing mac protocol for wireless ad hoc networks,” inProceeding of the first IEEE International Symposium on New Frontiers in Dynamic Spectrum Access Networks(DySPAN 2005), November 2005

• C. Cordeiro, K. Challapali, “C-mac: A cognitive mac protocol for multi-channel wireless networks,” in Proceedingof the 2nd IEEE International Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN2007), April 2007

• L. Le, E. Hossain, “Osa-mac: A mac protocol for opportunistic spectrum access in cognitive radio networks,” inProceeding of the IEEE Wireless Communications and Networking Conference (WCNC 2008) , April 2008

• Y. R. Kondareddy, P. Agrawal, “Synchronized mac protocol for multi-hop cognitive radio networks,” inProceeding of IEEE International Conference on Communications (ICC 2008) , May 2008



References

• Claudia Cormio , Kaushik R. Chowdhury, “A survey on MAC protocols for cognitive radio networks,” Ad HocNetworks journal, 2009

IEEE Standards Activities• C. Stevenson, G. Chouinard, L. Zhongding, H. Wendong, S. Shellhammer, W. Caldwell, “IEEE 802.22: The first

cognitive radio wireless regional area network standard”, IEEE Communications Magazine, 2009• IEEE P802.22™/ DRAFTv1.0 Draft Standard for Wireless Regional Area Networks Part 22: Cognitive Wireless

RAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Policies and procedures foroperation in the TV Bands, April 2008

Documents

Cognitive Radio Networks - Startseite TU Ilmenau · PDF fileIntegrated HW&SW Systems Group Cognitive Radio Networks 2 Outline • Introduction • Cognitive Radio Technology − Spectrum