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Integrated Communication Systems GroupIlmenau University of Technology

Cognitive Radio Networks

Advanced Mobile Communication Networks

Integrated Communication Systems Group

Advanced Mobile Communication Networks, Master Program

Outline

2

• Introduction

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

• Cognitive MAC Protocols

• Conclusions

• Example From ICS Research

• References



Introduction

3



Introduction

• Limited spectrum• Inefficiency in spectrum usage

– Utilization of the assigned licensed spectrum varies between 15% and85%

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

Cognitive radio technology

4



Cognitive Radio Technology

5



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 communication needs as a function of use context, and to provide radio resources and wireless services most appropriate to

those needs

6



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)

7



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

8



Cognitive Radio Network Architecture

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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 network 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

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Cognitive Radio Cycle

Spectrum Decision

Spectrum Sharing

Spectrum Sensing

Spectrum Mobility

Radio environment

Spectrum characterizations

RFstimuli

Spectrum holeChannel

capacity

Transmittedsignal

Decisionrequest

Primary userdetection

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Spectrum Sensing

12



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

CyclostationaryFeature Detection

13



Transmitter Detection

• Location of primary receiver is not known (no signaling betweenprimary and secondary users) detection of the weak signal froma primary transmitter through local observations of secondaryusers

• Mechanisms Matched filter detection Energy detection Cyclostationary feature detection

14




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

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

15




• 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

16

Can not detect transmitter



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 secondary userobservations, e.g. transmitter detection. Moreover, multi-pathfading and shadowing effects are mitigated

17



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

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Challenges

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

They are not aware 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 primary users(secondary networks coexist with each other as well as with primarynetworks)

– 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)

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Spectrum Decision

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Managing Available Spectrum

• 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 functions arerequired

• Spectrum management includes following steps1. Getting data from spectrum sensing2. Performing spectrum analysis3. Finally making a spectrum decision

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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 increased more 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

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Spectrum Decision Entity or CR Engine

• 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– ...

23



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 appropriately

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

unlicensed bands is challenging

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Spectrum Mobility

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Spectrum Mobility

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

• 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 is required with which protocolsof many layers cooperate to support mobility,

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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

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Spectrum Sharing

28



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

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Spectrum Sharing Techniques 1/3

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

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• Spectrum allocation behavior– Cooperative

- Observation results of each node are shared with other nodes spectrum allocation is done based on these measurements

- These techniques result in better spectrum utilization at the cost ofconsiderable signaling 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

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• Spectrum access technology– Interweave spectrum sharing (sometimes called “overlay” in the non-

information-theoretic literature)- Secondary nodes access spectrum holes not used by primary networks

Interference 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- Overlay spectrum sharing

- Cooperation with the primary transmission- SUs devote part of its transmit power to enhance the primary signal- Interference level may be increased

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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

Inter-network spectrum sharing

Intra-network spectrum sharing

Secondary user

(operator1)Secondary

user(operator2)

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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.

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Cognitive Radio MAC Protocols

35



Classification

Cognitive Radio (CR) MAC Protocols

Random Access MAC Protocols

Time Slotted MAC Protocols Hybrid MAC Protocols

(Partially time slotted and

partially random access)

(CSMA/CA-like random access for control packets

and data)

(Time synchronized control and data

slots)

Sing

leR

adio

Mul

tiR

adi

o

SCA-MACHC-MAC

DOSSDSA-MAC

C-MAC OS-MAC

SYN-MAC

Sing

leR

adio

CR

Cen

tral

ized

MA

CC

R A

d H

oc

MA

C

CSMA-MAC IEEE 802.22 DSA Driven MAC

36



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 among secondaryusers

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

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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

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CSMA-MAC Protocol

• Case (a): CRN out of the range of PS– 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): CRN in the range of PS but not of the PS transmitter

– 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): PS experiences internal collision

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

– Secondary user can start transmission• Case (d): Channel is not used by PS

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

39



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 Ch5

Slot for Ch4

RTS1 CTS1 Data

IE

Primary user active

IE

Available channels

RTS CTS DATA Information Event (IE) Backoff40



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

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CR Standards

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IEEE 802.11afSuper Wi-Fi (Feb. 2014)

IEEE 802.22 (July 2011)

TV white space spectrum in the VHF and UHF, 54 – 790 MHz, OFDM

Cognitive sensing, flexible adjustment of bandwidth, modulation, power and coding

Database driven approach: Access granted by Geolocation DataBase (GDB)Local sensing is used in certain special casesUp to 1 km, 35.6 Mbit/s for 8 MHz channels

30-100 km, 19 Mbit/s for 6 MHz channels

Access points (AP) and stations Base stations (BS) and customer-premises equipment (CPE)

Wireless local area network (WLAN) Wireless regional area network (WRAN)


Advanced Mobile Communication Networks, Master Program 43

Example from ICS ResearchFlexible MAC as Spectrum Mobility Solution

by André Puschmann



Motivation

Reference: TDMA Protocol Requirements for Wireless Sensor Networks, IEEE 2008

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TDMA CSMA/CA

Power consumption Low High

Bandwidth utilization Maximum Low

Preferred traffic level High Low

Dynamics (network change)

Poor Good

Effect of packet failure High (latency) Low

Synchronization Crucial -



Proposed Improvement

Flexible Link Layer Architecture [1]

Features:• Multiple configurations• Component reuse• Complexity reduction• Reuse of existing MACs• Separated control logic• Frame Error Rate (FER) /

Channel Busy Fraction (CBF) switching metric

[1] Puschmann et al., “A Component-based Approach for Constructing Flexible Link-Layer Protocols“, CROWNCOM 2013, Washington DC, USA.

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Results

• Gain:

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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 equipment 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, IEEE 802.11af are standards for cognitive radionetworks

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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 (WCNC2007), 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,” in Proceedingof IEEE International Conference on Communications (ICC 2008) , May 2008

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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

• IEEE 802.11af™-2013 “Standard for Information Technology - Telecommunications and Information ExchangeBetween Systems - Local and Metropolitan Area Networks - Specific Requirements - Part 11: Wireless LANMedium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 5: TV White SpacesOperation”, February 2014

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