Αντώνησ Πρελορέντζοσ...Infrastructure-based networks Dynamic Spectrum Access...

Ονοματεπώνυμα φοιτητών:

Άγγελοσ Αρχοντοβαςίλησ & Αντώνησ Πρελορέντζοσ

Διδάσκων:

Γ. Πολύζοσ

Επιβλέπων:

Β. Δούροσ

Αςύρματα Δίκτυα και Κινητέσ Επικοινωνίεσ

Msc Επιςτήμη ΥπολογιςτώνΟικονομικό Πανεπιςτήμιο Αθηνών

15/12/2009

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

Cognitive Radio Networks 2

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

◦ Applications: WLAN, Wi-Fi


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

◦ Spectrum stays unused in some areas and periods of time


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

◦ no legal protection from interference


Increasing demand for radio spectrum

Much of the spectrum is idle for a period of time and at large numbers of locations

Goal: Increase the efficiency of spectrum usage


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

Secondary users jump from one white space to another


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◦ a SU changes its frequency of operation when a PU appears in the same band


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 cluster-based architectures for local 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◦ Optimization of Cooperative 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 adjust operating parameters


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 use of non-uniforms channels by different CR users makes it difficult


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◦ Adaptive Framework for Spectrum Handoff


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 network’s detection sensitivity


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

(–) user reliability?


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- Partially slotted transmission, in which the control signaling generally

occurs over synchronized time slots


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

transmission


Random access protocols (CSMA)- Ensures coexistence among the CR users and the PUs by

adapting the transmission power and rate of the CR network

- 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)

- Νo clear assignment of the transmit power, coding scheme, transmission rate to the CR users


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

-Achieved by the coexistence beacon protocol (CBP)


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

iv) Spectrum Negotiationv) Data transfer


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)

Load balancing (load statistics from the analysis of beacons)


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

optimizes CR network performance


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 Zhang, IEEE Communications Society, ICC 2008 proceedings PDF


http://www.cse.ust.hk/~lars/paper/slidingwindow.pdf

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Αντώνησ Πρελορέντζοσ...Infrastructure-based networks Dynamic Spectrum Access...

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