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December 27, 2012

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OFDM for Cognitive Radio: Merits and Challenges

Hisham A. Mahmoud, Tev k Y cek, and H seyin Arslan

u u

Department of Electrical Engineering, University of South Florida

**** *. ****** ******, ***-118, Tampa, FL, 33620

E-mail:{hmahmoud, yucek}@mail.usf.edu, ******@***.***.***

is one of the most widely used technologies in current wireless

Abstract Cognitive radio (CR) is a novel concept that allows

wireless systems to sense the environment, adapt, and learn from communications systems. OFDM has the potential of ful lling

previous experience to improve the communication quality. How- the aforementioned requirements of CR inherently or with

ever, CR needs a exible and adaptive physical layer in order to

minor modi cations. Because of its attractive features, OFDM

perform the required tasks ef ciently. In this paper, CR systems

has been successfully used in numerous wireless standards and

and their requirements of the physical layer are discussed and

technologies. We believe that OFDM will play an important

orthogonal frequency division multiplexing (OFDM) technique

is investigated as a candidate transmission technology for CR. role in realizing CR concept as well by providing a proven,

The challenges that arise from employing OFDM in CR systems scalable, and adaptive technology for air interface.

are identi ed. The cognitive properties of some OFDM-based

In this paper, OFDM technique is investigated as a candidate

wireless standards are also discussed in order to indicate the

for CR systems. CR features and requirements are discussed

trend toward a more CR.

in detail, and OFDM s ability to satisfy these requirements is

explained. In addition, we go through the challenges that arise

I. I NTRODUCTION from employing OFDM technology in CR.

With emerging technologies and with the increasing number The article is organized as follows. In Section II, OFDM

of wireless devices, the radio spectrum is becoming increas- technology is introduced and a basic system model for OFDM-

ingly congested everyday. On the other hand, measurements based CR is presented. Section III discusses the merits of

show that wide ranges of the spectrum are rarely used most OFDM technology and its advantages when employed by CR

of the time, while other bands are heavily used. Depending systems. Challenges to a practical OFDM-based CR system

on the location, time of the day, and frequency bands, the and possible solutions are addressed in Section IV. Section V

spectrum is actually found to be underutilized. However, those looks into present and future technologies that use OFDM with

unused portions of the spectrum are licensed and thus cannot Cognitive features. Section VI concludes the article.

be used by systems other than the license owners. Hence, there

is a need for a novel technology that can bene t from these

II. OFDM-BASED CR

opportunities. cognitive radio (CR) arises to be a tempting

OFDM is a multicarrier modulation technique that can

solution to spectral crowding problem by introducing the

overcome many problems that arise with high bit rate com-

opportunistic usage of frequency bands that are not heavily

munications, the biggest of which is time dispersion. The data

occupied by licensed users (LU) [1]. CR can be de ned as

bearing symbol stream is split into several lower rate streams

an intelligent wireless system that is aware of its surrounding

and these streams are transmitted on different carriers. Since

environment through sensing and measurements; a system that

this splitting increases the symbol duration by the number

uses its gained experience to plan future actions and adapt to

of orthogonally overlapping carriers (subcarriers), multipath

improve the overall communication quality and meet user s

echoes affect only a small portion of the neighboring symbols.

needs.

Remaining inter-symbol interference (ISI) is removed by ex-

A main aspect of CR is to autonomously exploit locally

tending the OFDM symbol with a cyclic pre x (CP). Using

unused spectrum to improve spectrum utilization. Other as-

this method, OFDM reduces the dispersion effect of multipath

pects include interoperability across several networks, de-

channels encountered with high data rates and reduces the

vices, and protocols; roaming across borders while being

need for complex equalizers. Other advantages of OFDM

able to stay in compliance with local regulations; adapting

include high spectral ef ciency, robustness against narrow-

the system, transmission, and reception parameters without

band interference (NBI), scalability, and easy implementation

user intervention; and having the ability to understand and

using fast Fourier transform (FFT).

follow actions and choices taken by their users to learn and

become more responsive over time. The focus of this paper In this paper, we assume a CR system operating as a

is the rst aspect, i.e. CR s ability to sense and be aware secondary user in a licensed band. The CR system identi es

of its operational environment, and dynamically adjust its available or unused parts of the spectrum and exploit them.

radio operating parameters accordingly. For CR to achieve this The goal is to achieve maximum throughput while keeping

objective, the physical layer (PHY) needs to be highly exible interference to primary/licensed users to a minimum. An

and adaptable. A special case of multicarrier transmission example of such a CR system could be the IEEE 802.22

known as orthogonal frequency division multiplexing (OFDM) standard-based system where the spectrum allocated for TV

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Fig. 1. OFDM-based CR system block diagram. All of the layers can interact with the Cognitive engine. OFDM parameters and radio are con gured by the

Cognitive engine.

TABLE I

OFDM- BASED WIRELESS STANDARDS .

Parameters

Standard FFT Size CP Size Bit per symbol Pilots Bandwidth Multiple Accessing

CSMAa

IEEE 802.11 (a/g) 64 1/4 of FFT size 1, 2, 4, 6 4 20 MHz

128, 256, 512, 1/4, 1/8, 1/16, 1/32

OFDMA /TDMA

IEEE 802.16 (d/e) 1, 2, 4, 6 variable 1.75 to 20 MHz

1024, 2048 of FFT size

IEEE 802.22 1024, 2048, 4096 variable 2, 4, 6 96, 192, 384 6, 7, and 8 MHz OFDMA/TDMA

1/4, 1/8, 1/16, 1/32

DVB-T 2048, 8192 2, 4, 6 62, 245 8 MHz NA

of FFT size

a carrier sense multiple accessing (CSMA), orthogonal frequency division multiple access (OFDMA), time division multiple access (TDMA)

channels is reused. In this case, the TV channels are the mental characteristics, by simply changing the con guration

primary users and the standard-based systems are the sec- parameters of the OFDM system (see Table I for some example

ondary users (see section V-B for more details). A block parameters) and the radio frequency (RF) interface. Note that

diagram of the CR-OFDM system considered in this paper coding type, coding rate, interleaver pattern, and other medium

is shown in Fig. 11 . The cognitive engine is responsible for access control (MAC) and higher layer functionalities etc.

making intelligent decisions and con guring the radio and should also be changed accordingly.

PHY parameters. The transmission opportunities are identi ed

III. W HY OFDM IS A G OOD F IT FOR CR

by the decision unit based on the information from policy

OFDM s underlying sensing and spectrum shaping capa-

engine as well as local and network spectrum sensing data. As

bilities together with its exibility and adaptivity make it

far as the PHY layer is concerned, CR can communicate with

probably the best transmission technology for CR systems. In

various radio access technologies in the environment, or it can

the following, we present some of the requirements for CR and

improve the communication quality depending on the environ-

explain how OFDM can ful ll these requirements. A summary

of these requirements and strength of OFDM in meeting them

1 Some OFDM functions are skipped or simpli ed in order to keep the

are presented in Table II.

gure simple.

TABLE II qvxqr zxqyxtw

Sensing...

OFDM CR

CR Requirements OFDM s Strength }

Inherent FFT operation of OFDM eases spec-

Spectrum sensing m n mo no mp np

trum sensing in frequency domain. {xzyw vxwvvutsrq

Shaping...

Waveform can easily be shaped by simply

Ef cient spectrum

turning off some subcarriers where primary

utilization

}

users exist.

OFDM systems can be adapted to different

m n mo no mp np

transmission environments and available re- {xzyw vxwvvutsrq

Adaptation/Scalability sources. Some adaptable parameters are FFT

size, subcarrier spacing, CP size, modulation,

coding, subcarrier powers. Fig. 2. Spectrum sensing and shaping using OFDM.

Techniques such as multiple-input multiple-

Output (MIMO) are commonly used with

Advanced antenna

OFDM mainly because of the reduced equal-

techniques performance [5]. In these sensing algorithms, the availability

izer complexity. OFDM also supports smart

of FFT circuitry in OFDM systems eases the requirements on

antennas.

the hardware. Moreover, the computational requirements of the

With WLAN (IEEE 802.11), WMAN (IEEE

802.16). WRAN (IEEE 802.22), WPAN (IEEE spectrum sensing algorithm is reduced as the receiver already

Interoperability 802.15.3a) all using OFDM as their physical

applies FFT to the received signal in order to transform the

layer techniques, interoperability becomes eas-

received signal into frequency domain for data detection.

ier compared to other technologies.

Support for multiuser access is already inher-

Multiple accessing ited in the system design by assigning groups

B. Spectrum Shaping

and spectral allocation of subcarriers to different users (i.e. orthogonal

frequency division multiple access (OFDMA)).

After a CR system scans the spectrum and identi es active

NBI affects only some subcarriers in OFDM LUs, other rental users, and available opportunities, comes

NBI Immunity systems. These subcarriers can be simply

the next step: spectrum shaping. Ideally, it is desired to allow

turned off.

cognitive users to freely use available bands in the spectrum.

It is desired to have a exible spectrum mask and control

over waveform parameters such as signal bandwidth, power

A. Spectrum Sensing and Awareness

level, and center frequency. OFDM systems can provide such

One of the most important elements of CR concept is the exibility thanks to the unique nature of OFDM signaling. By

ability to measure, sense, learn, and be aware of important disabling a set of subcarriers, the spectrum of OFDM signals

operating conditions. This includes parameters related to the can be adaptively shaped to t into the required spectrum

radio channel characteristics, availability of spectrum, inter- mask2 . Assuming the spectrum mask is already known to the

ference temperature, and radio s operational environments. In CR system, choosing the disabled subcarriers is a relatively

addition, the system should be aware of user requirements simple process.

and applications, available networks infrastructures and nodes, An example of spectrum sensing and shaping procedures in

local policies and other operating restrictions. CR should be OFDM-based CR systems is illustrated in Fig. 2. The two LUs

able to identify and exploit the unused parts of the spectrum are detected using the output of the FFT block, and subcarriers

in a fast and ef cient way. In OFDM systems, conversion that can cause interference to these LUs are turned off. The

from time domain to frequency domain is achieved by using transmitter then uses the unoccupied part of the spectrum for

FFT. Hence, all the points in the time-frequency grid of the signal transmission.

OFDM system s operating band can be scanned without any

extra hardware or computation thanks to the hardware reuse

C. Adapting to the Environment

of FFT cores. Using the time-frequency grid, the selection of

bins that are available for exploitation (spectrum holes) can be Adaptivity is one of the key requirements of CR. By

carried out using simple hypothesis testing. In [2, 3], FFT is combining gathered information (awareness) with knowledge

applied to the received signal. By using the output of FFT, the of current system capabilities and limitations, CR can perform

receiver tries to detect the existence of a primary user in the various tasks. CR can adapt its waveform to interoperate with

band. In [3], more than one FFT output (averaging in time) other friendly communication devices, choose the most appro-

is used. However, averaging in time increases the delay or priate communication channel or network for transmission,

temporal overhead. In [4], the averaging size (number of FFTs) and allocate best frequency to transmit in a free band of

is adapted in order to increase the ef ciency in a cooperative the spectrum. The system waveform can also be adapted to

sensing environment. Primary user s signal is usually spread compensate for channel fading, and null any interfering signal.

over a group of FFT output samples as the bandwidth of OFDM offers a great exibility in this regard as the number

primary user is expected to be larger than the considered of parameters for adaptation is quite large [6].

bandwidth divided by the FFT size. Using this fact, the FFT

2 See

output is ltered for noise averaging in order to obtain a better Section IV-E for more details and for more advanced algorithms.

An OFDM-based system can adaptively change the mod- E. Interoperability

ulation order, coding, and transmit power of each individual Interoperability is de ned as the ability of two or more

subcarrier based on user needs or the channel quality [7]. This systems or components to exchange information and to use the

adaptive allocation can be optimized to achieve various goals information that has been exchanged [12]. Since CR systems

such as increasing the system throughput, reducing bit error may have to deal with LUs as well as other cognitive users,

rate (BER), limiting interference to LUs, increasing coverage, the ability to detect and encode existing users signals can

or to prolong unit battery life. In multiuser OFDM systems, expedite the adoption and improve the performance of CR

subcarriers allocation to users can be done adaptively as well systems. Furthermore, some recent unfortunate disasters man-

to achieve the same goals [8]. ifested the importance of interoperability in terms of wireless

One of the attractive features of OFDM for broadband communications for the rst responders. CR has the potential

communications is its ability to operate using simple one tap to improve the disaster relief operations by developing the

equalizers, in the frequency domain. To maintain this feature, coordination among rst responders [13].

the subcarrier spacing in set to be less than the channel To achieve interoperability, OFDM is one of the best

coherence bandwidth. In addition, to avoid ISI, the system signaling candidates. OFDM signaling has been successfully

appends a CP to each symbol with a duration longer than the used in various technologies including IEEE 802.11a and IEEE

channel maximum delay spread. Based on estimated channel 802.11g Wireless LAN standards, digital audio broadcast-

parameters, an OFDM-based CR system can adaptively change ing (DAB), digital video broadcasting (DVB), and WiMAX.

the length of the CP to maintain an ISI-free signal while OFDM has been used in both short range and long range

maximizing the system throughput [9]. communication systems. Hence, a CR system employing

Similarly, OFDM system can adaptively change the sub- OFDM can communicate with systems using other OFDM-

carrier spacing to reduce inter-carrier interference (ICI) or based technologies with much ease. Only the knowledge of

peak-to-average-power ratio (PAPR) [9], the data subcarrier signal parameters of intended users is needed (see Table I).

interleaving to reduce BER [10], or even the used pilot However, for such task to be successful, the system needs

patterns [11]. to know all standard-related information required to decode

The adaptivity in OFDM systems can be performed either the signal, such as the data and pilot mapping to the fre-

at algorithm level or at parameter level. In classical wireless quency subcarriers, frame structure, and the coding type and

systems, usually algorithm parameters, e.g. coding rate, have rate. More importantly, the RF circuitry of the CR system

been adapted in order to optimize the transmission. However, needs to be exible enough to accommodate different signal

in cognitive OFDM systems, algorithm type, e.g. channel bandwidths and center frequencies. As a result, CR should be

coding type, can also be adapted in order to achieve interop- built around a software-de ned radio architecture to provide

erability with other systems and/or to further optimize system required exibility to the system.

performance. To achieve such adaptivity, a fully con gurable

hardware platform would be needed. IV. C HALLENGES TO C OGNITIVE OFDM S YSTEMS

As an intelligent system with features such as awareness,

D. Multiple Accessing and Spectral Allocation

adaptivity and learning, CR represents the future of wireless

The resources available to a cognitive system have to be

systems with the promise of offering solutions to various

shared among users. Several techniques can be used to achieve

communication problems. However, with this new technology,

such a task. OFDM supports well-known multiple accessing

new challenges appear, raising interesting research topics.

techniques such as time division multiple access (TDMA),

These challenges can be grouped into three categories as

frequency division multiple accessing (FDMA) and carrier

illustrated in Fig. 3. The rst category includes the challenges

sense multiple accessing (CSMA). Moreover, code division

that are unique to classical OFDM systems such as PAPR,

multiple access (CDMA) can be used together with OFDM,

and sensitivity to frequency offset and phase noise. The second

in which case the transmission is known as multi-carrier code

category includes problems faced by all CRs such as spectrum

division multiple access (MC-CDMA) or multicarrier direct

sensing, cross layer adaptation, and interference avoidance.

spread code division multiple access (DS-CDMA).

Our main focus in this article is on the third category:

orthogonal frequency division multiple access (OFDMA), a

challenges that arise when OFDM technique is employed by

special case of FDMA, has gained signi cant attention recently

CR systems. In the following, we discuss major challenges

with its usage in xed and mobile Worldwide Interoperability

to a practical system implementation as well as some of the

for Microwave Access (WiMAX). In OFDMA, subcarriers are

proposed approaches for solving these challenges.

grouped into sets each of which is assigned to a different user.

Interleaved, randomized, or clustered subcarrier assignment

A. Multiband OFDM System Design

schemes can be used. Therefore, OFDMA offers very exible

multiple accessing and spectral allocation capability for CR So far we have considered the more conventional single

without any extra hardware complexity. The allocation of band systems. In single band CR-OFDM systems, the available

subcarriers can be tailored according to the spectrum avail- portion of the spectrum is occupied by a single OFDM signal.

ability. The exibility and support of OFDM systems for If LU exist within the used band, the CR system shapes

various multiple accessing techniques enable interoperability the OFDM signal as to avoid interference to those users

and accelerate the adoption of CR in future wireless systems. as shown in Fig. 2. For systems utilizing wide bands of

broadband transmit/receive switch at the antenna, desensitiza-

tion due to adjacent LU interferers, and fast band hopping to

avoid interference to occupied bands [17]. Frequency synthe-

sizers that can operate in seven bands [18, 19], nine bands [16],

and even 12 bands [17] have been presented recently.

If single-band transmission is employed, there might be

many subcarriers that are deactivated. In such a case, the

ef ciency of FFT algorithms can be increased and the ex-

ecution time can be decreased by removing operations on

input values which are zero; a process known as pruning.

Designing effective pruning algorithms speci c to CR-OFDM

is an important subject for achieving higher performance [20].

Fig. 3. Research challenges in CR and OFDM.

B. Location Awareness

Geolocation information can be used to enable location-

the spectrum, multi-band signaling approach where the total

based services, optimize the network traf c, and adapt the

bandwidth is divided into smaller bands can prove to be

transceiver to the environment. Applications utilizing location

more advantageous over using single band signaling. This

information can be grouped into four categories; location-

appears to be more signi cant if the detected free parts of

based services, network optimization, transceiver algorithm

the spectrum are scattered over a relatively wide band. While

development and optimization, and environment character-

using a single band simpli es the system design, processing

ization. Although some of the existing wireless networks

a wide band signal requires building highly complex RF

have a miniature utilization of location information, CR is

circuitry for signal transmission/reception. High speed analog

expected to have a more comprehensive location information

to digital converters (ADC) are required to sample and digitize

utilization [21, 22].

the wideband signal. In addition, higher complexity channel

OFDM signaling can be used to obtain the geolocation

equalizers are also needed to capture suf cient multipath signal

information in CRs [23] without the need for any external

energy for further processing. On the other hand, multi-band

positioning systems. Pilot sequences (preambles), which are

signaling relaxes the requirements on system hardware as

commonly used in OFDM systems for synchronization, can

smaller portions of the spectrum are processed separately.

be used for acquisition and tracking of units locations. In

Dividing the spectrum into smaller bands allows for better

the literature, both time and frequency domain techniques

spectrum allocation as well.

are proposed to estimate the time of arrival (TOA) using

For OFDM-based CR, the question becomes when to use

received OFDM signal. Existing wireless local area network

multi-band and when to use single band. Given a certain

(WLAN) systems are being studied for indoor positioning

scanned spectrum shape, choosing the number of bands de-

applications while MB-OFDM based UWB is proposed for

pends on various parameters. Required throughput, hardware

high precision applications. Such positioning capabilities help

limitations, computational complexity, number of spectrum

OFDM to ful ll another requirement of CR.

holes and their bandwidth, and interference level are examples

of what could affect a cognitive system choice.

C. Signaling the Transmission Parameters

It is worth mentioning that multi-band OFDM (MB-OFDM)

is employed in ultra wide band (UWB) systems. Instead of In a CR system, communication units sense the surrounding

using a single band UWB signal, the spectrum is divided into environment and gather up information that can be used to im-

sub-bands (with approximately 500 MHz bandwidth each) and prove the communication link. Based on gathered information,

OFDM signals are used to transmit data over each band [14]. the system selects transmission parameters such as LU bands,

However, while UWB is one of the applications of MB- spectral mask, operating frequency, coding, and modulation.

OFDM, it is only limited to a speci c scenario where all While some of these parameters can be detected blindly by

sub-bands have almost equal size, and OFDM signals used intended receivers, other parameters need to be known prior

in sub-bands are identical in other parameters such as CP size to establishing a communication link. Distributing information

and subcarrier spacing. among communication units rather than using local sensing

From a practical point of view, designing a cost effective reduces the complexity and improves the performance of the

multi-band transceiver with high performance has been studied system. Thus, it is crucial for the success of CR to successfully

in the literature [15 19]. On most proposed transceivers, direct distribute such information to other cognitive units.

conversion architecture is used to eliminate the need for image One approach is to dedicate a communication channel to

rejection lters, and relax the bandwidth requirements for the exchange measured information and transmission parameters

baseband lters and converters [15, 16]. The challenges that among cognitive units. However, this requires that a channel

face the implementation of a broadband multi-band OFDM be prede ned (or licensed) for that purpose. As a result, the

system includes the need for wide range frequency synthe- ability of cognitive units to adaptively operate within any

sizers, broadband circuits and matching, gain switch in the given unlicensed band becomes dependent on the availability

low noise ampli er (LNA) without degrading the input match, of such channel. Moreover, as the number of units in the

same cell increases, the amount of information that needs to neighboring subcarriers. However, the obvious disadvantage

be distributed increases as well. This can result in a huge of this method is again the reduction of spectral ef ciency.

overhead that the dedicated channel can not handle. A method that reduces interference to spectrum holes

Other approaches solve the distribution problem by either while keeping high spectrum ef ciency is proposed in [27]

reducing the information overhead or by improving the per- and [28] and is referred to as active interference cancellation

formance of blind detectors. For example, in OFDM systems, and cancellation carriers, respectively. Instead of disabling

based on the scanned channel, waveform is adjusted by turning subcarriers adjacent to spectrum holes, a much smaller number

off some subcarriers in order to exploit the available spectrum of those adjacent subcarriers is used to reduce the interference

holes (see Fig. 2). The receivers, however, should be informed leaked to spectrum holes. The cancellation subcarriers are

about detected spectrum holes (or which subcarriers are deacti- pre-calculated to reduce subcarrier sidelobes inside spectrum

vated). The overhead is reduced by sending a vector containing holes. This technique achieves signi cant reduction of adjacent

disabled subcarriers rather than sending the spectrum sensing channel interference. The disadvantage of this technique is the

results. One method to further reduce the overhead is proposed increase in overall system complexity due to the calculation

in [24]. The activation/deactivation of subcarriers is performed of cancellation carrier values for each symbol. In addition, for

over a block of subcarriers instead of individual subcarriers. larger spectrum holes, more cancellation carriers are needed to

Hence, the signaling overhead can be reduced by a factor of the maintain the desired interference level. Analog or digital lters

block size. On the other hand, instead of sharing the spectrum can also be used to lter-out the unwanted spectral components

sensing information, tone-boosting can be used [25]. Once a of the OFDM signal prior to transmission. However, since

cognitive unit detects a LU signal within the band, it sends a the spectrum mask on a CR signal needs to be adaptive,

tone with maximum power but with a very short time duration the use of analog lters is not practical. On the other hand,

over the detected signal band. The purpose is to inform other digital lters introduce an increase in the system computational

users that a LU exists within this band. Thus, the probability complexity and processing delay. Other methods to reduce

of interference to LUs is reduced, which is one of the main OFDM interference to adjacent channels are presented in [29,

purposes of spectrum sensing. Meanwhile, the short duration 30].

of these tones causes no interference to LUs. Fig. 4 shows an example of a CR system which is using

an OFDM signal with FFT size of 256 subcarriers and cyclic

pre x of 8 samples. A LU signal spanning three subcarriers 25,

D. Synchronization

26, and 27 is detected. It is desired to minimize the interference

Synchronization is an important issue that needs to be to the LU. In case I, the CR system disables subcarriers 23

addressed in OFDM system design. With the introduction though 29. A spectrum hole with 15 dB depth is achieved.

of CR, conventional synchronization methods become insuf - In case II, the OFDM symbols are windowed using a raised

cient. The NBI, which can interfere with the preamble, is one cosine window with a roll-off factor of 0.25 while keeping

of the problems [26]. Furthermore, the incomplete subcarrier subcarriers 23 through 29 disabled. The cyclic pre x is also

set might be an issue for preambles. Pilots as well may fall extended to 64 samples in order to preserve the orthogonality

into unused subcarriers. Moreover, if multiple accessing is em- of the signal. In this case, the interference power is reduced

ployed, subcarriers are assigned to different users. To keep the to 30 dB of the original signal power. Finally, in case III, the

orthogonality between subcarriers and to avoid interference, CP is kept to 8 samples, subcarriers 25 to 27 are disabled,

all users should be synchronized to the receiver. In [26], it and subcarriers 23, 24, 28, and 29 are used as cancellation

is shown that longer preambles are required in CR-OFDM subcarriers. A signi cant spectrum hole deeper than 70 dB is

systems as compared to conventional systems. In addition, new achieved in this case.

preamble structures are introduced and their performances for

time and frequency synchronization are investigated.

V. A S TEP T OWARDS C OGNITIVE -OFDM: S TANDARDS

AND T ECHNOLOGIES

E. Mutual Interference As CR concept is attracting more interest everyday, recently

developed standards are considering more cognitive features.

The sidelobes of modulated OFDM subcarriers are known

Dynamic frequency selection (DFS), transmit power control

to be large. As a result, there is power leakage from OFDM

(TPC), and spectrum sensing are just a few examples of

signals to adjacent channels. In addition, used subcarriers

features that are included in some of the current standards.

power leaks to nulled subcarriers which causes interference,

These standards can be considered as a step towards the

known as mutual interference, to LUs. Various techniques are

future implementation of a CR. In this section, some example

proposed in the literature to reduce this leakage and to enable

OFDM-based standards which utilize cognitive features are

co-existence of cognitive-OFDM systems with primary users.

introduced.

One technique is to window the time domain OFDM symbols.

However, spectrum shape improvement comes at the cost of

longer OFDM symbol duration and thus reduces the spectrum

A. IEEE 802.16

ef ciency of the system. Another approach is to increase the

number of nulled subcarriers to achieve lower interference One of the technologies that is getting a fair amount of

levels to LU bands as most of the interference is caused by interest lately in both academia and industry is IEEE 802.16

B. IEEE 802.22

Licensed

IEEE 802.22 standard is known as CR standard because

10 user band

of the amount of cognitive features that are employed. These

cognitive features include channel sensing, LUs detection,

10 DFS, and TPC. Even though IEEE 802.22 standard is not

nalized yet, the current draft proposal is based on OFDM

PSD (dB)

transmission and it is anticipated that the nal version will be

the same. The IEEE 802.22 standard is designed for a xed

point-to-multipoint communication topology where the base

station (BS) acts as the master mandating all the operation

parameters of users within the cell. And while the users

(slaves) can share sensing information with the BS through

Case I

Case II

distributed sensing, it is up to the BS to change a user transmit

Case III

power, modulation, coding or operating frequency.

**-**-**-**-**-** 30 32 34

One of the most distinctive feature of IEEE 802.22 standard

Normalized frequency (subcarrier index)

is its sensing requirements which is based on two stages: fast

and ne sensing. In the fast sensing stage, a coarse algorithm

Fig. 4. PSD of cognitive system OFDM signal with a spectrum hole over

LU band. is employed, e.g. energy detector. The ne sensing stage is

initiated based on the previous stage results. However, a more

detailed and powerful sensing methods are used in this stage.

(WiMAX). OFDMA PHY mode is probably the most interest- A BS can distribute sensing load among subscriber stations

ing mode supported by WiMAX. In this mode, users can be (SS). The results are returned to BS which uses these results

assigned different bandwidths, time durations, transmit power for managing transmissions.

levels, and modulation orders based on various parameters Another challenge in designing the IEEE 802.22 standard is

such as user carrier-to-interference-plus-noise ratio (CINR), the initialization of new users who desire to communicate with

received signal strength indicator (RSSI) or the available the BS. Unlike current wireless technologies, frequency and

bandwidth. Moreover, OFDMA PHY offers multiple FFT time duration of the initialization channel is not prede ned.

sizes, CP sizes, and pilot allocation schemes. The FFT size In other words, initial users have to scan parts (if not all)

can be selected as 128, 256, 512, 1024 or 2048 depending of the TV bands to nd the BS operating frequency and

on the transmission bandwidth3. Similarly, the CP length time. In addition, users should be able to differentiate between

can be set to 1/4, 1/8, 1/16 and 1/32 times the OFDM incumbent signals and the BS signal. This could prove to

symbol length. The CP size can be changed depending on the be very challenging especially if the BS is operating over a

environment characteristics. With all these adaptive features, combination of multiple frequency bands.

WiMAX has the ability to adapt to various channel conditions

and communication scenarios. C. IEEE 802.11

WiMAX standard is also rich in terms of advanced an-

The WLAN standard, IEEE 802.11a/g, is probably the most

tenna techniques as well. Available methods include adaptive

commonly known OFDM-based standard. The main standard

antenna systems (AAS), space time coding (STC), selected

is upgraded to have cognitive features with IEEE 802.11h and

mapping (SM), collaborative SM, antenna selection, antenna

IEEE 802.11k standards. IEEE 802.11h is designed to allow

grouping, MIMO precoding, STC sub-packet combining, fre-

estimation of channel characteristics and DFS. In addition,

quency hopping diversity combining (FHDC), and adaptive

TPC is incorporated as well, providing the system with more

MIMO switch. The standard use of these techniques is not

control over signal range and interference level. The purpose

directly related to CR, but rather to increase the spectral

of the IEEE 802.11h standard is to allow WLAN systems to

ef ciency and increase the overall throughput of the system.

share the 5-GHz spectrum with primary users (e.g. military

However, advanced antenna techniques could be used to

radar systems).

achieve some of the CR goals as well. For example, the

Note that the DFS proposed for the aforementioned stan-

CR transmitter can exploit location awareness to focus i

.dvi

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