The  emerging  feature-rich  and  high-data-rate  wireless  applications  have  put  increasing demand  on  radio  spectrum.  The  scarcity  of  spectrum  and  the  inefficiency  in  its  usage,  as caused  by  the  current  radio  spectrum  regulations,  necessitate  the  development  of  new dynamic spectrum allocation policies to better exploit the existing spectrum. The current spectrum allocation regulations assign specific bands to particular services, and grant licensed band access to licensed users only. Cognitive Radio (CR) will revolutionize the way spectrum is allocated. In a CR network, the intelligent radio part allows unlicensed users (secondary users) to access spectrum bands licensed to primary users, while avoiding interference   with   them.   In   this   scheme,  a   secondary  user   can   use   spectrum   sensing hardware/software to locate spectrum portions with reduced primary user activity or idle spectrum slots, select the best available channel, coordinate access to this channel with other secondary users, and vacate the channel when a primary user needs it. To achieve this, the transceiver  in  a  CR  system  should  have  awareness  of  the  radio  environment  in  terms  of spectrum  usage,  power  spectral  density  of  transmitted/received  signals, and  wireless protocol  signaling,  should  be  able  to  adaptively  tune  system  parameters  such  as  transmit power,  carrier  frequency,  and  modulation  strategy.  The  transceiver  should  also  be  ended with  an  antenna  system  that  can  simultaneously  operate  over  a  wide  frequency  band (sensing)  and  a  chosen  narrow  band  (communication),  or  operate  over  an  ultra-wide frequency band while possibly blocking signals in a narrow frequency range. Ultra-wideband (UWB) is a transmission technique that uses pulses with a very short time duration across a very large frequency portion of the spectrum. UWB is different from other radio frequency communication techniques in that it does not use RF carriers, but instead employs  modulated high  frequency  pulses  of  low  power  with  a  duration  of  less  than  1 nanosecond. From the perspective of other communication systems, the UWB transmissions are part of the low power background noise. Therefore UWB promises to enable the usage of licensed spectrum without harmful interference to primary communication systems, and can be used as an enabling technology for implementing CR. In  this  chapter,  we  offer  a  general  overview  of  Cognitive  Radio  and  dynamic  spectrum access,  discuss  the  advantages  of  using  UWB  as  an  enabling  technology  for  CR,  and  give special  attention  to  the  latest  research  on  Antenna  Design  for  CR.  The  chapter  will  be organized  as  follows.  Section  2  will  discuss  the  current radio  spectrum  regulations  and spectrum usage, and will focus on the availability of radio spectrum and the efficiency of its use.  Section  3  will  deal  with  the  concepts  of  Spectrum  Sharing  and  Dynamic  Spectrum Access.   Section   4   will  investigate   the   advantages   of   integrating   CR   with   the   UWB technology  and  the  recent  research  on  the  topic.  Section  5  will  review  the  recent  work  in antenna design for Cognitive Radio. Finally, a conclusion will be given in Section 6.

Current radio spectrum allocations/regulations

Radio  spectrum  refers  to  the  electromagnetic  frequencies  between  3  kHz  and  300  GHz. Table 1 lists the bands that make up this spectrum. Access to the spectrum is restricted by a radio  regulatory  regime,  which  licenses  most  of  it  to  be  exclusively  used  by  traditional communications systems and services. The spectrum allocation regime guarantees that the radio communications systems are protected against interference from other radio systems.

Frequency  allocation  or  spectrum  allocation  refers  to  licensing  parts  of  the  spectrum  for exclusive or shared usage, and to declaring other parts as unlicensed or as open spectrum. The  process  of  spectrum  allocation  is  organized  by  national  and  international  institutions usually  called  regulators.  Internationally,  frequency  allocation  processes  are  harmonized with the help of the International Telecommunication Union, which is the United Nations agency for information and communications technologies. In Europe, spectrum regulation is the  responsibility  of  the  Electronic  Communications  Committee  (ECC)  of  the  European Conference   of   Post   and   Telecommunications   Administrations   (CEPT).   In   the   United Kingdom, the Office of Communications (Ofcom) handles spectrum regulation, whereas in the   United   States   of   America,   this   responsibility   is   shared   between   the   Federal Communications    Commission    (FCC)    and    the   National    Telecommunications    and Information Administration (NTIA). In China and Japan, spectrum allocation is respectively conducted  by  the  Ministry  of  Information  Industry  (MII)  and  the  the  Ministry  of  Internal Affairs and Communications (MIC). Four approaches can differentiate the regulation of radio spectrum (Berlemann & Mangold,

2009). These are

1) the licensed spectrum for exclusive usage,

2) the licensed spectrum for shared usage,

3) the unlicensed spectrum, and

4) the open spectrum.

In  the  first  approach,  which FCC  calls  the exclusive  use model,  the licensee  has  exclusive usage rights for a specific spectrum. An example of a spectrum licensed for exclusive usage are  the  frequency  bands  sold  for  use  in  the  Universal  Mobile  Telecommunication  System (UMTS) in Europe. The second approach (called the command-and-control model by FCC) restricts the licensed spectrum for shared usage to a specific technology. An example of this approach could be the spectrum used for public safety services. In the unlicensed spectrum approach, which FCC calls the commons model or open access, the  spectrum  is  available to all  radio  systems  operating  according  to regulated  standards. The  Industrial,  Scientific  and  Medical  (ISM)  2.4  GHz  band  and  the  Unlicensed  National Information  Infrastructure  (U-NII)  5–6  GHz  bands  are  examples  of  unlicensed  or  license- exempt spectrum.

The fourth and last approach, the open spectrum, allows anyone to access any range of the spectrum without any restriction. Yet, a minimum set of rules from technical standards or etiquettes that are required for sharing spectrum should be respected in this approach.

With  the  licensed  spectrum  approaches,  spectrum  resources  could  often  be  wasted.  For example,  portions  of  the  spectrum  could  become  unused  because  the  communications systems licensed to operate in this spectrum have become more spectrum efficient due to technology  advancements,  and  thus  these  communications  systems  can  operate  in  only  a percentage of the spectrum initially licensed for that specific service. In another direction, if a service to which spectrum is licensed is not economically successful, its licensed spectrum remains largely unused. Furthermore, the spectrum dedicated to public safety and military radio systems is only occasionally used, which means it is unused most of the times. As a result, large parts of the spectrum are currently used inefficiently. Paradoxically, 90 to 95% of the licensed radio spectrum is not in use at any location at any given time (Berlemann & Mangold, 2009), especially that the current radio regulatory regime is too complex to handle the increasingly dynamic nature of emerging wireless applications. Added to the fact that the demand for additional spectrum is growing fast, even faster than costly technologies like Multiple Input Multiple Output (MIMO) and Space Division Multiple Access (SDMA) can improve spectrum efficiency, the conclusion is that the current spectrum regulations should be  fundamentally  rethought  in  order  to  solve  the  spectrum  scarcity  and  limited  radio resources problems. Could the solution be in more unlicensed spectrum? There is definitely a strong motivation for more unlicensed spectrum, due to its commercial success and the many different radio communications systems that operate within such spectrum. However, as more parties and technologies  utilize  unlicensed  spectrum,  it  is  becoming  more  crowded  and  consequently less  available  to  all.  This  again  necessitates  the  availability  of  more  spectrum,  or  more efficient radio systems.

Dynamic spectrum access and cognitiveradio

The increasing demand for wireless connectivity and current crowding of unlicensed spectra necessitate a new communication paradigm to exploit the existing spectrum in better ways. The  current  approach  for  spectrum  allocation  is  based  on  assigning  a  specific  band  to  a particular service. This is illustrated by the FCC frequency allocation chart shown in Fig. 1. The

     Determine  the  portion  of  spectrum  that  is  available,  which  is  known  as  Spectrum sensing.

•      Select the best available channel, which is called Spectrum decision.

•    Coordinate  access  to  this  channel  with  other  users,  which  is  known  as  Spectrum sharing.

•    Vacate  the  channel  when  a  licensed  user  is  detected,  which  is  referred  as  Spectrum mobility.

To  fulfill  these  functions  of  spectrum  sensing,  spectrum  decision,  spectrum  sharing  and spectrum mobility, a CR has to be cognitive, reconfigurable and self-organized. An example of  the  cognitive  capability  is  the  CR’s  ability  to  sense  the  spectrum  and  detect  spectrum holes (also called white spaces), which are those frequency bands not used by the licensed users  or  having  limited  interference  with  them.  The  reconfigurable  capability  can  be summarized   by   the   ability   to   dynamically   choose   the   suitable   operating   frequency (frequency  agility),  and  the ability  to adapt the  modulation/coding  schemes  and  transmit power  as  needed.  The  self-organized  capability  has  to  do  with  the  possession  of  a  good spectrum  management  scheme,  a  good  mobility  and  connection  management,  and  the ability to to support security functions in dynamic environments.

Dynamic spectrum allocation models

Dynamic  spectrum  access  (DSA)  represents  the  opposite  direction  of  the  current  static spectrum  management  policy.  It  is  broadly  categorized  under  three  models:  the  dynamic exclusive  use  model,  the  open  sharing  model,  and  the  hierarchical  access  model.  The taxonomy of DSA is illustrated in Fig. 2.

The open sharing model employs open sharing among peer users as the basis for managing a  spectral  region.  Supporters  of  this  model  rely  on  the  huge  success  of  wireless  services operating in the ISM band. A  hierarchical access  structure  with  primary  and  secondary  users  is  adopted  by  the  third model.  Here,  the  spectrum  licensed  to  primary  users  is  open  to  secondary  users  while limiting the interference perceived by primary users. Two approaches to spectrum sharing between  primary  and  secondary  users  have  been  considered:  spectrum  underlay  and spectrum overlay. In the underlay approach, secondary users should operate below the noise floor of primary users,  and  thus  severe  constraints  are  imposed  on  their  transmission  power.  One  way  to achieve  this  is  to  spread  the  transmitted  signals  of  secondary  users  over  an  ultra-wide frequency  band,  leading  to  a  short-range  high  data  rate  with  extremely  low  transmission power. Assuming that primary users transmit all the time (worst case scenario), this approach does not rely on detection and exploitation of spectrum white space. The  spectrum  overlay  (also  termed  opportunistic  spectrum  access  or  OSA)  approach imposes  restrictions  on  when  and  where  secondary  users  may  transmit  rather  on  their transmission power. In this approach, secondary users avoid higher priority users through the use of spectrum sensing and adaptive allocation. They identify and exploit the spectrum holes defined in space, time, and frequency

The  underlay  and  overlay  approaches  in  the  hierarchical  model  are  illustrated  in  Fig.  3. They   can   be   employed   simultaneously   for   further   spectrum   efficiency   improvement. Furthermore, the hierarchical model is more compatible with current spectrum management policies and legacy wireless systems as compared to the other two models.

Ultra-widebandcognitiveradio

Ultra-wideband  (UWB)  is  any  wireless  technology  that  has  a  bandwidth  greater  than  500 MHz  or  a  fractional  bandwidth  greater  than  0.2.  Ultra-wideband  systems  have  been attracting  an  intense  attention  from  both  the  industry  and  academic  world  since  FCC allowed the unlicensed usage of UWB in 2002. UWB is a promising technology for future short- and medium-range high-data-rate wireless communication  networks.  The  most  appealing  property  of  UWB  is  that  it  is  an  underlay system, meaning that it can coexist in the same temporal, spatial, and spectral domains with other  licensed/unlicensed  radios.  Other  interesting  features  of  UWB  include  that  it  has  a multi-dimensional  flexibility  involving  adaptable  pulse  shape,  bandwidth,  data  rate,  and transmit  power.  On  top  of  these,  UWB  has  a  low  power  consumption,  and  it  allows significantly  low  complexity  transceivers  leading  to  a  limited  system  cost.  Another  very important feature of UWB is providing secure communications. The power spectrum of a UWB transmission is embedded into the noise floor, thus it is very hard to detect. Combined with   other   higher   layer   encryption   techniques,   this   feature   introduces   very   secure transmission. UWB   systems   are   allowed   to   operate   in   the   3.1–10.6   GHz   band   without   a   license requirement  (according  to  the  current  FCC  regulations  in  the  USA),  but  under  very  strict transmission power limits. Both indoors and outdoors, UWB systems are not permitted to transmit  more  than  -42  dBm/MHz  in  the  specified  band.  This  limitation  ensures  that  the UWB systems do not interfere to the licensed operators that use various frequency bands in the  UWB  frequency  range.  However,  FCC  regulations  could  be  revised  and  regulatory agencies  may  consider  to  allow  UWB  systems  to  transmit  with  higher  powers  and  offer more  freedom  to  UWB  if  UWB  is  combined  with  CR  to  give  it  the  ability  to  sense  the spectrum to ensure the absence of licensed users operation in their target bands. There  are  two  common  technologies  for  implementing  UWB:  the  Orthogonal  Frequency Division  Multiplexing  based  UWB  (UWB-OFDM)  and  the  impulse  radio  based  UWB  (IR- UWB). IR-UWB is carried out by transmitting extremely short low-power pulses that are on the order of nanoseconds. An advantage of IR-UWB is that it enables to use various types of modulations,  including  on-off  keying  (OOK),  pulse  amplitude  modulation  (PAM),  pulse shape  modulation  (PSM),  pulse  interval  modulation  (PIM),  pulse  position  modulation (PPM), and phase shift keying (PSK).

Time hopping (TH) codes that are specific to each user can be employed by IR-UWB systems for multi-user  access. The TH codes,  which are specific pseudo-random noise (PN) codes, enable  the UWB  system  to  provide  access  to  multiple  users  conveniently.  The  multi-user parameters  can  be  adaptively  modified  according  to  the  change  in  number  of  users.  To enable more users to communicate, for example, the UWB system can increase the number of chips in each frame at the expense of decreasing each user’s data rate. Coherent receivers (such as Rake and correlator receivers) as well as non-coherent ones, such as energy detector and  transmitted  reference  receivers,  can  be  utilized  for  IR-UWB  communications.  Along with the flexibility in modulation methods and receiver types, IR-UWB also offers a variety of  options  regarding  the  shapes  of  the  transmitted  pulses.  IR-UWB  systems  have  an excellent multi-path resolving capability because of the extremely wide frequency band that they occupy. This property makes IR-UWB a precise radar technology as well as a highly accurate ranging and positioning system, in addition to being a communication system. In  OFDM-based UWB,  orthogonal  subcarriers  are  employed  to  modulate  the  transmitted data. In the current multi-band OFDM planning, which divides the entire UWB band into 14 sub-bands,  each  subband  is  considered  to  be  528  MHz  and  contains  128  subcarriers.  The subcarrier spacing is usually chosen to be less than the channel coherence bandwidth. This makes  each  subcarrier  go  through  a  flat  fading  channel.  As  a  result,  the  UWB-OFDM receiver  needs  a  simple  equalizer  implementation  to  recover  the  originally  transmitted signal.  With  UWB-OFDM,  it  is  easy  to  avoid  interference  to  licensed  systems.  By  simply turning  off  the  subcarriers  that  overlap  with  the  spectra  of  the  licensed  system,  a  UWB- OFDM transmitter can avoid jamming a licensed signal.

UWB features meeting cognitive radio requirements

Though usually associated with the underlay mode, UWB offers the possibility of also being implemented in the overlay mode (Arslan & Sahin, 2007). The difference between the two modes is the amount of transmitted power. In the underlay mode, UWB has a considerably restricted  power,  which  is  spread  over  a  wide  frequency  band.  When  a  UWB  system  is operating in the underlay mode, it is quite unlikely that any coexisting licensed system is affected from it. On top of this, underlay UWB can employ various narrowband interference avoidance methods. In the overlay mode, however, the transmitted power can be much higher. It actually can be increased to a level that is comparable to the power of licensed systems. But this mode is only applicable if two conditions are met: 1) if the UWB transmitter ensures that the targeted spectrum is completely free of signals of other systems, and 2) if the regulations are revised to allow this mode of operation. UWB can also operate in both underlay and overlay modes simultaneously. This can happen by shaping the transmitted signal so as to make part of the spectrum occupied in an underlay mode and some other parts occupied in an overlay mode. Apparently,   in   any   mode   of   operation,   UWB   causes   negligible   interference   to   other communication  systems.  This  special  feature  of  UWB  makes  it  very  tempting  for  the realization of cognitive radio.CR should have a high flexibility in determining the spectrum it occupies, because the bands that will be utilized for cognitive communication could vary after each periodic spectrum scan. Flexible spectrum shaping is a part of UWB’s nature. In IR-UWB, the occupied spectrum can directly be altered by varying the duration or the form of the short transmitted pulses. In UWB-OFDM,  on  the  other  hand,  spectrum  shaping  can  be  conveniently  accomplished  by turning some subcarriers on or off according to the spectral conditions.

CR  systems  are  should  be  able  to  adjust  their  data  rates  according  to  the  available bandwidth, which varies according to the utilization of the bands by the licensed systems. CR  systems  are  also  expected  to  provide  a  solution  for  the  cases  when  the  available bandwidth is so limited that the communication cannot be continued. UWB  systems  are  able  to  make  abrupt  changes  in  their  throughput.  For  example,  an  IR- UWB system can respond to a decrease in available bandwidth by switching to a different wider pulse shape, and can do the opposite if there is more band to use. In UWB-OFDM, the adjustment of the occupied bandwidth is even simpler. The subcarriers that overlap with the newly  occupied  bands  are  turned  off,  and  this  way  the  data  rate  is  decreased,  or  more subcarriers are used to occupy newly available bands, thus increasing the data rate.

Furthermore, UWB provides an exceptional solution regarding the dropped calls. If UWB is performed  in  overlay  mode,  and  in  cases  when  it  becomes  impossible  to  continue  the communication,  UWB  can  switch  to  the  underlay  mode.  Thus,  UWB  can  maintain  the communication link even though it is at a low quality since licensed systems are not affected by UWB operated in the underlay mode.

The  spectral  masks  that  are  imposed  by  the  regulatory  agencies  (such  as  the  FCC  in  the USA) are also determinative in spectrum usage in that they set a limit to the transmit power of wireless  systems.  UWB  offers  a  satisfactory  solution  to  the  adaptable  transmit  power requirement  of  cognitive  radio.  Both  UWB-OFDM  and  IR-UWB  systems  can  comply  with any  set  of  spectral  rules  mandated  upon  the  cognitive  radio  system  by  adapting  their transmit power. CR  networks  should  be  able  to  provide  multi-user  access since  there  will  be  a  number  of users willing to make use of the same spectrum opportunities at the same time. CR is also required to be able to modify its multiple access parameters to cope with the changes that may occur in the overall spectrum occupancy, or with the possible fluctuations in the signal quality observed by each user. From the point of adaptive multiple access, UWB is a proper candidate for CR applications and  is  very  flexible  in  terms  of  multiple  access.  For  example,  in  IR-UWB,  the  number  of users can be determined by modifying the number of chips in a frame. In UWB-OFDM, on the  other  hand,  more  users  can be  allowed  to  communicate  by  decreasing  the  subcarriers assigned to each user.UWB has information security in its nature. Hence, it can be considered a strong candidate for  CR  applications  in  terms  of  information  security.  Underlay  UWB  is  a  highly  secure means  of  exchanging  information.  If  a  UWB  system  is  working  in  the  underlay  mode, because of the very low power level, it is impossible for unwanted users to detect even the existence  of  the  UWB  signals.  Overlay  mode  UWB,  on  the  other  hand,  can  also  be considered a safe communication method. In overlay IR-UWB, multiple accessing is enabled either by time hopping or by direct sequencing. Therefore, receiving a user’s information is only  possible  if  the  user’s  time  hopping  or  spreading  code  is  known.  UWB-OFDM  also provides security by assigning different subcarriers to different users. The level of security can be increased by periodically changing these subcarrier assignments. Apparently, UWB is a secure way of communicating in both its underlay and overlay modes.UWB  communication  can  be  accomplished  by  employing  very  low  cost  transmitters  and receivers.  The  transceiver  circuitries  required  to  generate  and  process  UWB  signals  are inexpensive, and the RF front-end required to send and capture UWB signals are also quite uncomplicated  and  inexpensive.  This  property  of  UWB  makes  it  very  attractive  for  CR, which aims at limited infrastructure and transceiver costs.

Adaptive UWB spectral mask

Fig.   4   depicts   the   UWB   spectrum   and   the   bands   for   some   existing   and   dedicated narrowband services. UWB signals, which spread over a very wide spectral region at low power  levels  near  the  noise  floor,  will  overlap  with  some  of  these  systems,  such  as  fixed satellite, radio astronomy, and the services operating in the U-NII bands. Despite the very low power level, there have been some concerns that UWB would increase the interference floor and degrade the performances of licensed users.

transceiver   of   the   CR   device   should   be   ended   with   an   antenna   system   that   can simultaneously  sense  the  channel  over  a  wide  frequency  range  and  communicate  over  a narrow band once the operating frequency is determined. If  UWB  is  used  as  the  CR  enabling  technology,  UWB  antennas  can  be  employed  at  the transceiver    front-end.    In    the    underlay    mode,    a    UWB    antenna    can    be    used    to transmit/receive  the  very-low-power  pulse.  In  the  overlay  mode,  the  UWB  higher-power pulse  could  be  transmitted  if  the  targeted  spectrum  is  completely  free  of  signals  of  other systems  (time  white  space),  or  this  pulse  should  be  unique  and  adaptive  with  spectral notches  to  avoid  strong  interference  to  existing  narrow-band  wireless  services.  In  both modes,  UWB  antennas  could  still  be  used,  however  in  the  overlay  mode  UWB  antennas with controllable frequency notches are more robust.

CR antennas for opportunistic spectrum access

Antennas designed for CR devices operating using the OSA approach are usually dual-port antenna systems with one port used for sensing and the other used for communication. An example of such an antenna is given by (Kelly et al., 2008). This antenna system integrates a UWB antenna (used for sensing) with a narrowband one (used for communication). The UWB antenna, with the ability to sense the spectrum from 3 to 11 GHz, is a coplanar waveguide (CPW)  fed  wine-glass shape  monopole, whereas  the  other  one,  with  narrowband  operation over 5.15 to 5.35 GHz, is a shorted microstrip patch antenna.The  CR  antenna  system  design  presented  by  (Al-Husseini  et  al.,  2010)  is  comprised  of  two microstrip-line-fed monopoles sharing a common partial ground. The configuration of the two antennas  is  shown  in  Fig.  6.  The  sensing  UWB  antenna  is  based  on  an  egg-shaped  patch, obtained  by  combining  a  circle  and  an  ellipse  at  their  centers.  A  small  tapered  microstrip section is used to match the 50-Ω feed to the input impedance of the patch. The UWB response of the sensing antenna is guaranteed by the design of the patch, the partial ground plane, and the feed matching section. The return loss of the sensing antenna is shown in Fig. 7.

The communicating antenna is a combination of a long strip line connected to a 50-Ω feed line via a matching section, and a small triangular conducting part. Two electronic switches are incorporated along the strip line part of the antenna, and a third identical one connects the  strip  line  to  the  triangular  part.  By  controlling  these  three  switches,  the  length  of  the antenna  is  changed,  thus  leading  to  various  resonance  frequencies  in  the  UWB  frequency range.  Four  switching  cases  are  considered.  The  resulting  measured  return  loss  plots  are given in Fig. 8, which shows clear frequency reconfigurability and a coverage of most of the UWB range using only three switches.The design is simple, low in cost, and easy to fabricate. Both the sensing and communicating antennas   have   omnidirectional   radiation   patterns,   good   peak   gain   values,   and   good isolation between their two ports

Antennas for UWB-CR

UWB  antennas  can  be  used  with  UWB-enabled  CR.  The  literature  is  full  of  research  and work pertaining to the design of UWB antennas. (Low et al., 2005) presented a UWB knight’s helm  shape  antenna  fabricated  on an  FR4  board  with  a  double  slotted  rectangular  patch tapered from a 50-Ω feed line, and a partial ground plane flushed with the feed line. Three techniques are applied for good impedance matching over the UWB range: 1) the dual slots on the rectangular patch, 2) the tapered connection between the rectangular patch and the feed line, and 3) a partial ground plane flushed with feed line. Consistent omnidirectional radiation patterns and a small group delay characterize this UWB antenna.The effect of the ground plane on the performance of UWB antennas is discussed by (Al- Husseini  et  al.,  2009a).  Here,  two  CPW-fed  antennas  based  on  the  same  egg-shaped conductor and same substrate are presented and compared. In the first, the ground plane features  a  large  egg-shaped  slot,  and  in  the  second,  the  ground  plane  is  partial  and rectangular in shape. The configuration of the two designs is given in Fig. 9. Both designs exhibit  UWB  response,  with  Design  I  offering  a  larger  impedance  bandwidth  and  better omnidirectional radiation pattern but a slightly smaller efficiency.

The  paper  by  (Al-Husseini  et  al.,  2009a)  presents  a  low-cost  UWB  microstrip  antenna featuring a microstrip feed line with two 45° bends and a tapered section for size reduction and  matching,  respectively.  The  ground  plane  is  partial  and  comprises  a  rectangular  part and a trapezoidal part.   The patch is a half ellipse, where the cut is made along the minor axis.  Four  slots  whose  location  and  size  relate  to  a  modified  Sierpinski  carpet,  with  the ellipse as the basic shape, are incorporated into the patch. The configuration of this antenna is shown in Fig. 10.

Four  techniques  are  applied  for  good  impedance  matching  over  the  UWB  range: 

1)  the specially designed patch shape,

2) the tapered connection between the patch and the feed line,

3) the optimized partial ground plane, and

4) the slots whose design is based on the knowledge of fractal shapes.

As a result, this antenna has an impedance bandwidth over the 2–11  GHz  range,  as  shown  in  Fig.  11,  and  thus  can  operate  in  the  bands  used  for  UMTS, WLAN, WiMAX, and UWB applications.

In the overlay mode of UWB-enabled CR, UWB antennas with notched bands are tempting. Many UWB antennas with frequency band notches are available in the literature. However, (Al-Husseini et al., 2010a) present a design where the notch is switchable, meaning that it can  be  turned  ON  or  OFF  by  controlling  an  electronic  switch.  The  design  makes  use  of complementary split-ring resonators (CSRRs). A CSRR is etched in the patch of the antenna, and an electronic switch is mounted across its slot. The OFF-state of the switch leaves the corresponding CSRR active, thus causing a band notch. Putting the switch to its ON-state deactivates  the  resonance  of  the  CSRR  and  cancels  the  notch.  The  configuration  of  the antenna is illustrated in Fig. 13.The return loss of the antenna is shown in Fig. 14. The frequency band notch appears when the  CSRR  is  active  (switch  OFF),  and  disappears  when  the  CSRR  is  inactive  (switch  ON), thus retrieving the UWB response of the antenna. This original UWB response is guaranteed by the design  of the original antenna, mainly the use of the partial ground plane and the good matching attained by etching a slit in the ground below the feed. A variation of this design is obtained by including another CSRR, of a slightly different size, in  the  patch  of  the  antenna.  The  combined  effect  of  the  two  CSRRs  leads  to  a  larger  and stronger  band  notch,  which  still  can  be  canceled  by  turning  both  switches  (on  the  two CSRRs)  ON.  One  implication  of  this  variation  is  the  ability  to  control  the  width  of  the notched band.

Conclusion

This   chapter   has   discussed   the   current   spectrum   allocation   regulations   and   their shortcomings and has given a general overview of Cognitive Radio and dynamic spectrum access,  mainly  the  exclusive  use,  the  open  sharing,  and  the  hierarchical  access  models  of DSA  and  the  difference  between  the  underlay  and  overlay  approaches  in  the  hierarchical access model. The chapter has also discussed the advantages of using UWB as an enablingtechnology  for  CR.  A  major  part  of  the  chapter  has  been  a  survey  of  the  latest  work  on antenna  design  for  cognitive  radio.  Two  categories  of  antennas  have  been  included: antennas that could be used for CR when the OSA approach is implemented, and antennas that  could  be  used  for  UWB-CR.  Further  research  on  both  categories  of  antennas  is  still under way.