Tian Feng,Cheng Shilun,Yang Zhen

（Communication Engineering Institute， Nanjing University of Posts and Telecommunications， Nanjing 210003， China）2

T he IEEE 8O2.22 Working Group has been formed in November 2OO4 with the task of developing a standard for the Wireless Regional Area Network（WRAN）based on Cognitive Radio（CR）technologies.The standard includes Physical Layer（PHY）and Media Access Control（MAC），using the already allocated fallow spectrums to broadcast TVwith no interference.The WRAN employs CRtechnologies to sense and estimate the television frequencies and uses the dynamic spectrum management to find and allocate fallow spectrums.The CR technologies will become an important trend for future wireless communications.In last issue，（first part of two）the WRANs and IEEE 8O2.22，CRtechnologies were introduced.This article on WRAN and CR technologies is the second part of two.In this issue，the applications of CR technologies are introduced.

3 IEEE 802.22 Systems Co-existence and CR Applications

The Wireless Regional Area Network（WRAN）standard stipulated by the IEEE 8O2.22 Working Group is different from other standards.It operates on the existing spectrums occupied by fixed users.The co-existence in the air interface protocols and algorithms shall be defined as a part of the standard.The issue of systems co-existence plays an important role in the 8O2.22 air interface.

Being the means and technologies for future intelligent communications，Cognitive Radio（CR）will be widely applied to all communication networks to greatly improve their performance.

Next，we will address the co-existence issue of the IEEE 8O2.22 system as well as the CR-based applications in wireless communications.

3.1 Co-existence of the IEEE 802.22 System and Other Systems

The TVbroadcast spectrums on which the IEEE 8O2.22 WRAN operates have been used by the TVbroadcast or wireless microphone systems.As there are different air interfaces and communication modes on the same band，it is necessary to consider the co-existence mechanism at the beginning of drafting the standard.

3.1.1 Antenna

The most basic objective of the IEEE 8O2.22 is to find a technology that can not only deliver potential services but also ensure no interference to existing authorized services.That means，each Customer Premier Equipment（CPE）needs two separate antennas:directional antenna and omnidirectional antenna.The directional antenna acts as the operating antenna for communication between the CPEand the Base Station（BS）.It prevents energy from being emitted to other unnecessary directions and thus reduces interference.Moreover，it can improve power control efficiency to ensure the implementation of system co-existence.The omnidirectional antenna is used for sense and estimation.It enables the CPEto search among its adjacent nodes in all rather than a single direction for signals of the original authorized Primary Users（PUs）to avoid system conflict as much as possible.

3.1.2 Sense and Protection of TVor Wireless Microphone Users

The BSas well as the CPEin the IEEE 8O2.22 has the responsibility to protect the authorized services of the Primary Users.As a single CPEmay not sense reliably，the BSadopts the periodically distributed sensing mechanism as well as data integration and retrieval technologies to obtain reliable information about spectrum occupation.

（1）Sense Threshold

The BSand the CPEin the IEEE 8O2.22 use each direction and polarization of the omnidirectional antenna to sense the authorized users'transmission，or detect the PUs.If thedetected value of authorized signals is higher than the preset threshold，the BS will vacate the channel.

▼Table 1. Maximum CPE power

▲Figure 5. CPE EIRP profile.

（2）Response Time

The response time refers to the interference time that TVbroadcast or wireless microphone systems can bear before the IEEE 8O2.22 system vacates the related channels.Usually the TVor broadcast stations operate all night long and a bit longer response time is not quite important.If the distributed sensing mechanism is used to detect TVor broadcast stations，the bearable response time can be severalminutes to tens of minutes.However，when TVor broadcast stations operate in on-and-off mode instead of continuous mode（for example，stop operation at night），it is necessary to have a faster sensing speed and a lower response time.

（3）Spectrum Availability

In the IEEE 8O2.22 standard，the BS needs to maintain a table of channel availability.In this functional table the channels are classified by the state of availability，such as occupied（PUs'signals are being transmitted），available（able to be occupied by IEEE 8O2.22 users）and unavailable（cannot be occupied by IEEE 8O2.22 users）.The table can be updated by the system administrator（for example，certain channels are set to unavailable）or controlled by the IEEE 8O2.22 sensing mechanism.

（4）Maximum Power Limit

The problem of interference possibly caused by the BSor the CPEis very important.For this reason，the IEEE 8O2.22 Working Group made numerous hypotheses.For details about them，readers can refer to relative documents.Based on the hypotheses，we can reach a conclusion that the BSconforms to the IEEE 8O2.22 BSneeds to control the CPE and ensures its transmit power doesn't exceed the value given in the last column of Table 1.Therefore，the CPEmust provide the Effective Isotropic Radiated Power（EIRP）limit as shown in Figure 5.Similarly，Table 2 and Figure 6 give the transmit power limit for this BS.

（5）Out-band Emission Shielding

In order to protect normal operation of the digital TVor wireless microphone systems，the IEEE 8O2.22 standard requires the BSand the CPEthat provide the EIRPvalue of up to 4 Wmust meet the requirements described in Table 3 to reduce the out-band interference.

3.2 CR Applications and Spectrum Sharing

▼Table 2. Maximum BS power

The CRtechnologies are applied to theIEEE 8O2.22 system in various modes including distributed spectrum sensing，estimation，detection algorithm，and spectrum management.The combination of all these modes offers fast response to urgent events.The CRis considered the most promising means for future communications.The early FCCO3-322 Notice of Proposed Rulemaking（NPRM）has set forth the advantages of using CR for spectrum management and detailed four CRapplications:rural markets and unregistered devices，public spectrum leasing，dynamic spectrum sharing，and interaction between a communication system and a Mesh network.The CR technologies have enormous potentials in wireless communications.For example，CRusers can negotiate with other users for efficient spectrum sharing，and the CRcan act as a bridge for interaction between communication systems operating on different spectrums.Furthermore，CRcan drive the secondary market development of spectrum resources and enable spectrum access to rural areas.The CRtechnologies show their application potentials in central networks，distributed networks，Ad hoc，and Mesh structure to meet the needs of authorized and unauthorized users.Next，we will detail the specific application functions and problems of the listed various potential CRtechnologies.

▲Figure 6. BS EIRP profile.

▼Table 3. WRAN adjacent channel interference limit

3.2.1 Radio Scene Analysis and Interference Suppression The CRsystem shall firstly make a radio scene analysis before transmitting signals.The excitations generated by a radio transmitter are unstable and Space-time（ST）signals because their statistic features depend on time and space.The related radio scene analysis involves STprocessing.It includes various functions related to adaptive spectrum detection such as Interference Temperature（IT）estimation and spectrum-hole detection，and adaptive beam shaping for interference suppression.

（1）ITDefinition and Estimation

The FCCrecommends a method for interference estimation that is based on the adaptive real-time interaction between the transmitter and the receiver.This change gives rise to a new measurement called IT.As shown in Figure 7，the ITequation is TI=PI/kB，where the Boltzmann constant k is equal to 1.38×1O-23J/K，B is the related bandwidth，and PIis interference power（generated by the accumulated internal noise sources and external RFenergies）.The equation is used to quantify and manage interference sources in the radio environment.Moreover，the ITlimit describes the worst conditions in the Radio Frequency（RF）environment of a specified frequency band and a specified geographical location.Within the ITlimit，the receiver is expected to work satisfactorily.On any given frequency band，if the measured ITat the receiver of a communication system doesn't exceed a certain limit，users waiting for service can use that frequency band.In addition，the ITlimit will be considered as the upper limit of RFpower on that frequency band.

In CR，it is very important for the receiver to provide a reliable estimation of ITpower spectrum.The Multi-taper Method（MTM）can be adopted to estimate ITpower spectrum（generated by the accumulated internal noise sources and external RFenergies）.The multi-taper spectrum estimation combined with singular value decomposition provides an effective means for estimation of lower-limit noise power spectrum in the RFenvironment.

▲Figure 7. IT estimation.

（2）Spectrum-hole Sensing

The spectrum hole refers to the frequency band allocated to a certain initial authorized user but unused by the user on a specified time and location.We can divide the spectrum area into three types:black area that is usually occupied by partial high-energy interference；grey area that is occupied by low-energy interference for a part time；and white area that is only occupied by environment noise rather than RF interference.Users waiting for service can use the white area and the limited grey area.Currently two methods are available for sensing and detecting PUs（the presence of PUs means the absence of spectrum holes）:energy-based sensing and waveform-based sensing.The latter is better than the former，because the latter provides such performances as cycle-stationary spectrum estimation and feature detection.

（3）Interference Suppression Based on Adaptive Antenna Beam Shaping Technology

In order to optimize spatial characteristics of RFexcited signals，the adaptive antenna beam shaping technology can be employed to suppress interferences at the CR receiver.It can be implemented through the following two measures:

·Using a directional antenna to avoid transmitting signals to all directions and save power

·Transmitting in a fixed direction to minimize interferences generated by other transmitters

For interferences from the known transmitter，the beam shaping technology is adopted at the receiver to perform adaptive residual interference cancellation.In addition，the robust Generalized Side Lobe Canceller（GSLC）is used to protect the target RFsignals and set space and zero signals in the interference direction.

3.2.2 Estimation of Channel State and Capacity

Traditionally，two methods are available to solve the problem of channel state estimation:differential detection and pilot transmission.Based on the combination of the two methods，the semi-blind training technique can be used to compromise performances and bandwidth resources.The semi-blind training is different from either the differential detection with all-blind processing or the training sequence transmission with directive processing.

A state space model can describe the time varying of channel state information.The features of dynamic noise and measure noise determine what sort of state space model is used.In a Gaussian noise environment，the Gaussian state space model is used and a traditional Kalman filter can be used to track the channel state.In a non-Gaussian noise environment，the non-Gaussian state space model is used and a particle filter can do the tracking.The results of channel state estimation can be used to calculate channelcapacity and control signal energy at the transmitter.We calculate the channel capacity C by using the Shannon theorem.Moreover，in the channel capacity analysis，we consider the impact of system feedback delay and use a high-order Markov model.

▲Figure 8. Dynamic spectrum allocation of the CR system.

3.2.3 Power Control

The CRsystem can employ the power control technology implemented in the BS.However，without the central control mode，some optional modes should be found to control the transmitting power.The problem of power control for a multi-user CRsystem can be considered as a problem of game theory.If the competition is excluded，it can be considered as a cooperative game and thus simplified as a problem of optimal control theory.If each user maximizes its efficiency，the problem of power control using the game theory willbe concluded as a non-cooperative game.The game theory and the water-fillinging scheme based on the information theory can be used to solve the problem of power control.By contrast，the iterative water-filling scheme provides many ways to handle multi-user environment.The two water-filling schemes can be combined to improve system performance.

3.2.4 Dynamic Spectrum Access and Sharing

Dynamic spectrum management is also called dynamic spectrum allocation，as shown in Figure 8.Without doubt，in combination with power control，the spectrum management aims to develop an adaptive policy for effective utilization of RFspectrum.A special spectrum management algorithm is designed as follows:detect outputs of each transmitting power controller through the spectrum holes established by the radio scene analyzer，select an appropriate modulation policy to adapt to the time-varying RFenvironment and always ensure a reliable communication on the channel.The appropriate spectrum allocation enables spectrum sharing between the CRsystem and authorized users.

The Orthogonal Frequency Division Multiplexing（OFDM）technology can be used for modulation.OFDM is a flexible and effective technology that enables information to be respectively modulated onto each carrier，so it is especially suitable for information transfer over frequency-selective or variable channels.Considering communication traffic，the dynamic spectrum management algorithm for theCDMA-based CRsystem will naturally focus on the user allocation.It first allocates the"white spaces"in the spectrum with a lower interference level and then the"grey spaces"with a higher interference level.Moreover，to meet the requirement，it must include a traffic model for the primary user occupying the"black spaces".

Specially，the dynamic management of spectrum sharing can create important"white spaces"to greatly improve spectrum utilization in the dynamically changing radio environment.

4 Conclusions

The IEEE 8O2.22 Working Group is drafting the first universal air interface standard based on CRtechnologies.The new standard operating on TVbands will adopt such technologies as spectrum sensing，authorized service detection，conflict avoidance，and spectrum management to enable effective service co-existence and radio resources sharing in existing authorized services.This article gives a complete and in-depth introduction to the IEEE 8O2.22 Working Group's status quo of the CR research，and concludes that the wireless communication technologies based on CRwill enjoy a promising development potential.With the IEEE 8O2.22 standards being drafted，the CR technologies will give an enormous impetus to the growth of WRAN and other networks.Meanwhile，WRAN will certainly have a far-reaching impact on future communication services and markets.The CRtechnologies will undoubtedly become an important trend in future wireless communications.