Challenges of WRAN

1. Quality of Service (QoS)

One of the most challenges in 802.22 is providing QoS requirement while ensuring no harmful interference to incumbent users. Most of CR technologies are based on multichannel operation which means more than one data communication can be done simultaneously and it significantly enhances the throughput of the network. However, 802.22 MAC is based on signal channel operation. Data transmission between Base Station (BS) and customer premise equipments (CPEs) is scheduled by BS and CPEs are only allowed to transmit in allocated time slots. Moreover, if the operating channel is shared by multiple WRANs in coexistence mode, the time frames are also shared and transmission opportunities for WRANs users will be much lower than normal mode. In this situation, ensuring QoS satisfaction for WRANs services is great challenge.

In order to protect the incumbent users, 802.22 system needs to sense the channels and detect the incumbents signals. In 802.22 systems, channel sensing is done by both BS and CPEs within quiet periods which are carefully scheduled by BS. To get the reliable and accurate channel sensing results, no transmission is allowed in quiet periods and only channel sensing must be performed. So that, any signal which is detected in quiet periods can be determined as incumbent signals. However, in order to get more accurate and reliable sensing, the quiet periods need to be scheduled quite frequently and significantly long. On the other hand, scheduling the quiet periods significantly affects the throughput of network. Therefore, optimizing the trade off between throughput and reliable channel sensing will be a great research issue for 802.22 system. Some researchers have put their effort to perform data transmission and spectrum sensing simultaneously to improve the throughput and satisfy the QoS requirements.

One possible solution to enhance the throughput is developing the multichannel concepts and apply in 802.22 WRAN system since many multichannel technologies for CR networks have been developed. In 802.22 system, all transmission are done only in operating channel, but in the available channel set, backup channels are also free. Therefore, data transmission can be accomplished in other channels such as backup channels. To apply the multichannel techniques, BS may needs more transceivers to support parallel transmission and receiving. Moreover, more sophisticated channel management mechanism, scheduling and sensing algorithms will be required. However, from the CPEs’ point of view, only fast channel switching may be required.

2. Coexistence with Heterogeneous Networks

Many research efforts are currently ongoing to enable secondary access to TV white spaces (TVWS) such as IEEE 802.11af and 802.19.1 Task Groups. Any Cognitive Radio (CR) based technology that aims to operate in TVWS should consider coexisting with not only incumbent users but also other secondary networks. 802.22 system addresses self-coexistence issue, but no mechanism for coexistence with other heterogeneous secondary networks is supported. We will mainly discuss on coexistence of 802.22 system and 802.11af which has been developing to operate the Wi-Fi (WLAN) on TVWS. Fig.1 represents the coexistence scenario of heterogeneous secondary networks especially 802.22 and 802.11af.

        

                 Fig.1. Heterogeneous coexistence scenario of different secondary networks.

i.  Network Discovery

The first challenge to coexist with different networks is secondary network discovery. Among multiple 802.22 WRAN systems, network discovery can be done by coexistence beacon protocol. Any CPE receives beacon from other WRANs, it will decode the beacon and report to its BS. Then BS changes the network operation into self-coexistence mode. Upcoming WLAN standard like 802.11af may use the beacons for network discovery. However, the problem is different standards may use different MAC frame formats and different PHY modulations. Although CPE or WLAN user receives the beacon of other networks, it may not be able to decode it and it may be confused with incumbent signal. Therefore, developing reliable network discovery mechanisms and sophisticated incumbent detection algorithms will be great challenges.

ii. Spectrum Sharing

The most efficient way to utilize the spectrum is operating different networks on different channels independently. However, this might not be always possible especially when available channels are not sufficient. Spectrum sharing between different secondary networks must be considered in advance.

One of the most great challenges in spectrum sharing is different CR technologies might use different MAC strategies. 802.22 MAC is TDM-based with resource allocation while 802.11af will possibly use contention based protocols such as carrier sense multiple access (CSMA). CSMA mechanism is based on listen before talk, so that 802.11af users can back off if they sense the spectrum is occupied by a 802.22 system. But, 802.22 users do not need to listen before transmission and it can cause harmful interference to WLAN users.

Spectrum sharing among heterogeneous networks in the time domain was proposed. This mechanism has already developed for sharing the spectrum among multiple overlapping 802.22 systems. As can be seen in Fig.2, certain time slots are allocated for 802.22 system and others are reserved for WLAN.

  

                     Fig. 2. Spectrum sharing between 802.22 WRAN and 802.11af WLAN

3. Self-Coexistence

One of the major challenges for WRAN systems is how to efficiently schedule both channel sensing and data transmission, as a channel cannot be simultaneously used for sensing and data transmission. The non hopping mode, which is the basic mode of IEEE 802.22 systems, requires that a WRAN cell operating on a single channel should interrupt data transmissions every 2 seconds for sensing. This periodic interruption decreases the system throughput and can significantly affect the quality of service (QoS) for IEEE 802.22 systems (e.g., voice transmissions can tolerate up to 20 ms interruption).

                                                             
                                      Fig. 3. Coexistence problem in IEEE 802.22

Coexistence beacon protocol is used for neighbor discovery, control information exchange among WRANs cells and inner-cell communications. Coexistence beacons are transmitted in self-coexistence window (SCW) and it includes channel information and specific time schedules. If a CPE receives a coexistence beacon from other WRANs cells, it decodes it and reports to its BS. Once BS receives the report, it changes its operating mode from normal to coexistence mode. The overlapping WRANs cell can share the frames by using on-demand frame contention protocol. A WRAN cell can request frames from other WRAN which is currently occupying the frames by using on-demand frame contention protocol and the requests are sent with coexistence beacon in SCW.

Various solutions proposed by different research groups are discussed below:

i. Dynamic-frequency-hopping (DFH)

This scheme, data transmission is performed using one of the available channels, while the other channels are simultaneously sensed. After 2 seconds, a WRAN cell hops to a new working channel and vacates the previously used one. To achieve efficient performance of DFH, multiple adjacent WRAN cells have to coordinate their hopping behavior. This coordination reduces the effect of a well-known problem in WRAN systems, namely coexistence problem.

ii. Fixed-scheduling DFH (FDFH)

Neighboring WRAN cells determine a fixed schedule for selecting the next working channel. A fixed schedule is used to select the next working channel by neighboring WRAN cells.

iii. Cooperative DFH (CDFH) 

To overcome the static nature of this scheme, another scheme is proposed ,namely cooperative DFH (CDFH), that cooperatively selects working channels - this implements  cooperative selection of working channels, which avoids static nature of in FSDFH.

iv. sectoral DFH (SDFH)

 As the latter scheme requires coordination overhead between base stations (BS), an overhead-free scheme is proposed called sectoral DFH. SDFH divides a WRAN cell into sectors to decrease the chances of having several WRAN users in overlapped sectors between neighboring BSs that are served by same operating channels è  The use of coordination overhead in CDFH to avoid WRAN overlap is replaced by dividing the WRAN cell  in to different sectors for decreasing the number of users that use the same channel in the overlapped region.

v. Fixed-scheduling sectoral DFH (FSDFH)  

Finally, the FDFH and SDFH are integrate into a new scheme, called fixed-scheduling sectoral DFH (FSDFH), which exploits the advantages of both schemes with no additional overhead. It takes the best behaviors of FDSH and SDFH with no additional overhead.

5. Incumbent Protection

Incumbent protection is one of the most important functions in 802.11. Incumbent users represent licensed users of the spectrum such as analog TV, digital TV and wireless microphones. In order to protect the incumbents users, 802.22 system needs information about the usage of the TV channels. This can be done by two different techniques, incumbent database and spectrum sensing. Incumbent databases are maintained by regulatory bodies and it contains about information of spectrum usage of incumbent users. This database can be accessed by any 802.22 device. The 802.22 system sends the request with the geo-location of the BS and its associated CPEs and the database provides available channels list. These incumbent databases are also updated periodically.

Spectrum sensing can be done by both BS and CPEs. Normally, spectrum sensing is done within quiet periods which are scheduled by BS. However, BS may request to specific CPEs to perform spectrum sensing in normal operation.

If any CPE detects the incumbent signal on operating channel, it reports to BS. According to these reports, BS manages the channel by choosing some options; (1) perform channel switching or (2) request specific CPE to perform channel sensing or (3) waits more report from other CPEs. If BS decides to switch the channel, it broadcasts the channel switching request and the whole WRAN cell shall switch its operating channel to the first backup channel. Basically, 802.22 support two channel management modes, embedded mode and explicit mode. In embedded mode, the channel management messages are broadcast in every frame to the CPEs. In explicit mode, the channel management messages are sent to specific CPEs and BS dynamically manages the channel operations explicitly.


References:
1. "Overview of 802.22 WRAN Standard and Research Challenges", Zaw Htike and Choong Seon Hong
2. "Coexistence Problem in IEEE 802.22 Wireless Regional Area Networks", Raed Al-Zubi, Mohammad Z. Siam, and Marwan Krunz
3. IEEE 802.22 Wireless Regional Area Networks - Enabling Rural Broadband Wireless Access Using Cognitive Radio Technology - IEEE P802.22 Wireless RANs
4. "IEEE 802.22 Standard Approved for White Space Development", Hsien-Tang Ko, Chien-Hsun Lee, and Nan-Shiun Chu

Challenges for Industrial Development

 

 

1. Spectrum Planning in Each Country

Differences in local conditions and spectrum usage mean that each country has taken a different approach to White Space adoption. UK and the US, for example, have decided to make White Space available for unlicensed use after extensive research and testing. Japan however, is leaning towards a secondary spectrum transaction approach that the spectrum owners such as broadcaster can resell or lease their vacant spectrum to users. Differences in how the license is issued will decide the entry threshold for the industry. While an unlicensed approach will help industry development, it may also lead to chaos without effective management and industry self-regulation. Adopting a secondary spectrum transaction model will generate new spectrum brokering and transaction opportunities from both long/short-term leasing and resale as well as boost the utilization of vacant spectrum.

2. Geolocation System

Geolocation (e.g. GPS) is one of the key functions of White Space terminal devices. The device must identify and report its location to the back-end database, so the database can notify the device what channels are available at its location. One of the inherent limitations of GPS is the poor in-door or obstacle crossing reception. This greatly hampers the accuracy of current location interpretation and will be particularly acute for personal mobile/portable devices. Integrating a spectrum sensing function may alleviate the geolocation problem but will greatly increase the manufacturing cost and then hamper service adoption. Hence, a simple and cost effective auxiliary geolocation  solution is what the industry urgently needs to provide today.

 

3. Spectrum and Geolocation Database

The preferred approach to White Space detection and application for spectrum regulators and the industry has switched from spectrum sensing to the database model. Once a unified national spectrum and geolocation database is established, it can be made available to fixed and personal mobile/portable devices. It can also be used for other fields such as radar spectrum sharing, femtocell operation and even temporary sharing of 3G/4G spectrum among mobile operators, for example, telecommunication companies may reuse their vacant3G/4G spectrum in remote area by leasing or resale as the White Space approach, creating new business models and opportunities. The construction of spectrum and geolocation databases has therefore become an important issue.The content of the database will mainly consist of local spectrum usage and geolocation information. Apart from information on local broadcast spectrum usage, most data will need to be provided by the terminal equipment. The available spectrum and recommended power output is sent back to the terminal device. As the terminal device must periodically update geolocational data to avoid interference,the setting of the refresh frequency becomes extremely important. Data security and authentication are other key areas of development. The industry still needs to devise a way to prevent fake database and geolocation data from undermining information integrity. The US and UK governments both plan to transfer the responsibility of database setup and management to the private sector and this is a sought-after role for major international vendors. While not requiring a license indicates the government and operators may not collect fees from releasing or leasing the spectrum, the establishment of database by private sector may offer a source of revenue and create promising business opportunities in future White Space applications. 

 

4. Competing Technologies

Before White Space deregulation brings new possibilities for the communications industry, the relevant technical standards must be established first. Careful thought must also be given to guaranteeing the interoperability and co-existence with different technologies. A plethora of different standards based on cognitive radio technology including IEEE 802.22, IEEE 802.11af, IEEE 802.16h, IEEE 802.19, IEEEP1900/SCC41, ECMA-392 and ETSI RSS (Reconfigurable Radio Systems) has presented an immense market potential of different applications. Most of these standards have now been formally adopted.When compared to other standards, 802.22 is developed specifically for the TV spectrum and makes more effective use by taking advantage of the UHF/VHF spectrum. The approval of the standard does not necessarily mean that802.22 will dominate the White Space market however. The standard has been in development since 2004, and such along gestation process raises doubts about the marketability and future potential. As no alliance or forum has been formed to back this standard to date, many obstacles remain to overcome. The latest entry to the field is the IEEE 802.11af standard supported by the Wi-Fi Alliance that is still under development. Wi-Fi 802.11x wireless transmission technology is finding an increasing range of applications and the jump in demand for network connectivity from terminal devices represents it has gradually become an influential standard feature of consumer electronic devices in the industry. A White Space standard derived from this approach can generate synergies from integration with existing 802.11x standards and may well become the greatest competitor of802.22 standard in the future.

References:
1. "Overview of 802.22 WRAN Standard and Research Challenges", Zaw Htike and Choong Seon Hong
2. "Coexistence Problem in IEEE 802.22 Wireless Regional Area Networks", Raed Al-Zubi, Mohammad Z. Siam, and Marwan Krunz
3. IEEE 802.22 Wireless Regional Area Networks - Enabling Rural Broadband Wireless Access Using Cognitive Radio Technology - IEEE P802.22 Wireless RANs
4. "IEEE 802.22 Standard Approved for White Space Development", Hsien-Tang Ko, Chien-Hsun Lee, and Nan-Shiun Chu

Future Trends in Application Development

 1.  Wireless Broadband Network Solution  for Remote Regions

In the UK,  for example, BT estimated that 15% of UK residents (approximately 2.75 million households) could not have broadband access (over 2Mbit/s). By utilizing TV White Space, the residential broadband access coverage can be increased around 25% (or 687,000 households), or as high as one million households in practice. Furthermore, approximate 30% of  EU households are also with poor broadband connection. Broadband infrastructure in remote regions has always been a problem for governments and  telecommunication companies to solve. Nevertheless, the rollout and operating costs for solutions based on fixed line connection (FTTx, xDSL, Cable Modem etc.) or wireless access (e.g. 3G/4G, WiMAX and Wi-Fi) were all too high to be profitable. Most operators are reluctant to providebroadband service in remote regions without governmentsubsidies.

2.  Internet of things (IoT) Applications

 IoT has been one of the key areas of development in the ICT industry in recent years. The future development of IoT applications such as smart transportation, tele-medicine, smart grid, and surveillance and terminal transmission technologies such as ZigBee and RFID have all attracted strong commercial interest. Most of the feedback mechanisms for collected data require the wired or wireless networks operated by  telecommunication companies, however. Unless the service is promoted by the  telecommunication company itself, the cost of leasing the feedback network becomes a very heavy burden. The roll out of White Space networks may effectively reduce the cost of network leased and offer greater flexibility in actual use.

3. Emergency Disaster Communications

 When the March 11 earthquake disrupted most communications networks in Japan, the critical wide coverage of TV spectrum could keep residents in the disaster area continually informed through the 1-seg mobile TV system. In the past,broadcast TV channels used for emergency disaster notification systems emphasized one-way dissemination of information. If the system can incorporate the bi-directional data transfer of White Space, a transmission platform could be quickly set up at disaster sites to accelerate the flow of disaster information and the process of disaster response.

4. Integrated Applications for Heterogeneous Broadcast/Mobile Networks

Most governments plan to assign their recovered analog TV spectrum to emerging services such as digital TV, next-generation mobile communications and mobile TV. The existence of White Space opens up new development possibilities for broadcasting, mobile and even emerging service providers such as Femtocell data offloading, broadcast TV feedback, broadcasting/mobile multimedia services, mobile advertising, electronic signage and mobile commerce. For example, physical stores can use White Space to broadcast their advertisements and promotions to all kinds of mobile devices and boost their marketing effects. Advertisers canal so lower their operating costs by using White Space to transmit updates in real-time to electronic signage. These applications may not be considered innovative and have all been proposed before. However, the use of White Space and related technologies provide vendors with a more flexible and cost-effective solution that should lead to a more profit-able business model.

References:
1. "IEEE 802.22 Standard Approved for White Space Development", Hsien-Tang Ko, Chien-Hsun Lee, and Nan-Shiun Chu
2. "IEEE 802.22 Standard Finalized, White Space Internet Gets Closer", - http://news.softpedia.com/news/IEEE-802-22-Standard-Finalized-Whitespace-Internet-Gets-Closer-213778.shtml

IEEE 802.22 Standard Approved for White Space Development

 

The 802.22 Wireless Regional Area Networks (WRAN) standard was formally approved on July 22, 2011. The new standard will operate in the UHF and VHF spectrum used for broadcasting, and will support single-channel 22 Mbps broadband wireless access with a theoretical range of up to 100 km. The 802.22 technology will be particularly suitable for areas with low population density and avoid the interference with terrestrial TV broadcast signals. To optimize spectrum utilization, the UK and US have been actively examining ways of exploiting the unused "White Space" in the TV spectrum. The passing of the 802.22 standard is expected to accelerate the development of "White Space" applications such as remote area communications, IoT (Internet of  Things) communications, and emergency disaster communications.

On September23, 2010, the FCC has removed the mandatory requirement on the White Space devices (WSD) to include a sensing technology for the detection of signals from TV stations and low-power auxiliary service stations (wireless microphones). The reason behind this move is that geolocation and database access methods and other provisions will provide adequate and reliable protection fr incumbent devices.


There is no particular reason for the development of  the White Space WRAN standard under IEEE. IEEE has took the initiative to do so when the FCC has sent a request for the development of the standard. Other institutes also had the chance to work on it.

Capabilities of Cognitive Radio Networks

1. Cognitive Capabilities

Spectrum Sensing: possibility to individuate spectrum holes

Spectrum Sharing: possibility of sharing spectrum under the terms of an agreement between a licensee and a third party. Parties may eventually be able to negotiate for spectrum use on an ad hoc or real-time basis, without the need for prior agreements between all parties

Location Identification: ability to determine the MT location and the location of other transmitters, and then select the appropriate operating parameters such as the power and frequency allowed at its location

Network/System Discovery: for a cognitive radio terminal to determine the best way to communicate, it shall first discover available networks around it

Service Discovery: service discovery usually accompanies with network/system discovery

2. Self-Organized capabilities

Spectrum/Radio Resource Management:  manage and organize spectrum holes information among cognitive radios

Mobility and Connection Management:  due to the heterogeneity of CRNs, routing and topology information is more and more complex. Good mobility and connection management can help neighborhood discovery, detect available Internet access and support vertical hand-offs, which help cognitive radios to select route and networks
Trust/Security Management: trust is thus a prerequisite for securing operations in CRNs. 

What does the future hold for IEEE 802.22 WRAN

Even thought it has some challenges that require solution before it can get to market, it has many advantages that help it to be realized in the near future. For there is no worldwide uniformity in channelization for TV services, the standard shall accommodate various international TV channel bandwidths of 6, 7 and 8 MHZ. The spectrum where the IEEE 802.22 WRAN will use is already in use by TV services and wireless microphones. This has penalty as interference is unbearable unless a strict compliance is followed on radio emission.  The CR technology becomes handy here in solving the interference problem by spectrum sensing.

The main competitive advantages for the IEEE 802.22 WRAN are free access to the spectrum, very favorable propagation characteristics (large area of coverage and better material penetration) which will induce in reduction of cost, . Since this technology is intended for the rural areas which doesn't have a broadband access, there is no question for market.

Membership

Members participating in the development of the 802.22 standard come from a diverse background, which is primarily due to the unique requirement of incumbent protection of the final 802.22 standard. Hence, the key to the success of 802.22 depends not only on representatives from wireless companies but also from the incumbent community. Thus, members of the IEEE 802.22 WG include the more traditional corporations (e.g., Philips, Intel, Motorola, ST Micro, CRC, Samsung, Nokia) as well as delegates from the incumbent world (e.g., Fox, CBS, NAB, MSN, Shure Inc.). 

Spectrum Sensing Technologies

The spectrum sensing  technology has the following roles:

• Provide spectrum occupancy information to MAC
• Identify type of incoming signal
• Fast tracking time to improve data throughput
• Flexible resolution for adaptive and scaling searching
• Simple computation for low power
• Easy implementation for low cos

These functionalities are grouped into four set of categories:

1. Spectrum sensing - responsible for detecting unused spectrum
• In order to avoid interference the radio scans the spectrum for free channels. The most efficient way is to detect the primary users of the spectrum.
2. Spectrum management - responsible for capturing the best spectrum available for the required QoS of communication
• First analysis is made on the spectrum for interference, path loss, channel holding time etc and then making decisions on which channel to operate
3. Spectrum mobility - responsible for maintaining seamless communication during spectrum transition by using the channel information exchanges between the BSs and CPEs
4. Spectrum sharing - responsible for providing fair spectrum scheduling method for sharing the available open spectrum

Frame structure

The current 802.22 draft MAC employs the superframe structure as shown below.
At the beginning of every superframe, the BS sends special preamble and SCH
(superframe control header) through each and every TV channel (up to 3 contiguous)
that can be used for communication and that is guaranteed to meet the incumbent 
protection requirements. CPEs tuned to any of these channels and who synchronizes
and receives the SCH, are able to obtain all the information it needs to associate
with the BS. During the lifetime of a superframe, multiple MAC frames are transmitted
which may span multiple channels and hence can provide better system capacity, range,
multipath diversity, and data rate. Note, however, that for flexibility purposes the MAC
supports CPEs which are capable of operating on a single or multiple channels. During
each MAC frame the BS has the responsibility to manage the upstream and
downstream direction, which may include ordinary data communication, measurement
activities, coexistence procedures, and so on.

Generalized Superframe

The MAC frame structure is shown below. As we can see, a frame is comprised
of two parts: a downstream (DS) subframe and an upstream (US) subframe. The
boundary between these two segments is adaptive, and so the control of the
downstream and upstream capacity can be easily done. The downstream subframe
consists of only one downstream PHY PDU with possible contention intervals for
 coexistence purposes. An upstream subframe consists of contention intervals
scheduled for initialization (e.g., initial ranging), bandwidth request, UCS (Urgent
Coexistence Situation) notification, and possibly coexistence purposes and one 
or multiple upstream PHYPDUs, each transmitted from different CPEs.

Time/frequency structure of a MAC frame

Media Access Control (MAC) Layer

This layer will be based in Cognitive Radio Technology. It needs to be also able to adapt dynamically to changes in the environment by sensing the spectrum. The MAC layer will consist of two structures: Frame and Superframe. A superframe will be formed by many frames. The superframe will have an SCH (Superframe Control Header) and a preamble. These will be sent by the BS in every channel that it's possible to transmit and not cause interference. When a CPE is turned on, it will sense the spectrum, find out which channels are available and will receive all the needed information to attach to the BS.

Two different types of spectrum measurement will be done by the CPE: in-band and out-of-band. The in-band measurement consists in sensing the actual channel that is being used by the BS and CPE. The out-of-band measurement will consist in sensing the rest of the channels. The MAC layer will perform two different types of sensing in either in-band or out-of-band measurements: fast sensing and fine sensing. Fast sensing will consist in sensing at speeds of under 1ms per channel. This sensing is performed by the CPE and the BS and the BS's will gather all the information and will decide if there is something new to be done. The fine sensing takes more time (approximately 25 ms per channel or more) and it is used based on the outcome of the previous fast sensing mechanism.These sensing mechanisms are primarily used to identify if there is an incumbent transmitting, and if there is a need to avoid interfering with it.


To perform reliable sensing, in the basic operation mode on a single frequency band as described above (the “listenbefore-talk” mode) one has to allocate Quiet Times, in which no data transmission is permitted. Such periodic interruption of data transmission could impair the quality of service (QoS) of cognitive radio systems. This issue is addressed by an alternative operation mode proposed in IEEE 802.22 called Dynamic frequency hopping (DFH) where data transmission of the WRAN systems are performed in parallel with spectrum sensing without any interruption.