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