6.3Coexistence Control of ETSI BRAN HiperLAN/2 and IEEE 802.11

The CCHC concept as discussed before has the potential to serve as basic approach to solve the coexistence problem between 802.11 and HiperLAN/2 stations, as well as the overlapping QBSS problem identified in Section 5.4, p. 108. Coexistence of WLANs using different protocols is difficult to achieve as long as the stations are not coordinated by a single device that is capable of all protocols. This is exactly what the CCHC approach offers. The CCHC as central coordinator of all stations within a QBSS allows a time-sharing of one frequency channel by two (or more) different WLANs. The coexistence control is difficult to perform when more than one CCHC operate at the same frequency channel, in the same area, as explained in Section 5.4, p. 108, for the coexistence of HCs in 802.11e networks, i.e., overlapping QBSSs each employing their own centrally controlled resource coordination.

6.3.1Conventional Solutions to Support Coexistence of WLANs

Until today, coexistence is handled by using DFS and by selecting different frequency channels upon detecting a competing QBSS. As described earlier, DFS is available for HiperLAN/2 as part of the standard. DFS is available too for 802.11 as part of the 802.11h supplementary standard defined in IEEE 802.11 WG (2002b), an extension to the 802.11 MAC and the 802.11a PHY. For this reason, DFS will be available to be applied for coexistence control in an integrated protocol as well; the CCHC would also be capable of applying DFS. Handling the coexistence problem based on DFS requires a number of free frequency channels to be available. This may not always be the case: under high traffic load or with a large number of active stations, or with many overlapping QBSSs, it may be advantageous and spectrum efficient that stations of different wireless networks share a single frequency channel instead of occupying different frequency channels. If all the frequency channels are already occupied by colocated WLANs, it appears to be advantageous to share a single frequency channel by stations of different WLANs. Of course, the same or similar level of QoS should be available then as if they would operate on exclusive frequency channels.

Developing a new technique to allow coexistence of CCHCs operated on the same frequency channel, is focus of the rest of this thesis.

6.3.2Coexistence as a Game Problem

In the following, a scenario of two overlapping QBSSs coordinated by CCHCs is assumed, as illustrated in Figure 6.5.

The CCHCs are assumed to be able to detect each other all the time, and all stations operate at the same frequency channel. Two CCHCs with their QBSSs share a finite capacity radio channel. For each CCHC, independent QoS requirements are assumed to exist, that both CCHCs attempt to serve throughout a certain communication phase. This phase is a time interval with finite duration, as is the continuing resource allocation process in the coexistence scenario.

CCHCs in general are able to allocate resources whenever required. However, when both CCHCs attempt to allocate resources at the same time, both experience significant QoS degradations in what they observe after the resource allocation process.

This mutual interdependency of the CCHCs is considered as a game problem (Mangold, 2000). It is therefore proposed to analyze the CCHC coexistence problem with the theory of games, as explained in detail in the next chapter. A competition for resources across QBSSs exists with and without information exchange between CCHCs, i.e., with and without their interworking.

The game problem exists also when the CCHCs that interact with each other are capable to interwork and are able to notify each other about their individual QoS requirements. They still would have to negotiate how to allocate resources: the actual competition remains present.

In what follows, exchanging information between CCHCs is assumed to be not permitted at any time in the coexistence scenario. The use of radio resources allocated by one CCHC is observed by the opponent CCHC. Any spectrum coordination by announcing the actual QoS requirements and/or upcoming radio resource allocations is excluded here.

The reason for this restriction is that in the following the thesis is established that, in order to achieve successful coexistence between interfering CCHCs, it is not required that an explicit communication by data exchange between the CCHCs must take place. The CCHC coexistence scenario discussed in the following, and the control concepts derived from game theory that will be presented in the next chapters, will serve as example for tackling the problem of WLAN coexistence in the unlicensed spectrum. In the unlicensed spectra, WLANs following different air interface standards should be able to share radio resources similarly to how the CCHCs share resources. However, different radio networks in an unlicensed spectrum are generally not capable of exchanging information because of the

lack of a common protocol. Therefore, although technically feasible, the exchange of information for spectrum coordination between CCHCs is assumed to be not allowed, throughout the analysis in the rest of this thesis.

The following chapters provide an in-depth analysis of the CCHC coexistence problem, and offer candidate solution concepts for the general problem of coexistence of radio networks that operate in unlicensed bands.

CCHC's detection ranges

Figure 6.5: Two QBSSs each coordinated by a CCHC, both operating at the same frequency channel. Each CCHC coordinates both, interworking HiperLAN/2 and 802.11 stations and coexistence of the respective QBSSs.