1. Field of the Invention
Embodiments of the present invention relate, in general, to inter-cell spectrum sharing in cognitive radio networks and particularly to beacon period framing for inter-cell communication of on-demand spectrum contention resolution.
2. Relevant Background
Cognitive Radio is an enabling technology that allows unlicensed radio transmitters to operate in licensed bands at locations when that spectrum is temporarily not in use. Based on cognitive radio technology, Institute of Electrical and Electronics Engineers (“IEEE”) 802.22, following a Federal Communication Commission (“FCC”) Notice of Proposed Rulemaking in 2004, is an emerging standard for Wireless Regional Area Networks (“WRAN”) aiming to provide alternative broadband wireless access in, among other places, rural areas. Cognitive radio operates on a license-exempt and non-interference basis in the Television (“TV”) band (between 47-910 MHz) without creating harmful interference to the licensed services, which include, among others, Digital TV (“DTV”) and Part 74 devices (e.g. wireless microphones).
One of the key design challenges in cognitive radio network systems, such as the one specified in 802.22, is how to establish reliable and efficient communication links between the coexisting network cells. This is important to ensure proper operations of the coexisting cells in many aspects (such as quiet period synchronization, network discovery, and negotiation for channel use) in order to achieve the goals of coordinated incumbent protection and dynamic spectrum sharing.
In a typical deployment scenario, multiple WRAN cells, each of which comprises a base station (“BS”) and associated customer premise equipments (“CPE”), may operate in the same vicinity while coexisting with DTV and Part 74 devices. In order to effectively avoid harmful interference to these licensed incumbents, the set of channels on which the WRAN cells are allowed to operate could be quite limited. For example as shown in FIG. 1, residing within the protection contours of DTV 140 and wireless microphones 150, both WRAN1 110 and WRAN3 130 are only allowed to operate on channel A, while WRAN2 120 may occupy either channel A or B, assuming that in total only 3 channels (channel A, B and C) are available. If WRAN1 and WRAN3 (or WRAN1 and WRAN2) attempt to perform data transmissions on channel A simultaneously, mutual interference between these collocated WRAN cells could degrade the system performance significantly.
As well known to one skilled in the prior art, IEEE 802.22 provides a basic inter-cell communication mechanism called coexistence beaconing protocol (“CBP”), which allows the coexisting network cells to exchange coexistence beacons using the synchronized beacon periods (“BP”) scheduled at the end of the Medium Access Control (“MAC”) layer frames. To access the BPs by multiple coexisting network cells, well-know contention-based schemes such as Carrier Sense Multiple Access (“CSMA”) can be employed.
FIG. 2 shows a general depiction of a super-frame data structure as known to one skilled in the relevant art. The present depiction is of three channels 210, channel A, B, and C. All channels 210 have synchronized super-frames 215, which consist of 16 frames 2201, 2202, 2203, . . . 22016 with fixed frame size 240. Each frame can be divided into two parts: Data Transmission Period (“DTP”) 250 and an optional fixed sized Beacon Period 260. When an allocated beacon period is not utilized in a frame, the network cell can use the whole frame for data transmission without engaging in any inter-cell communication.
FIG. 3 depicts an example as known to one skilled in the relevant art showing how WRAN1 110 and WRAN2 120, operating on two different channels (Channel A 310 and B 320) can exchange CBP packets across channels. CBP however only allows a single network cell to transmit CBP packets in the BPs on its operating channel; in other words, off-channel BPs can only be used in the receive mode. As shown in Frame n 330 Cell 1 110 is transmitting while Cell 2 120 is receiving. In the same manner in Frame n+1 340 Cell 2's 320 BP is transmitting and Cell 1 310 is receiving. This successful illustration of communication is not always present. For example Frames n+2, n+3, and n all show examples of unsuccessful communication. In Frame n+2 350 both Cell 1 110 and Cell 2 120 are transmitting. Since neither cell is in capture mode the transmission fails. Similarly in Frame n+3 both Cell 1 110 and Cell 2 120 are in receiving mode ready to capture a transmission. However no transmission is present, again resulting in communication failure. Finally in Frame n 370 it is shown that the entire frame can be occupied by data transmission eliminating the possibility of transmission or capture.
FIG. 4 shows an example of CBP communications as known to one skilled in the relevant art where two coexisting network cells, WRAN 1 110 and WRAN 2 120, operating on the same channel (Channel A 410) are transmitting or receiving CBP packets during the synchronized BPs 420. For example in Frame 430 Cell 1 110 is transmitting and Cell 2 120 is receiving. Thus an effective communication link has taken place. Similarly in Frame n+1 440 Cell 2 120 is transmitting and Cell 1 110 is receiving. However in Frame n+2 both Cell 1 110 and Cell 2 120 are transmitting resulting in a communication failure. Likewise in Frame n+3 460, when both cells are in capture mode, no communication takes place. Lastly in Frame n+4, when the entire frame is occupied by data transmission, no communication between cells is possible.
In order to avoid performance degradation of CBP transmission due to multipath fading and shadowing, it is desirable to perform inter-cell communications (either transmitting or receiving) in a distributive fashion in which all stations in a network cell participate. Moreover, from a system design point of view, scheduling all stations in a cell to perform either transmitting or receiving a CBP packet during a BP can significantly reduce the design complexity.
Although the design of CBP has the advantage of simplicity, the CBP packets are transmitted by a network cell in non-deterministic instances that are unknown to other neighboring network cells. This non-coordinated nature of CBP communication causes a degradation of system performance and efficiency for handling intensive single- and multi-channel inter-cell communications.
One of the issues is packet loss due to collision of CBP packet transmissions, shown in Frame n+2 450 of FIG. 4. Since beacons are transmitted through random access during the BPs, when inter-cell communication becomes more intensive (i.e. more cells participate and more beacons are transmitted), there is a higher probability that two or more co-channel coexisting cells will transmit beacons in the same BP and cause collision at a receiving station. CBP packet collisions will lead to packet loss, therefore increasing the communication latency (the communication will not complete until a new CBP packet has been successfully received) and reducing the system efficiency (the bandwidth for transmitting and receiving the CBP packets that are dropped is wasted).
Another issue is bandwidth wastage. As mentioned above, collisions will cause the bandwidth used for transmitting and receiving the beacon to be wasted. On the other hand, a BP may be scheduled by a network cell in order to capture a randomly arrived beacon packet. If no beacon is captured in the scheduled BP, the bandwidth is wasted as well. Similarly, bandwidth wastage will happen when a transmitted CBP packet is not received by a target network cell that is not prepared to receive such a package. This “no capture” effect is shown in FIGS. 3 and 4 in Frame n+3 360, 460 respectively. Note that in IEEE 802.22 as an example, each frame has a fixed duration of 10 ms (24˜28 OFDM symbols) and the BP size is 4˜5 OFDM symbols. Therefore the overhead for inter-cell communication using CBP is quite large (about 13%˜20.8%) when a BP is scheduled in a frame, and enhancing bandwidth efficiency should be a key design goal for developing inter-cell communication protocol.
Cognitive radio network cells typically perform network discovery on a regular basis to identify neighboring cells that are operating co-channel or cross-channel. This process is usually performed by capturing beacon packets broadcasted by the neighboring cells. As shown, using the basic CBP, due to the random characteristic of the CBP packet transmission, a discovering network cell would have to continuously stay on a channel and monitor CBP packets for a potentially large number of frames. Obviously this causes significant overhead in bandwidth usage (and wastage) on one hand, and on the other hand, results in a long delay in discovering neighbors are undesirable for certain inter-cell coordination algorithms (such as load balancing, quiet period synchronization, etc.). Needed, therefore, is a protocol that enables reliable, efficient, and scalable inter-network communication in support of inter-network coordination functions.