1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication, particularly at radio frequencies, has become commonplace in a large and rapidly increasing number of technologies. For example, wireless communication is used for transmissions between base stations and mobile units such as cellular telephones, personal data assistants, smart phones, pagers, text messaging devices, global positioning devices, navigation systems, network interface cards, notebook computers, desktop computers, and the like. Wireless technologies may also be used to provide access to various networks using shorter range transceivers that operate according to protocols such as Bluetooth, WiFi, and the IEEE 802.11 protocols. For example, a keyboard may transmit information indicative of keystrokes to a desktop computer using radiofrequency transmissions in accordance with the Bluetooth protocol. For another example, a notebook computer that includes a wireless interface card that operates according to the WiFi protocol may access the Internet via an access point located in an airport terminal.
Many conventional wireless communication systems, such as cellular telephone systems, operate in reserved frequency bands. For example, wireless communication systems that implement Universal Mobile Telecommunication System (UMTS) may operate in a reserved frequency band that extends from about 1900 MHz to about 2200 MHz. Consequently, interference between different wireless communication systems may be minimized by insuring that these systems do not transmit in overlapping frequency bands. However, the proliferation of wireless applications has led to intense competition for increasingly scarce spectrum. Awarding the remaining spectrum in a fair, consistent, and objective manner has consequently become progressively more difficult under the centralized command and control (C&C) licensing regime. The C&C approach is also inefficient and has exacerbated the scarcity of spectrum.
Techniques for sharing scarce spectrum among different wireless technologies and systems are being developed. For example, telecommunications regulatory authorities around the world (particularly Ofcom in the UK and the FCC in the US) are studying techniques for implementing more flexible, less “hands on,” spectrum management policies. Some examples of lighter spectrum management regimes that have already been implemented (albeit on a small scale) include the creation of shared spectrum commons. In some cases, the shared spectrum is licensed, e.g., the shared spectral bandwidth is a private commons and the license holder can allow second party access to the spectrum. Alternatively, the shared spectrum may be an unlicensed portion of the spectrum (e.g., the shared spectral bandwidth is a public commons, such as the ISM band) and so usage by different devices is totally unrestricted.
One of the main problems with spectrum commons is interference. A diverse range of devices and radio technologies may operate in a spectrum commons (or license-exempt bands), which tends to decrease the effectiveness of conventional politeness protocols for mitigating interference. Politeness protocols typically require that each device “listen” to the transmissions currently present within the transmission bandwidth and determine whether or not other devices are using the spectrum. If other devices are transmitting, the device may wait a random amount of time before beginning a transmission. However, devices transmitting in a spectrum commons may not share the same technology and therefore may not be able to determine whether or not other devices are present. Different devices would therefore not be able to determine if their transmissions were interfering with other devices. This would be the case even in the absence of hidden terminal problems, since devices may not obey an explicit politeness protocol.
Moreover, even if a device could determine that interference from another device was present, the device may not be able to communicate with the interfering device. Due to the different technologies and/or protocols in use in the spectrum commons, devices would not necessarily be able to identify the interferer. Even if the device could identify the interferer, the device may not possess the hardware and/or software required to decode an interfering signal from the interfering device. To illustrate this point, Table 1 provides a non-exhaustive list of different radio technologies and protocols that currently co-exist in the same radiofrequency bands, but which are incompatible. Catastrophic interference may result if devices using more than one of these technologies are transmitting at the same time and in the same geographic area. There are no well-established techniques for mitigating interference with a spectrum commons, but there are ongoing efforts to develop enabling technologies for spectrum commons.
TABLE 1List of co-existing technologies.TechnologyFrequency BandIEEE 802.11bISMIEEE 802.11gISMIEEE 802.15.1 (Bluetooth)ISMIEEE 802.15.3ISMIEEE 802.15.4ISMCordless PhoneISMVideo TransmitterISMMicrowave OvenISMIEEE 802.11aU-NIIIEEE 802.15.3a (UWB)TbdIEEE 802.16a2-16 GHzCordless PhoneU-NIINon-OFDM Video TransmitterU-NIIISM: Industrial, Science and Medical 2.4 GHz band: 2401-2483 MHz.U-NII: Unlicensed National Information Infrastructure 5.0 GHz band: 5150-5350 MHz and 5785-5825 MHz.
Common multi-radio medium access control layer (CMR-MAC) is an approach for controlling diverse radio access technologies. In CMR-MAC, devices that implement different access technologies and standards operate a common, compatible MAC. Current research efforts concentrate on enabling the cooperation of access technologies that are mutually non-interfering (i.e. operating on different spectrum bands), for example transmitting data via multiple access technologies to a destination. Therefore, CMR-MAC can potentially be a way of coordinating the co-existence of multiple technologies, it is not able to mitigate interference between devices transmitting in the same spectrum. To be effective, the CMR-MAC protocols would also have to be adopted by everyone as a standard. Moreover, CMR-MAC would reside in the lower layers of the protocol stack and hence would influence the higher layers, which would result in a tight coupling between CMR-MAC and the access technology. This makes it difficult to implement for all access technologies because the access technology would have to be built around the CMR-MAC.
Cognitive radio through opportunistic access is a proposed technique that requires that the transceiver make changes to particular transmission or reception parameters to perform a particular task in shared spectrum. The changes may be made based on observations of the radio spectrum. Software-defined radios are considered a key technology for enabling cognitive radio. A software-defined radio uses software technologies for the dynamic reconfiguration of the radio, so that a single transceiver can understand and use multiple radio access technologies. However, cognitive radio is still an immature technology and significant developments in many different fields are required before it becomes technologically and economically feasible. Software-defined radios are not mature and would result in more complex (and therefore expensive) transceivers with higher power consumptions, which would reduce battery-life in battery-powered devices. Receivers are also currently unable to reconfigure themselves fast enough to scan all available radio technologies.