The way wireless spectrum is used is changing in the future. The strict allocation of bandwidth to a certain standard will be changed, at least in some frequency bands, to be more flexible. The band, which has been primarily allocated to one wireless standard, might also have other secondary users operating there under certain rules, such as for example that secondary users are not allowed to cause any harmful interference to the primary user. This typically means that the secondary user must detect and avoid the primary users which use the allocated frequency bands. Since it is up to the secondary user to avoid interference and it is seen as impractical for a central node to routinely inform the secondary user which spectrum it might access, the secondary user must be cognizant of the spectrum and is therefore termed herein for brevity as a cognitive user or cognitive radio CR.
CR technology is supposed to implement negotiated or opportunistic spectrum sharing over a wide frequency range covering multiple mobile communication standards, and so the CR link should intelligently detect the usage of a frequency segment in the radio spectrum and jump into any temporarily unused spectrum rapidly without interfering the communication between other authorized (e.g., primary) users. CR technology is promising for the friendly coexistence of the heterogeneous wireless networks, i.e., cellular, wireless Personal Area Network (PAN), wireless Local Area Network (WAN), and wireless Metro Area Network (MAN), etc. In the US, the FCC has encouraged the development of the CR technology for unlicensed operation in the TV broadcasting bands, and CR technology has been adopted as a core feature in the emerging wireless access standards such as the IEEE 802.22-Wireless Regional Area Network (WRAN).
The operation under a primary user (or in a band where there can be several wireless standards with no fixed allocations) means that the CR has to find free (unoccupied) frequencies in three dimensional boundaries: time, frequency and space (for brevity, TFS). The CR has to be aware of the radio environment and change its operation if any of the primary users are occupying the current TFS. The control and maintenance of this kind of system is challenging, and operating on a very wide bandwidth can be a time and power consuming operation.
One consideration addressed herein is how a CR device, when first powered up for the first time in a new location, will find a CR system when the CR device does not have any prior information of the spectrum occupancy/usage in the TFS domains. This is generally termed the so called acquisition problem. Typically, the CR devices may potentially spend vast amounts of time in an acquisition mode, and thus minimizing the power consumption in this mode has substantial implications to the overall power consumption of the CR device. Therefore, in the cognitive radio field a great emphasis has been placed on the spectrum sensing area, i.e. how to find the unused frequency spaces.
One relevant reference for the acquisition problem is by J. Laskar, et al, and entitled “RECONFIGURABLE RFICS AND MODULES FOR COGNITIVE RADIO” (IEEE, SiRF 2006). The Laskar paper asserts that the realization of CR requires two essential features: (i) wideband spectrum sensing, and (ii) frequency-agile operation. In order to find the vacant spectrum available, the CR system can recognize the existence of the signals with meaningful power levels throughout the wide frequency range from tens of MHz to several GHz. Additionally, it should have reliable detection performance with low power consumption for various types of interference signals. The proposal in the Laskar paper does spectrum sensing first by coarse sensing and then by fine sensing in the frequency domain. The coarse-sensing block detects the existence of any meaningful RF signals received by the wideband antenna. Since the impacts of the interferers depend on the signal types and the modulation schemes, Laskar asserts that the identification of the specific signal format is very important for reliable CR link performance. Hence, the fine-sensing block of Laskar further scrutinizes the detected spectrum segment to determine the type of the received interference signal. The resulting spectrum usage status is then reported to the MAC, which processes the reported usage data to allocate the available spectrum for safe CR link.
The Laskar teachings may well be an effective and efficient way to find the spectrum holes that an opportunistic CR can then use, but by the inventors' lights this alone misses an important aspect of the whole CR acquisition problem. Namely, how does a CR device first find the CR system? Once this is done then the CR device can begin its spectrum sensing that finds the free spectrum holes, but as Laskar admits this is a wideband problem. It therefore has the potential to consume excessive power, which is always a consideration in mobile devices. What is needed in the art is a way to find the CR system in the first place, in a manner more power-efficient than simple blind detection over a wideband potential spectrum. Once that is done in an efficient manner, then the terminal in the CR system can take advantage of the already detected spectrum usage and holes and continue to update this information in conjunction with other terminals in CR system.