Method to improve diversity gain in a cooperative spectrum sensing network

A cooperative sensing technique (300) operates by selecting a group of subscribers (302) from a secondary system, measuring a cooperative sensing metric (306) and then using the metric to identify an achievable spatial diversity gain for the group of selected subscribers (308). Once an achievable spatial diversity gain is determined for the group (308/412), it is compared to a required spatial diversity gain (310), and if the condition is met at (310), the validated group can begin spectrum sensing (314) to identify a vacant/suitable channel for operation. If the achievable spatial diversity gain is insufficient, then a new group of users is selected (312) within the secondary system and the process repeats itself.

FIELD OF THE INVENTION

The invention generally relates to communication systems and more particularly to cooperative spectrum sensing in cognitive radio networks.

BACKGROUND OF THE INVENTION

Wireless products and services have continued to expand to the point that finite resources of available communication spectrum are being overwhelmed. Industry has been forced to make dramatic changes, as it must adapt to accommodate the exponential demand on spectrum access, efficiency and reliability.

The Federal Communications Commission (FCC) in the United States, and its counterparts around the world, allocate radio spectrum across frequency channels of varying bandwidth. Various bands may cover, for example, AM radio, VH television, cellular phones, citizen's-band radio, pagers and so on. As more devices go wireless, an increasingly crowded amount of radio spectrum needs to be shared. Although the radio spectrum is almost entirely occupied, not all devices use portions of the radio spectrum at the same time or location. At certain times, a large percentage of the allocated spectrum may be sitting idle, even though it is officially accounted for. Regulatory authorities are beginning to permit usage of allocated spectrum on a secondary basis under certain strict constraints. For example, the FCC is beginning to permit the secondary usage of channels 21-51, also known as TV white space.

Cognitive radio is a term used to describe a suite of technologies with the potential to significantly alter the manner in which spectrum is utilized by future radio systems. A paradigm for wireless communication in which either a network or wireless device alters its transmission or reception parameters to avoid inference with licensed or unlicensed incumbent users, cognitive radio implements measures to avoid selecting an occupied frequency, so as to avoid interference that can possibly damage the incumbent device and /or reduce its signal reception quality. The alteration of parameters is based on active monitoring of several factors in the external and internal radio environment, such as radio frequency usage, user behavior and network state. Cognitive radio operation in TV White Space is strictly conditional on reliable detection of occupied and unoccupied spectrum and is also conditional on fast network recovery in the case of in-band incumbent detection.

Cooperative spectrum sensing is a technique used to increase the probability of detection of primary users leading to reduced interference to the primary users by the cognitive radio network. However, cooperative sensing has certain drawbacks when individual nodes experience correlated fading or shadowing effects. Shadowing or long term fading refers to variation in received power due to large obstacles between the transmitter and the receiver. The cooperative sensing network quickly becomes inefficient as the cooperative sensing gains diminish with correlated fading/shadowing.

Accordingly, improvements are sought in cooperative sensing techniques to improve network efficiency and reduce interference to primary users.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to spectrum sensing management and control techniques. Cognitive radios are typically secondary unlicensed users which temporarily utilize available unused spectrum to maximize overall spectrum usage. The key challenge in optimizing spectrum usage is to avoid interference with primary users at all costs.

Throughout the description the terms nodes and subscribers have been used interchangeably and are intended to represent the same entity. The cooperative sensing technique provided herein achieves a high probability of detection with robustness to deep fades by choosing a group of cooperating nodes that provides needed spatial diversity gain, while keeping the number of cooperating nodes to a minimum. Further, the invention does not require any location information of the cooperating nodes. Briefly stated, the cooperative sensing technique operates by selecting a group of subscribers belonging to a secondary system, estimating a cooperative sensing metric and then validating the metric to identify an achievable spatial diversity gain for the group of selected subscribers. The achievable spatial diversity gain is then compared to a required or target spatial diversity gain required to meet the desired probability of primary user detection given a specified false alarm rate. If the chosen group of cooperating nodes provide the necessary spatial diversity gain, then each node senses a channel for the presence of the primary user and a decision is made based on the sensing results of each node. If the group of devices declares that the channel has no primary activity, then communication can take place amongst the secondary users utilizing an unused channel usually dedicated to a primary system. On the other hand, if the achievable spatial diversity gain is insufficient, then a new group of users is selected within the secondary system and the process repeats itself.

Accordingly, the apparatus components and method steps to be described herein have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

FIG. 1is an example of two cognitive radio networks having different spatial separation between their respective subscriber units.FIG. 1shows how correlated shadowing significantly impacts the Cooperative Sensing (CS) performance. Shadowing or long term fading refers to variation in received power due to large obstacles between the transmitter and the receiver. The power variation has a normal distribution in dB scale with a variance σdBin the range of 6-10 dB. Two users who are spatially very close to each other, as in104undergo highly correlated shadowing, while highly separated users in102undergo uncorrelated shadowing. The correlation in shadowing is a function of the spatial separation between two users and the topography in which they operate. The effects of correlated shadowing can be modeled by an exponential function R(d), where d is the separation between the users and the constant d0takes different values according to the terrain (urban or suburban) as shown in equation (1).

Referring toFIG. 2, there is shown a cognitive radio (CR) network202operating in accordance with the cooperative sensing technique of the present invention. CR network202comprises a plurality of cognitive radio subscribers (CRs)204and a base station or access point (AP)206having access to CR databases, such as geo-location database208, local terrain database210and policy database212. The CR subscribers may be two-way radios, cellular phones, and/or combinations of mixed two-way radios and cell phones or the like. The plurality of CR databases208,210,212may be located outside of the network202or internally within base station206. The illustration of the databases is simply to show that the base station206has access to geo-location, terrain and spectrum policy information. Network202utilizes cooperative sensing so as not to interfere with primary/incumbent system220operating under policies governed by a regulatory authority or other higher prioritized secondary system.

The cognitive radio network202operating under the cooperative sensing technique of the present invention caters to a situation where the power of the primary/incumbent system220is unknown and can either be a high power or a low power device. The type of primary system220can be made up of either high power devices, such as a television station or low power devices, such as wireless microphone or even other cognitive radios from a different network. Location information of the primary system220is accessible via geo-location database208. Policy information, associated with the primary system220as well as the cognitive radio system202, is known and accessible via database212.

Cognitive radio network202operating in accordance with the present invention minimizes interference with primary source220while, at the same time, maximizing opportunistic spectral usage. Spectral sensing is an important aspect of the technique being applied to cognitive network202. The spectral sensing incorporates cooperation between users to improve primary user detection capability.

FIG. 3illustrates an example of the cooperative sensing technique300in accordance with an embodiment of the invention. Referring to the CR network202ofFIG. 2in conjunction with the sensing technique ofFIG. 3, base station (BS)202begins at step302by selecting an initial cooperative sensing (CS) group with Nc users from CRs204. The Nc users selected by the BS are notified to begin sensing and report X measurements of hard/soft information at step304. The sensing configuration may include the channels or frequencies to sense, type of sensing method and type of sensing feedback as well as known and unknown transmitters. The goal of obtaining sensing results from the CR nodes is to estimate the achievable spatial diversity of a given group of nodes. The fact that spatial diversity is independent of the transmitter locations, it is possible to utilize known sources (transmitters) for sensing, such as known TV stations, cell towers etc. The X measurements are taken over a period of time. The same type of measurements will be requested by the BS, so there are X measurements of same type from each user. All the Nc users scan the channel and report back sensed data in the form of hard decisions/soft information to the BS at306. A hard decision is 1/0 binary decision specifying whether activity is detected or not detected on a channel. Soft information for a channel can be signal-to-noise ratio (SNR) values, received signal strength indicator (RSSI), correlation data to name a few.

Upon receipt of the sensing results (hard/soft information) from Nc users, the base station206runs a validation algorithm to validate the spatial diversity of the user group based on sensing data at step308, as will be elaborated inFIG. 4. Briefly, validation308compares cooperative sensing index (CSI) metrics to threshold(s) to identify the achievable spatial diversity gain of the Nc group of CRs, wherein the CSI metric is calculated using sensing data collected from the CRs in the Nc group. Thus validation step308results in an achievable spatial diversity gain of the Nc group of subscribers.

Once the validation step308is complete, the technique moves to step310where the base station206checks if the achievable spatial diversity gain of the group of Nc subscribers meets a required spatial diversity gain or not. The required spatial diversity gain for system200is a function of the desired probability of detection (at a known false alarm rate) and may be specified within the databases,208,210or212or specified by an external source, such as a system engineer and as such will be considered a predetermined spatial diversity gain. If the predetermined spatial diversity gain is not met at310, then another group of users is selected at312, and the technique returns to step304to notify and begin cooperative sensing again using the new group of users. If the predetermined spatial diversity gain is met at310, the validated group can start spectrum sensing at314to identify a vacant/suitable channel for operation. Periodic triggers can be used to ensure that the group remains validated or gets updated. Maintaining diversity of the CRs participating in cooperative sensing in order to reliably detect primary incumbents as provided by method300allows a secondary system200to communicate over a channel typically dedicated to the primary system without interfering with the primary system.

Referring toFIG. 4, the validation method308is described in more detail in accordance with an embodiment. The term “node” will be used instead of subscriber simply to facilitate explanation. Validation308is performed by the base station206. Since the minimum number of nodes needed for cooperation is at least 2, an initial value of L=2 is chosen. Thus, two nodes are randomly selected from Nc users at402.

The base station then selects sets of sensing measurements from the randomly chosen L nodes out of Ncnodes at step404. Note, as shown in equation (2), that there will be:

(NcL)=Nc!(Nc-L)!⁢L!(2)
sets of L nodes out of a set of Ncnodes.

The base station206estimates the Cooperative Sensing Index (CSI) metric using the measurements taken by the randomly selected L nodes. CSI is based on the statistical Entropy metric which is a measure of the uncertainty or randomness. For the sake of clarity, (not a limitation of the invention), assume that the sensing information from every node is a hard decision (activity detected (1) or not (0)). The CSI metric for a given L is defined by equation (3) as:

CSI⁡(L)=H⁡(U)L,H⁡(U)=-∑j=12L⁢⁢pj⁢log2⁡(pj)(3)
where H(U) is the Shannon Entropy for a discrete random variable U that takes on 2Lvalues each with probability pj. These probabilities are estimated based on the sensing measurements taken from the sets of randomly chosen L nodes. Since the CSI metric is a function of entropy, it measures the randomness in the measurements which is proportional to the spatial diversity gain. If the CSI metric for a given L exceeds a threshold as will be described later, then the selected group of Ncusers are capable of providing a diversity gain of L.

The CSI metric is normalized between 0 and 1 and monotonically decreases with increase in the number of correlated or dependent observations within the chosen set of nodes. As mentioned earlier, correlated nodes reduce the cooperative sensing gain and as such should be avoided.

For a given group of nodes, the CSI metric is used to estimate the achievable spatial diversity gain by computing the CSI metric for increasing values of L. The value of L at which the CSI metric falls below a predetermined threshold is termed as achievable diversity gain.

The CSI metric for randomly chosen L nodes is estimated on the selected measurements at step406and compared to one or more CSI thresholds at step408. The CSI threshold could be one or many different thresholds depending on the type of measurements that are used to calculate the CSI metric being considered. If the CSI estimate metric exceeds the CSI threshold at408, then the achievable spatial diversity gain is considered to be higher than L, and the achievable spatial diversity gain is incremented as L=L+1 at410. The CSI metric is then re-estimated at406using sets of sensing measurements from randomly chosen L+1 nodes out of Ncnodes. The process is repeated until the CSI metric falls below the threshold(s). The value of L when the CSI metric falls below the threshold will be the achievable spatial diversity gain at412, which gets compared to the specification at310.

In accordance with further embodiments, there may arise situations where there might not be any known TV stations or cellular towers operating within the CR network. In such cases, the CR devices will perform both group validation as well as primary user sensing simultaneously. In other words, if there are no reference signals that can be sensed for validation then sensing for the primary signal itself is performed for validation (estimation of CSI etc.) and use cooperative sensing within the group to make a decision on whether a primary user is present or not.

Referring toFIG. 5, a box plot illustration500of the variation in the CSI metric as a function of the correlation between the sensing measurements from cooperating users in accordance with an example of various embodiments of the invention. The y-axis502shows the CSI metric and the x-axis504shows the percentage of correlated users. Variability of the CSI metric is represented by the various boxes, such as boxes506each box representing an interquartile range. Outlier points508represent statistical data falling outside of the interquartile ranges506. Simulation tests for a given configuration have shown that for a selected number of correlated and uncorrelated nodes (Nc) of 10 and randomly selecting sets of L=4 nodes and the measurement samples associated therewith, the CSI metric may show very high values when the percentage of correlated users is less than 60 percent. As the number of correlated users (within the set of Nc users) increases, the CSI metric begins decreasing. The downward trend of CSI metrics continues with increasing percentages of correlated users. Again, the selection of uncorrelated users leads to increased diversity gain with optimal network configuration.

Accordingly, there has been provided a cooperative sensing technique for a cognitive radio network that determines achievable diversity gain for selected groups of secondary users within the network. Once the achievable diversity gain meets a specified spatial diversity gain (required to detect a primary system), the selected group of CR users can detect the presence of primary activity. The spectral sensing advantageously incorporates cooperation between users to improve primary user detection capability by choosing a group of cooperating nodes that meet predetermined spatial diversity gain requirements, while keeping the number of cooperating nodes to a minimum. Further, the technique does not require any location information of the cooperating nodes which is desirable.

In the description herein, numerous specific examples are given to provide a thorough understanding of various embodiments of the invention. The examples are included for illustrative purpose only and are not intended to be exhaustive or to limit the invention in any way. It should be noted that various equivalent modifications are possible within the spirit and scope of the present invention. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced with or without the apparatuses, systems, assemblies, methods, components mentioned in the description.

Those skilled in the art will appreciate that the above recognized advantages and other advantages described herein are merely exemplary and are not meant to be a complete rendering of all of the advantages of the various embodiments of the present invention.