Abstract:
A method detects unused frequency bands in a cognitive radio network. Multiple frequency bands for RF signals are sensed using an antenna array including a plurality of elements coupled to a receive RF chain, in which the plurality of elements are individually controllable. For each frequency band, multiple spatial directions are sensed for the RF signals using the antenna array. A particular frequency band and a particular direction and a particular time slot are assigned as an available frequency band, an available direction and a particular time slot for transmitting RF signals in a cognitive radio if the RF signals are not sensed in the particular spatial direction for the particular frequency band, and in which the RF signals are transmitted by a transmit chain connected to the antenna array.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to spectrum sensing techniques in cognitive radio (CR) communications systems, and more particularly to CR devices with antenna arrays. 
   BACKGROUND OF THE INVENTION 
   In cognitive radio techniques, secondary users (CR transceivers) dynamically change transmit and receive parameters so that their signals do not interfere with signals of primary users (PU transceivers). Conventionally, cognitive radios consider radio frequency spectrum utilization, user behavior, and network state. Cognitive radios are considered in the IEEE 802.15, IEEE 802.19, and IEEE 802.22 standards. Conventionally, cognitive radio techniques conduct one-dimensional spectrum sensing by periodically scanning only the frequency domain to locate unused frequency bands (“spectrum holes”). The unused frequency bands can then be used for CR to transmit signals that do not interfere with the signals of the PUs. 
   To obtain accurate result, a duration T s (f) is used by the CR to sense the spectrum at the frequency band f. The sensing can be conducted by directly measuring the of PU signals (power-based sensing), or by analyzing statistics of the signals transmitted by PUs, (intelligent sensing). 
   Spectrum sensing can be categorized as focal sensing, and cooperative sensing. Local sensing is performed by a single CR. Cooperative sensing is performed by multiple CRs that share results. The performance of local sensing can be suboptimal due to shadowing and fading. Consequently, “hidden” PUs can exist because a single CR cannot detect the existence of all PUs that can receive interfering signals. In addition, locating a large number of unused frequency bands is better performed cooperatively. 
   In cooperative spectrum sensing, associated CRs can exchange local sensing results, so that a cognitive network obtains an accurate estimate of unused frequency band, or even, locations of the PUs. 
   SUMMARY OF THE INVENTION 
   The embodiments of the invention provide space-time-spectrum sensing for an RF spectrum in cognitive radio (CR). The CR is equipped with an antenna array. As defined herein, an antenna array includes multiple antenna elements. The antenna elements can be controlled individually. The antenna arrays can be used to scan the RF spectrum in frequency, time, and space domains to detect unused frequency bands. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a schematic of a cognitive radio network that uses the embodiments of the invention. 
       FIG. 1B  is a block diagram of a cognitive radio according to an embodiment of the invention. 
       FIG. 2  is a schematic of cognitive beamforming according to an embodiment of the invention. 
       FIG. 3A  is a graph of beamforming vector according to an embodiment of the invention. 
       FIG. 3B  is an energy pattern of an antenna array corresponding to the vector of  FIG. 3A ; and 
       FIG. 4  is a graph of space-time-frequency sensing according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   CR Network 
     FIG. 1A  shows radio networks in which the embodiments of our invention operates. The task is to detect unused frequency bands (“spectrum holes”  401  in  FIG. 4 ). The unused frequency bands can then be assigned as available frequency bands for a cognitive radio. 
   More specifically, we desire to detect the unused frequency bands using cognitive radios equipped with antenna arrays. As defined herein, an antenna array includes multiple antenna elements that can be individually controller over space, time and frequency dimensions. 
   The networks include a primary user (PU) network  104  and a PU transceiver  103 , and secondary users (CR)  101 - 102 . The PUs and CRs are located so that their signals can interfere with each other. As shown, the communication links do not need to be in the form of direct propagations, i.e., line-of-sight (LOS) connections. The embodiments of our invention can be applied to any scattering/fading environment. 
   As shown in  FIG. 1A , two CRs (CR 1   101  and CR 2   102 ) desire to communicate with each other. CR 1  is an access point (AP) and CR 2  is a mobile station (MS). It should be understood that other types of CR transceivers can communicate with each other according to the embodiment of our invention. 
   The PUs transmit and receive RF signals via a used frequency and f 1 . The CRs detect an unused frequency band f 2 . This frequency band that is unused by the PUs is then assigned to the secondary users. The frequency band f 2  should not interfere with the frequency band f 1 . 
   CR Transceiver 
     FIG. 1B  shows a cognitive radio (CR)  150  according to an embodiment of our invention. The CR includes an antenna array  152  connected directly, or via a switch  154 , to one or more transmit RF chains  156  and receive RV chains  158 , which are respectively connected to a transmit unit  157  and a receive unit  159 , operating respectively according to a transmit beamforming matrix (TxBF)  161  and a receive beamforming matrix (RxBF)  162 . A controller  170  controls overall operations of the various components as described herein. 
   The antenna array  152  includes multiple antenna elements  151  that can be individually controlled  170  via the RF chains. Depending on the connections between the RF chains and the antenna array, the elements can either transmit, receive or both. As described herein the radio signals transmitted by the antenna elements can vary in space, time and frequency. 
   Using the beamforming matrices  161 - 162 , the antenna arrays can be “steered” in particular spatial directions. The steering is accomplished by either time shifting in the spatial domain or phase shifting in the frequency domain the RF signals on the individual antenna elements  151 . This is called beamforming. 
   Channel Model 
   A matrix H kj   (CR)  (f) represents the CR channel state information (CSI) matrix from CR k to CR j at frequency f. A vector h k . . . n   (ifi)  (f) denotes an n th  interference channel vector form a particular PU with an index n, detected at CR k at a frequency f. In the case of multiple interfering signals transmitted from one PU, each signal serves as one virtual PU. Hence, the channel vector h k . . . n   (ifi)  (f) still applies. We denote N T . . . k  and N R . . . k  as the number of transmit and receive antenna elements of the array  152  at CR k, and N T . . . k =N R . . .k =N k , ∀k. 
   In the CRs, spectrum sensing via the RF chain  158  and data transmission via the RF chain  156  can be conducted in a time-division manner, i.e., alternatively unused frequency bands can be sensed in the time domain, or by different CRs in a cooperative manner in the frequency domain. The results can be formatted and distributed in the frequency domain using a media access control (MAC) layer. 
   Let N S . . . k  denote the number of antenna elements at CR k, in a time-division sensing scenario, where N S . . . k =N k . The detecting by different CRs, where N S . . . k ≠N k  is possible, is described below. 
   Therefore, the matrix H kf   (CR) (f) is of dimension N f ×N k , while the dimension of the vector h k . . . n   (ifi)  (f) is N k 1. With these settings, the n th  sensed signal vector from a PU detected by CR k at frequency f is:
 
 y   k . . . n   (ifi) (f)= h   k . . .n   (ifi) (f) x   n   (ifi) (f)+ v   k (f),  (1)
 
where x n   (ifi)  (f) is the signal transmitted by the n th  PU at frequency f, and the vector v k  (f) represents the additive noise at CR k.
 
   The CR communication link at frequency f can be represented by:
 
 y   kj   (CR) (f)= H   kj   (CR) (f) T   k (f) x   k (f)+ v   j (f),  (2)
 
where x k (f) is the L k ×1 transmitted vector form CR k, in which L k ≦min(N k ,N j ), T k (f) is the N k ×L k  transmit beamforming matrix at CR k.
 
   It is possible for a receiver to apply the RxBF matrix  159  before signal detection. This can be expressed by: 
                           s   kj     ⁡     (   f   )       =           R   j     ⁡     (   f   )       H     ⁢       y   kj     (   CR   )       ⁡     (   f   )                     =           R   j     ⁡     (   f   )       H     ⁡     [           H   kj     (   CR   )       ⁡     (   f   )       ⁢       T   k     ⁡     (   f   )       ⁢       x   k     ⁡     (   f   )         +       v   j     ⁡     (   f   )         ]                     =           H   kj       (   CR   )     ⁢   ′       ⁡     (   f   )       ⁢       T   k     ⁡     (   f   )       ⁢       x   k     ⁡     (   f   )         +       v   j   ′     ⁡     (   f   )           ,                 (     3.1   ,   3.2   ,   3.3     )               
where R j (f) is the N f ×L j  RxBF matrix  159 , (.) H  represents the matrix conjugate transpose H kj   (CR)′ =R j (f) H  H kj   (CR) , the noise vector v f ′(f)=R j (f) H v j (f), and s kj (f) is the L j ×1 equivalent received vector. Note that Equation (3) is generally similar to Equation (2) other than the details of the modified channel matrix and the noise vector. To correctly recover the transmitted data in x k (f), L j ≧L k  is required.
 
   Beamforming 
     FIG. 2  shows transmit and receive beamforming, TxBF and RxBF, in scattering/fading channels. Circles  201  and  202  represent respectively areas around CR 1  and CR 2  where signals can be scattered. 
   In the case of two-dimensional beamforming, L k =L j =2. At the frequency f, the strongest inter-cluster paths, which are orthogonal or non-interfering with each other, are denoted by paths  203  and  204 . The transmitter and/or the receiver can select to form their beams corresponding to these two paths, where t k . . .i (f) and r k . . .i (f) denote the i th  transmit and receive beamforming vectors at CR k, respectively. 
   Therefore, the TxBF matrix  161  and the RxBF matrix  162  can be expressed by:
 
 T   k (f)=[ t   k . . . 1 (f) t   k . . . 2 (f) . . .  t   k . . . L     k   (f)], and  R   j (f)=[ r   j . . . 1 (f) r   j . . .  (f) . . .  r   j . . . L     j   (f)].  (4.1, 4.2)
 
   It should be noted that any type of beamforming can be used. This includes the case where multiple CRs operate at the same frequency f, and where TxBF and RxBF are used for both multi-user and multi-stream interference reduction at each CR. 
   Space-Time-Frequency RF Spectrum Sensing 
   In prior art, one-dimensional local frequency sensing does not consider spatial directions. That is equivalent to sensing with a random-directional receive antenna array or a fixed RxBF matrix. 
   In space-time-frequency sensing of the RF spectrum sensing according to the embodiments of our invention, an adjustable N k ×1 RxBF vector b k  (f) is applied at an input to the antenna array  152 . The vector b k  (f) is varied over time at different frequency bands using a pre-determined “sweeping route.” The values assigned to the vector b k  (f) can vary for different applications. 
   As shown in  FIG. 3A , the vector b k  (f)  310  can be aimed in particular spatial direction according to azimuth and altitude angles (θ, φ), when a high spatial sensing resolution is required.  FIG. 3B  shows the equivalent antenna pattern. In this example, all of the antenna elements are omni-directional. In this case, the beamforming vector b can be expressed as: 
                       b   k     ⁡     (   f   )       =       β   ⁡     (   φ   )       ⁡     [         1             ⅇ     2   ⁢   π   ⁢           ⁢   j   ⁢       Δ   k     λ     ⁢   cos   ⁢           ⁢   θ               ⋮             ⅇ     2   ⁢   π   ⁢           ⁢     j   ⁡     (       N   k     -   1     )       ⁢       Δ   k     λ     ⁢   cos   ⁢           ⁢   θ             ]         ,           (   5   )               
where β(φ) is a function corresponding to φ, and Δ k  is the spacing between adjacent antenna elements. Note that the vector b k  (f) in Equation (5) can be varied to provide any spatial setting for the antenna array, and any pattern of each antenna element.
 
   As shown in  FIG. 3B , the relatively narrow main lob  301  of the equivalent antenna pattern can result in a larger number of quantized spatial directions to be sensed. Other values in the vector b k  (f) b k (f) can lead to wider main lobes, which correspond to lower sensing resolution and a reduced number of quantized spatial directions to be sensed. 
   The space-time-frequency spectrum sensing periodically adjust the vector b k  (f) so that the different frequency bands are sensed in different spatial directions. These directions correspond to different spatial directions of the main lobe  301  of the equivalent antenna pattern. For example, let a set Ψ k  (f) collect the PU signals sensed at CR k in a frequency band f. Then, under the directional antenna array setting of Equation (5), the sensing output at frequency f and direction (θ, φ) can be expressed by 
                           a   k     ⁡     (     f   ,   θ   ,   φ     )       =           b   k     ⁡     (   f   )       H     ⁢       ∑     n   ∈       Ψ   k     ⁡     (   f   )                   ⁢           ⁢       y   k_n     (   ifi   )       ⁡     (   f   )                         =           b   k     ⁡     (   f   )       H     ⁢       ∑     n   ∈       Ψ   k     ⁡     (   f   )                   ⁢           ⁢     (           h   k_n     (   ifi   )       ⁡     (   f   )       ⁢       x   n     (   ifi   )       ⁡     (   f   )         +       v   k     ⁡     (   f   )         )           ,                 (   6   )               
where α k (f, θ, φ) is a time-varying value.
 
   By sensing over a time duration T S (f), the CR can conduct the conventional spectrum sensing either by power-based or by other intelligent methods. In other words, the time-varying variable α k (f, θ, φ) acts like the observations of conventional CR without considering spatial parameters. Again, the spatial sensing is based on the values in the beamforming vector b(f)  310 , or the equivalent antenna pattern  301 . 
   Then, as shown in  FIG. 4 , the CR forms a “map” of the available space-spectrum holes  401 , stored in a time varying set Γ k (f) by collecting the current sensing results.  FIG. 4  shows an example of our space-spectrum sensing for three frequency bands f 1 f 2 and f 3 ., with space along the vertical axis and frequency on the horizontal axis, and time orthogonal to the other two axes. The unused frequency bands are the “holes”  401 . The remaining area  402  in the space and frequency plane is, perhaps, used by PU signals sensed by the CR. In  FIG. 4 , all three frequency bands are occupied, in part, by some PUs. 
   Without spatial sensing as is done in the prior art, e.g., by using only one omni-directional antenna, the CR will probably not locate any spectrum holes. Also, with a fixed directional antenna as in the prior art, e.g., in the area  403 , fewer spectrum holes are detected. Therefore, the space-time-frequency RF spectrum sensing according to the embodiments of our invention locates more available spectrum holes, compared with conventional one-dimensional CR sensing methods. 
   Additional spectrum holes can be located by sensing over smaller frequency bands. This can be realized, e.g., by sensing one part of the overall reachable bandwidth for the CR at each sensing phase, and then sensing at different parts of the overall frequency bands using different sensing phases. Consequently, space-time-frequency sensing of the RF spectrum does lengthen the sensing duration. Reducing the frequency sensing bandwidth can also simplify the hardware designs. 
   In the case of space-time-frequency sensing of the RF spectrum using the antenna array  152  of N k  elements  151  at CR k, the CR  150  can concurrently form N k  orthogonal beams according to:
 
 B   k (f)=[ b   k . . . 1 (f), b   k . . . 2 (f), . . . ,  b   k . . . N     i   (f)],   (7)
 
where, the inner product between two vectors            (b k . . . m (f),b k. . . n (f)         =0, ∀m≠n, and the beamforming matrix B k (f) is an N k ×N k  orthogonal matrix. Then, the RxBF matrix B k (f)  162  can be directly applied to derive N k  sensing statistics in the form of Equation (6). This decreases the time required for the sensing by a factor of N k .

   Cooperative CRs can exchange the sensing results to enable communications on the unused frequency bands. If Γ k (f)≠Φ, CR k reports an unused frequency f. 
   Space-Time-Frequency Sensing of the RF Spectrum with Beamforming 
   The space-time-frequency sensing can be combined with the TxBF  161  and/or the RxBF  162 . As described above, without sensing, the transmitter and the receiver can locate the optimal beamforming steering vectors/matrices according to the current channel state, over all possible spatial directions. 
   The basic idea of our space-time-frequency sensing and beamforming can be described as follows. 
   The CR k transmits to CF j at a frequency f, i.e., Γ k (f)≠Φ, Γ j (f)≠Φ). Therefore, vectors T k  (f) in the TxBF  161  are selected within the detected unused frequency bands Γ k (f). In addition, the CR receiver j should not observe any interference from the PUs. Hence, the RxBF  162  in Equation (3) is applied directly, where the RxBF vectors in R j (f) are selected within the detected unused frequency bands in Γ j (f). 
   If b k . . . 1 (f), . . . , b k . . . L     k   (f)∈Γ k (f), when T k (f)=α[b k . . . 1 (f), b k . . . 2 (f), . . . , b k . . . i     k   (f)], then the n th  PU will observe weak interference form CR k, because b k . . . n   (ifi) (f) H  T k (f)≈0. 
   At CR receiver j, the sensing results, (“space-spectrum holes”) imply:
 
 b   j . . . n (f) H   h   j . . . n   (ifi) (f)≈0.
 
   Hence, there is almost no interference form PU observed at CR j, if the vectors b j . . . n (f) H  is used for the matrix RxBF  162 . Consequently, CRs and PUs can operated in the same frequency bands f without interfering with each other. This can significantly improve the system level efficiency of the CR and PU networks. 
   According to Equation (3), the matrices TxBF and RxBF can be combined with any other transmitter and/or receiver designs, such in MIMO and/or multi-user systems, where Equation (3.3) represents the equivalent channel model. 
   In the case of beamforming over pre-determined (quantized) steering vector tables, e.g., by using linear precoding, the spatial sensing is performed over the vectors of the steering tables. Then, the CR can construct a new steering table with a reduced size, i.e., the steering vectors corresponding only to space-spectrum holes. Then, the beamforming can be conducted with this new table. This can reduce the complexity for locating the steering vectors. If the space-time-frequency sensing is not conducted within the original steering table, the updated table contains the steering vectors corresponding to the resultant space-spectrum holes. 
   Variations 
   Up to now, we have described electronic “steering” of the antenna arrays using beam forming. However, it should be understood, that the different spatial directions can also be sensed by physically adjusting the azimuth and altitude angles (θ, φ) of the antenna array elements. 
   The space-time-frequency spectrum sensing can be extended to cooperative sensing, where local space-time-frequency sensing results are combined by multiple CRs. The sensing can be performed in a cooperative manner, while the processing and the assignment of available frequency bands and directions can be performed centrally. 
   The space-time-frequency spectrum sensing is applicable to the case of multi-user beamforming at the CR transmitter or receiver, in the same frequency band, as described above. 
   The space-time-frequency spectrum sensing is applicable to both single carrier (SC) and orthogonal frequency-division multiplexing (OFDM) systems. In the case of SC, sensing over a frequency band is accomplished by changing the carrier frequency of the CR. In the case of OFDM, the same task can be performed by the combination of carrier sensing and monitoring the energy in different subcarriers of a wideband OFDM signal during the sensing phase. In OFDM, the beamforming schemes above can be conducted in each subcarrier. 
   In the case of space-time-frequency spectrum sensing at a transmitter CR k equipped with an antenna array, the difference between the RF responses of the transmit chains  156  and that of the receive chains  156  can reduce accuracy. This is because the channel matrices contain not only the physical propagation channels, but also the RF responses. To reduce these RF imbalances, hardware calibration and over-the-air calibration can be performed with the assistance of a peer station in the CR network. This can be assisted by media-access control (MAC) signaling. 
   When spectrum sensing and data transceiving are conducted by different users and different frequency bands, the space-time spectrum sensing can still be applied, but at a reduced sensing accuracy. Because different antenna arrays can experience non-identical fading characteristics, especially for small scale fading, the spatial sensing result can deviate from that experienced by the antennas used for data transceiving. If these two sets of antenna array are substantially colocated, and if spatial sensing results are mainly determined by local scattering and spatial parameters, which can be the same for both arrays, spatial sensing result is still effective. 
   In the case N S . . . k ≠N k , the space-time-frequency spectrum sensing results with steering vectors b k (f) of the dimension N S . . .  ×1, should be converted to steering vectors b k   ′ (f) of the dimension N k ×1, before conducting data transceiving. This conversion is conducted in such a manner that the change in the equivalent antenna pattern is minimum. For example, if b(f) is aimed in different directions (θ, φ) as in Equation (5), then b′ k (f) for the updated unused frequency bands is the same form as in Equation (5) with the same spatial parameters (θ, φ). 
   Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.