Beacon-Assisted Spectrum Access with Cooperative Cognitive Transmitter and Receiver

Spectrum access is an important function of cognitive radios for detecting and utilizing spectrum holes without interfering with the legacy systems. In this paper we propose novel cooperative communication models and show how deploying such cooperations between a pair of secondary transmitter and receiver assists them in identifying spectrum opportunities more reliably. These cooperations are facilitated by dynamically and opportunistically assigning one of the secondary users as a relay to assist the other one which results in more efficient spectrum hole detection. Also, we investigate the impact of erroneous detection of spectrum holes and thereof missing communication opportunities on the capacity of the secondary channel. The capacity of the secondary users with interference-avoiding spectrum access is affected by 1) how effectively the availability of vacant spectrum is sensed by the secondary transmitter-receiver pair, and 2) how correlated are the perceptions of the secondary transmitter-receiver pair about network spectral activity. We show that both factors are improved by using the proposed cooperative protocols. One of the proposed protocols requires explicit information exchange in the network. Such information exchange in practice is prone to wireless channel errors (i.e., is imperfect) and costs bandwidth loss. We analyze the effects of such imperfect information exchange on the capacity as well as the effect of bandwidth cost on the achievable throughput. The protocols are also extended to multiuser secondary networks.Comment: 36 pages, 6 figures, To appear in IEEE Transaction on Mobile Computin

Information Exchange Limits in Cooperative MIMO Networks

Concurrent presence of inter-cell and intra-cell interferences constitutes a major impediment to reliable downlink transmission in multi-cell multiuser networks. Harnessing such interferences largely hinges on two levels of information exchange in the network: one from the users to the base-stations (feedback) and the other one among the base-stations (cooperation). We demonstrate that exchanging a finite number of bits across the network, in the form of feedback and cooperation, is adequate for achieving the optimal capacity scaling. We also show that the average level of information exchange is independent of the number of users in the network. This level of information exchange is considerably less than that required by the existing coordination strategies which necessitate exchanging infinite bits across the network for achieving the optimal sum-rate capacity scaling. The results provided rely on a constructive proof.Comment: 35 pages, 5 figur

Production of the Exotic 1−−1^{--} Hadrons ϕ(2170)\phi(2170), X(4260) and Yb(10890)Y_b(10890) at the LHC and Tevatron via the Drell-Yan Mechanism

We calculate the Drell-Yan production cross sections and differential distributions in the transverse momentum and rapidity of the $J^{PC}=1^{--}$ exotic hadrons $\phi(2170)$, X(4260) and $Y_b(10890)$ at the hadron colliders LHC and the Tevatron. These hadrons are tetraquark (four-quark) candidates, with a hidden $s\bar{s}$, $c\bar{c}$ and $b\bar{b}$ quark pair, respectively. In deriving the distributions and cross sections, we include the order $\alpha_s$ QCD corrections, resum the large logarithms in the small transverse momentum region in the impact-parameter formalism, and use the state of the art parton distribution functions. Taking into account the data on the production and decays of these vector hadrons from the $e^+e^-$ experiments, we present the production rates for the processes $pp(\bar{p}) \to \phi(2170)(\to \phi(1020) \pi^+\pi^- \to K^+K^- \pi^+\pi^-)+...$, $pp(\bar{p}) \to X(4260)(\to J/\psi \pi^+\pi^- \to \mu^+\mu^-\pi^+\pi^-)+...$, and $pp(\bar{p}) \to Y_b(10890)(\to (\Upsilon(1S), \Upsilon(2S), \Upsilon(3S)) \pi^+\pi^- \to \mu^+\mu^-\pi^+\pi^-)+...$. Their measurements at the hadron colliders will provide new experimental avenues to explore the underlying dynamics of these hadrons.Comment: 4 pages, 2 Tables, 2 Figures; submitted to Physical Review Letter

A Wideband CMOS Linear Digital Phase Rotator

This paper presents a 10-bit wideband Cartesian phase rotator with a novel linear digital VGA implemented in a 0.13um CMOS process. The VGA topology is robust to device modeling uncertainties and PVT variations. The system provides 7.8dB voltage gain with -3dB bandwidth of 7.6GHz. A maximum phase error of 2º has been achieved for a phase shifting range of 360º with 32 phase steps of 11.25º. The capability to compensate for mismatched quadrature inputs is also demonstrated

Multiuser Diversity Gain in Cognitive Networks

Dynamic allocation of resources to the \emph{best} link in large multiuser networks offers considerable improvement in spectral efficiency. This gain, often referred to as \emph{multiuser diversity gain}, can be cast as double-logarithmic growth of the network throughput with the number of users. In this paper we consider large cognitive networks granted concurrent spectrum access with license-holding users. The primary network affords to share its under-utilized spectrum bands with the secondary users. We assess the optimal multiuser diversity gain in the cognitive networks by quantifying how the sum-rate throughput of the network scales with the number of secondary users. For this purpose we look at the optimal pairing of spectrum bands and secondary users, which is supervised by a central entity fully aware of the instantaneous channel conditions, and show that the throughput of the cognitive network scales double-logarithmically with the number of secondary users ($N$) and linearly with the number of available spectrum bands ($M$), i.e., $M\log\log N$. We then propose a \emph{distributed} spectrum allocation scheme, which does not necessitate a central controller or any information exchange between different secondary users and still obeys the optimal throughput scaling law. This scheme requires that \emph{some} secondary transmitter-receiver pairs exchange $\log M$ information bits among themselves. We also show that the aggregate amount of information exchange between secondary transmitter-receiver pairs is {\em asymptotically} equal to $M\log M$. Finally, we show that our distributed scheme guarantees fairness among the secondary users, meaning that they are equally likely to get access to an available spectrum band.Comment: 32 pages, 3 figures, to appear in the IEEE/ACM Transactions on Networkin