Activity recognition along with device position recognition can provide contextual cues suitable to infer user interruptibility and device accessibility. Our system fuses data from accelerometer and multiple light sensors to classify activities and device positions. Previously published results achieve robust activity recognition performance with multiple sensors attached to fixed body positions, a model suitable for use cases such as healthcare and fitness. We achieve comparable activity recognition performance using smartphones placed in unknown on-body positions including pocket, holster and hand. Results obtained from a diverse data set show that motion state and device position are classified with macro-averaged f-scores 92.6% and 66.8% respectively, over six activities and seven device positions. We demonstrate the performance of our classifier with an implementation running on the Android platform, that visitors can try out.

The fully connectedK-user interference channel is studied in a multipath environment with bandwidthW. We show that when each link consists ofDphysical paths, the total spectral efficiency can grow linearly withK. This result holds not merely in the limit of large transmit powerP, but for any fixedP, and is, therefore, a stronger characterization than degrees of freedom. It is achieved via a form of interference alignment in the time domain. A caveat of this result is thatWmust grow withK, a phenomenon we refer to as bandwidth scaling. Our insight comes from examining channels with single path links (D=1), which we refer to as line-of-sight (LOS) links. For such channels, we build a time-indexed interference graph and associate the communication problem with finding its maximum independent set. This graph has a stationarity property that we exploit to solve the problem efficiently via dynamic programming. Additionally, the interference graph enables us to demonstrate the necessity of bandwidth scaling for any scheme operating over LOS interference channels. Bandwidth scaling is then shown to also be a necessary ingredient for interference alignment in theK-user interference channel.

We consider the problem of two wireless networks operating on the same (presumably unlicensed) frequency band. Pairs within a given network cooperate to schedule transmissions, but between networks there is competition for spectrum. To make the problem tractable, we assume transmissions are scheduled according to a random access protocol where each network chooses an access probability for its users. A game between the two networks is defined. We characterize the Nash Equilibrium behavior of the system. Three regimes are identified: one in which both networks simultaneously schedule all transmissions, one in which the denser network schedules all transmissions and the sparser only schedules a fraction, and one in which both networks schedule only a fraction of their transmissions. The regime of operation depends on the path loss exponent α, the latter regime being desirable but attainable only for α > 4. This suggests that in certain environments, rival wireless networks may end up naturally cooperating. To substantiate our analytical results, we simulate a system where networks iteratively optimize their access probabilities in a greedy manner. We also discuss a distributed scheduling protocol that employs carrier sensing and demonstrate via simulations that again a near cooperative equilibrium exists for sufficiently large α.

The optimal diversity-multiplexing tradeoff curve for the intersymbol interference (ISI) channel is computed and various equalizers are analyzed using this performance metric. Maximum-likelihood signal decoding (MLSD) and decision feedback equalization (DFE) equalizers achieve the optimal tradeoff without coding, but zero forcing (ZF) and minimum mean-square-error (MMSE) equalizers do not. However if each transmission block is ended with a period of silence lasting the coherence time of the channel, both ZF and MMSE equalizers become diversity-multiplexing optimal. This suggests that the bulk of the performance gain obtained by replacing linear decoders with computationally intensive ones such as orthogonal frequency-division multiplexing (OFDM) or Viterbi, can be realized in much simpler fashion-with a small modification to the transmit scheme.

We consider the problem of two wireless networks operating on the same (presumably unlicensed) frequency band. Pairs within a given network cooperate with one another, but between networks there is competition for spectrum. To make the problem tractable, we assume transmissions are scheduled according to a random access protocol where each network chooses an access probability for its users. In this vein a game between the two networks is defined. We characterize the Nash Equilibrium behavior of the system. Three regimes are identified; a full-spread regime, where both networks choose to simultaneously schedule all transmissions; a partial-spread regime, where one network schedules all transmissions and the other only schedules a fraction; and a joint-spread regime, where both networks schedule a fraction of their transmissions. The regime of operation depends on the pathloss exponent alpha. The joint-spread regime, which has a cooperative flavor, is attainable only for alpha > 4, which suggests that in certain environments there may be a natural incentive for rival wireless networks to cooperate.

We introduce a new problem of broadcast source coding with a discrimination requirement - there is an eavesdropping user from whom we wish to withhold the true message in an entropic sense. Binning can achieve the Slepian-Wolf rate, but at the cost of full information leakage to the eavesdropper. Our main result is a lower bound that implies that any entropically efficient broadcast scheme must be "like binning" in that it also must leak significant information to eavesdroppers

This paper computes the optimal diversity multiplexing curve for intersymbol interference (ISI) channels and reveal it equals the matched filter bound. A simple transmission scheme that achieves this bound is presented and analyzed. The scheme provides a tractable method of extracting maximal diversity gain for any spatial multiplexing gain, when communicating over multipath fading channels.