Time-of-arrival estimation is a critical aspect of wireless systems, both for initial access and ongoing communications. The timing estimates are needed in uplink and downlink, where the initial timing estimation in LTE uplink is based on a physical random access channel (PRACH) preamble transmitted by the user. Block Interleaved Frequency Division Multiple Access (B-IFDMA) is a transmission scheme that recently emerged as an attractive choice for LTE operation in unlicensed spectrum due to its benefits in allowing increased power levels under unlicensed band regulations. In this paper, we introduce a new PRACH preamble format suitable for unlicensed operation based on B-IFDMA. In addition, we present a receiver structure for the PRACH preamble together with simulation results on its timing estimation performance. We show that the PRACH preamble design is robust against possible aliasing effects caused by frequency-domain properties of B-IFDMA and provides accurate timing estimates. We also propose non-uniform frequency interleaving for the PRACH preamble that further improves timing estimation performance.

The capability of nodes to broadcast their message to the entire wireless network when nodes employ cooperation is considered. We employ an asymptotic analysis using an extended random network setting under an additive white Gaussian channel model with path loss, and show that the broadcast performance strongly depends on the path loss exponent of the medium. In particular, the probability of broadcasting in a 1-D infinite network is zero for path loss exponents larger than one, and is equal to a nonzero value for path loss exponents less than one. In 2-D infinite networks, the same behavior is observed for path loss exponents above and below two, respectively.

We present achievable scaling results on the per-node secure throughput that can be realized in a large random wireless network of n legitimate nodes in the presence of m eavesdroppers of unknown location. We consider both one-dimensional and two-dimensional networks. In the one-dimensional case, we show that a per-node secure throughput of order 1/n is achievable if the number of eavesdroppers satisfies m = o(n/log n). We obtain similar results for the two-dimensional case, where a secure throughput of order 1/(√n log n) is achievable under the same condition. The number of eavesdroppers that can be tolerated is significantly higher than previous works that address the case of unknown eavesdropper locations. The key technique introduced in our construction to handle unknown eavesdropper locations forces adversaries to intercept a number of packets to be able to decode a single message. The whole network is divided into regions, where a certain subset of packets is protected from adversaries located in each region. In the one-dimensional case, our construction makes use of artificial noise generation by legitimate nodes to degrade the signal quality at the potential locations of eavesdroppers. In the two-dimensional case, the availability of many paths to reach a destination is utilized to handle collaborating eavesdroppers of unknown location.

We introduce a peak mitigation technique for reference based systems that is similar to the tone reservation scheme employed in orthogonal frequency division multiplexing (OFDM) systems but without the cost in data rate. A comparison of reference-based systems under either peak or average power constraints is presented.