Vidyut Naware

Stability and delay of finite-user slotted ALOHA with multipacket reception

The effect of multipacket reception (MPR) on stability and delay of slotted ALOHA based random-access systems is considered. A general asymmetric MPR model is introduced and the medium-access control (MAC) capacity region is specified. An explicit characterization of the ALOHA stability region for the two-user system is given. It is shown that the stability region undergoes a phase transition from a concave region to a convex polyhedral region as the MPR capability improves. It is also shown that after this phase transition, slotted ALOHA is optimal i.e., the ALOHA stability region coincides with the MAC capacity region. Further, it is observed that there is no need for transmission control when ALOHA is optimal i.e., ALOHA with transmission probability one is optimal. Next, these results are extended to a symmetric N>2 user ALOHA system. Finally, a complete characterization of average delay in capture channels for the two-user system is given. It is shown that in certain capture scenarios, ALOHA with transmission probability one is delay optimal for all stable arrival rates. Further, it is also shown that ALOHA with transmission probability one is optimal for stability and delay simultaneously in the two-user capture channel.

Signal processing in random access

In this paper, a cross-layer view for roles of signal processing in random access network and vice versa is presented. The two cases where cross-layer design has a quantifiable impact on system performance are discussed. The first case is a small network (such as wireless LAN) where a few nodes with bursty arrivals communicate with an access point. The design objective is to achieve the highest throughput among users with variable rate and delay constraints. The impact of PHY layer design on MAC protocol is examined and illustrates a tradeoff between allocating resources to the PHY layer and to MAC layer. The second case, in contrast, deals with large-scale sensor networks where each node carries little information but is severely constrained by its computation and communication complexity and most importantly, battery power. This paper emphasizes that the design of signal processing algorithms must take into account the role of MAC and the nature of random arrivals and bursty transmissions.

Asymptotic Detection Performance of Type-Based Multiple Access Over Multiaccess Fading Channels

The problem of communicating sensor readings over a multiaccess channel for detecting a target using type-based multiple access (TBMA) is considered. TBMA is analyzed in a general framework by considering non-i.i.d. data and nonidentical channel gains. An asymptotically optimal detector is proposed and its error-exponents for detection probabilities are characterized using tools from large deviations theory (LDT). In case of i.i.d. channel gains, it is shown that the performance of TBMA presents two distinct behaviors depending on whether the channel gains have zero mean. Numerical simulations are used to demonstrate that the error exponents provide reasonably accurate estimates of the performance of TBMA

Asymptotic detection performance of type-based multiple access in sensor networks

The problem of communicating sensor readings over a multiaccess channel for detecting a target is considered. A natural way of communication in target detection is to let sensors simultaneously transmit one of two predetermined frequency tones indicating whether the target is detected or not. Recently, this scheme has been generalized to consider non-binary sensor observations by letting sensors simultaneously transmit orthogonal waveforms depending on the value of their observations-type-based multiple access (TBMA). TBMA was shown to be asymptotically optimal in terms of detection-error probability under the idealistic assumptions that the sensor channel gains are identical, and the sensor data are conditionally independent and identically distributed (i.i.d.). In this paper, TBMA is analyzed in a more general framework by considering non-i.i.d. data and non-identical channel gains. An asymptotically optimal detector is proposed and its error-exponents for detection probabilities are characterized using tools from large deviations theory. Numerical simulations are used to demonstrate that the error exponents provide reasonably accurate estimates of the performance of TBMA.

Stability of slotted ALOHA with spatial diversity

We consider the problem of stability of slotted ALOHA for a system consisting of N users communicating with a common receiver, which employs spatial diversity to receive multiple transmissions simultaneously. We introduce a general packet reception model to incorporate multiple packet receptions since the collision channel model is no longer valid in such a scenario. We characterize the stability region of slotted ALOHA for the two user case explicitly. We also provide a sufficient condition for stability of slotted ALOHA for the N > 2 case, Finally, we apply our results to a simple system in which two users communicate with a base station equipped with a linear antenna array. Using stability region as a performance measure, we compare four different receivers/beamformers viz. matched filter, zero forcing, pseudo-MMSE and true MMSE employed by the base station to receive information from the users.