215 research outputs found
Type-Based Random Access for Distributed Detection Over Multiaccess Fading Channels
The problem of distributed detection in a sensor network over multiaccess fading channels is considered. A random-access transmission scheme referred to as the type-based random access (TBRA) is proposed and analyzed. Error exponents of TBRA under noncoherent detection are characterized with respect to the mean transmission rate and the channel-coherence index. For the zero-mean multiaccess fading channels, it is shown that there exists an optimal mean-transmission rate that maximizes the detection-error exponents. The optimal mean-transmission rate can be calculated numerically or estimated using the Gaussian approximation, and it gives a sensor-activation strategy that achieves an optimal allocation of transmission energy to spatial and temporal domains. Numerical examples and simulations are used to compare TBRA with the conventional centralized time-division multiple access (TDMA) scheme. It is shown that for the zero-mean multiaccess fading channels, TBRA gives substantial improvement in the low signal-to-noise ratio (SNR) regime whereas for the nonzero mean fading channels, TBRA performs better over a wide range of SNR
Delay Considerations for Opportunistic Scheduling in Broadcast Fading Channels
We consider a single-antenna broadcast block fading
channel with n users where the transmission is packetbased.
We define the (packet) delay as the minimum number of channel uses that guarantees all n users successfully receive m packets. This is a more stringent notion of delay than average delay and is the worst case (access) delay among the users. A delay optimal scheduling scheme, such as round-robin, achieves the delay of mn. For the opportunistic scheduling (which is throughput optimal) where the transmitter sends the packet to the user with the best channel conditions at each channel use, we derive the mean and variance of the delay for any m and n. For large n and in a homogeneous network, it is proved that the expected delay in receiving one packet by all the receivers scales as n log n, as opposed to n for the round-robin scheduling. We also show that when m grows faster than (log n)^r, for some r > 1, then the delay scales as mn. This roughly determines the timescale required for the system to behave fairly in a homogeneous network. We then propose a scheme to significantly reduce the delay at the expense of a small throughput hit. We further look into the advantage of multiple transmit antennas on the delay. For a system with M antennas in the transmitter where at each channel use packets are sent to M different users, we obtain the expected delay in receiving one packet by all the users
Dynamic algorithms for multicast with intra-session network coding
The problem of multiple multicast sessions with
intra-session network coding in time-varying networks is considered.
The network-layer capacity region of input rates that can be
stably supported is established. Dynamic algorithms for multicast
routing, network coding, power allocation, session scheduling, and
rate allocation across correlated sources, which achieve stability
for rates within the capacity region, are presented. This work
builds on the back-pressure approach introduced by Tassiulas
et al., extending it to network coding and correlated sources. In
the proposed algorithms, decisions on routing, network coding,
and scheduling between different sessions at a node are made
locally at each node based on virtual queues for different sinks.
For correlated sources, the sinks locally determine and control
transmission rates across the sources. The proposed approach
yields a completely distributed algorithm for wired networks.
In the wireless case, power control among different transmitters
is centralized while routing, network coding, and scheduling
between different sessions at a given node are distributed
Type-Based Random Access for Distributed Detection Over Multiaccess Fading Channels
The problem of distributed detection in a sensor network over multiaccess fading channels is considered. A random-access transmission scheme referred to as the type-based random access (TBRA) is proposed and analyzed. Error exponents of TBRA under noncoherent detection are characterized with respect to the mean transmission rate and the channel-coherence index. For the zero-mean multiaccess fading channels, it is shown that there exists an optimal mean-transmission rate that maximizes the detection-error exponents. The optimal mean-transmission rate can be calculated numerically or estimated using the Gaussian approximation, and it gives a sensor-activation strategy that achieves an optimal allocation of transmission energy to spatial and temporal domains. Numerical examples and simulations are used to compare TBRA with the conventional centralized time-division multiple access (TDMA) scheme. It is shown that for the zero-mean multiaccess fading channels, TBRA gives substantial improvement in the low signal-to-noise ratio (SNR) regime whereas for the nonzero mean fading channels, TBRA performs better over a wide range of SNR
Towards a System Theoretic Approach to Wireless Network Capacity in Finite Time and Space
In asymptotic regimes, both in time and space (network size), the derivation
of network capacity results is grossly simplified by brushing aside queueing
behavior in non-Jackson networks. This simplifying double-limit model, however,
lends itself to conservative numerical results in finite regimes. To properly
account for queueing behavior beyond a simple calculus based on average rates,
we advocate a system theoretic methodology for the capacity problem in finite
time and space regimes. This methodology also accounts for spatial correlations
arising in networks with CSMA/CA scheduling and it delivers rigorous
closed-form capacity results in terms of probability distributions. Unlike
numerous existing asymptotic results, subject to anecdotal practical concerns,
our transient one can be used in practical settings: for example, to compute
the time scales at which multi-hop routing is more advantageous than single-hop
routing
On network coding and routing in dynamic wireless multicast networks
We compare multicast network coding and routing
for a time-varying wireless network model with interference-
determined link capacities. We use dynamic back pressure
algorithms that are optimal for intra-session network coding
and routing respectively. Our results suggest that under such conditions, the gap in multicast capacity between network coding and routing can decrease relative to a collision-based wireless model with fixed link capacities
- …