Transmitter channel state information (CSIT) is crucial for the multiplexing gains offered by advanced interference management techniques such as multiuser multiple-input multiple-output (MIMO) and interference alignment. Such CSIT is usually obtained by feedback from the receivers, but the feedback is subject to delays. The usual approach is to use the fed back information to predict the current channel state and then apply a scheme designed assuming perfect CSIT. When the feedback delay is large compared to the channel coherence time, such a prediction approach completely fails to achieve any multiplexing gain. In this paper, we show that even in this case, the completely stale CSI is still very useful. More concretely, we show that in an MIMO broadcast channel with transmit antennas and receivers each with 1 receive antenna, K/1+1/2+···+1/K (>;1) degrees of freedom is achievable even when the fed back channel state is completely independent of the current channel state. Moreover, we establish that if all receivers have independent and identically distributed channels, then this is the optimal number of degrees of freedom achievable. In the optimal scheme, the transmitter uses the fed back CSI to learn the side information that the receivers receive from previous transmissions rather than to predict the current channel state. Our result can be viewed as the first example of feedback providing a degree-of-freedom gain in memoryless channels.

Completely Stale Transmitter Channel State Information is Still Very Useful

Transmitter channel state information (CSIT) is crucial for the multiplexing gains offered by advanced interference management techniques such as multiuser MIMO and interference alignment. Such CSIT is usually obtained by feedback from the receivers, but the feedback is subject to delays. The usual approach is to use the fed back information to predict the current channel state and then apply a scheme designed assuming perfect CSIT. When the feedback delay is large compared to the channel coherence time, such a prediction approach completely fails to achieve any multiplexing gain. In this paper, we show that even in this case, the completely stale CSI is still very useful. More concretely, we show that in a MIMO broadcast channel with $K$ transmit antennas and $K$ receivers each with 1 receive antenna, $\frac{K}{1+1/2+ ...+ \frac{1}{K}} (> 1) $ degrees of freedom is achievable even when the fed back channel state is completely independent of the current channel state. Moreover, we establish that if all receivers have independent and identically distributed channels, then this is the optimal number of degrees of freedom achievable. In the optimal scheme, the transmitter uses the fed back CSI to learn the side information that the receivers receive from previous transmissions rather than to predict the current channel state. Our result can be viewed as the first example of feedback providing a degree-of-freedom gain in memoryless channels.

In a cooperative network with multiple potential relays and multiple simultaneous transmissions, we present selection cooperation wherein each source pairs with a single "best" relay. We analyze the outage probability of a simple and completely distributed selection scheme, requiring some feedback but no centralization, and show that it outperforms distributed space-time codes for networks with more than three relaying nodes. These gains are due to the more efficient use of power in networks using selection. We suggest two other more complex selection schemes based on increasing system intelligence and centralization, and show that for smaller network sizes their performance improvement over the simple selection scheme is not significant.

Selection cooperation in multi-source cooperative networks

We consider the problem of optimal scheduling and routing in an ad-hoc wireless network with multiple traffic streams and time varying channel reliability. Each packet transmission can be overheard by a subset of receiver nodes, with a transmission success probability that may vary from receiver to receiver and may also vary with time. We develop a simple backpressure routing algorithm that maximizes network throughput and expends an average power that can be pushed arbitrarily close to the minimum average power required for network stability, with a corresponding tradeoff in network delay. When channels are orthogonal, the algorithm can be implemented in a distributed manner using only local link error probability information, and supports a ''blind transmission'' mode (where error probabilities are not required) in special cases when the power metric is neglected and when there is only a single destination for all traffic streams. For networks with general inter-channel interference, we present a distributed algorithm with constant-factor optimality guarantees.

Optimal backpressure routing for wireless networks with multi-receiver diversity

In this paper, we are interested in minimizing the delay and maximizing the lifetime of event-driven wireless sensor networks for which events occur infrequently. In such systems, most of the energy is consumed when the radios are on, waiting for a packet to arrive. Sleep-wake scheduling is an effective mechanism to prolong the lifetime of these energy-constrained wireless sensor networks. However, sleep-wake scheduling could result in substantial delays because a transmitting node needs to wait for its next-hop relay node to wake up. An interesting line of work attempts to reduce these delays by developing "anycast"-based packet forwarding schemes, where each node opportunistically forwards a packet to the first neighboring node that wakes up among multiple candidate nodes. In this paper, we first study how to optimize the anycast forwarding schemes for minimizing the expected packet-delivery delays from the sensor nodes to the sink. Based on this result, we then provide a solution to the joint control problem of how to optimally control the system parameters of the sleep-wake scheduling protocol and the anycast packet-forwarding protocol to maximize the network lifetime, subject to a constraint on the expected end-to-end packet-delivery delay. Our numerical results indicate that the proposed solution can outperform prior heuristic solutions in the literature, especially under practical scenarios where there are obstructions, e.g., a lake or a mountain, in the coverage area of the wireless sensor network.

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Minimizing delay and maximizing lifetime for wireless sensor networks with anycast

This paper presents a novel ARQ scheme for multiple unicast flows in a multi-user system, which enhances the aggregate throughput efficiency. The core idea is to exploit knowledge of previously overheard packets by unintended users, and perform wise selection and joint encoding of any retransmitted information. Intended users may decode the jointly encoded packet by exploiting the previously overheard packets. It is found that the throughput efficiency in a K-user system approaches, the fairly remarkable result of, unit value when K goes to infinity. Moreover, complexity and overhead of the scheme is studied and a link to (feedback free) network coding is established.

Multi-User ARQ

Multihop packet radio networking promises increased data rates and enhanced coverage in wireless networks. In this paper, we propose a new high performing low complexity multihop forwarding scheme, denoted multiuser diversity forwarding (MDF), that efficiently exploits inherent radio channel characteristics. The scheme is founded on a cross-layer paradigm, which enables a transmitting node to perform instant rate adaptation as well as to select among relays and among flows, thereby taking full advantage of the inherent multiuser diversity. The relative performance of MDF is benchmarked against two reference schemes, nearest with forward progress (NFP) and selection diversity forwarding (SDF). The simulations indicate that the proposed scheme yields a substantial throughput increase.

Multiuser diversity forwarding in multihop packet radio networks

The present invention relates to the use of beaconing or 'hello' messages in wireless multihop or ad hoc communication networks. In the method according to the present invention beacon messages (HELLO messages) are transmitted between a plurality of radio nodes (205, 215) in an ad hoc or multihop network. The rate of which the radio nodes transmit their beacons is based on an estimate of the network dynamics. Also the transmit power of the beacons are preferably based on an estimate of the network dynamics. The radio nodes bases their estimate of the network dynamics on beacons received from neighboring radio nodes.

Method and arrangement in wireless ad hoc or multihop networks

Today a multitude of radio access technologies exist and are continuously evolved to support higher bit rates, lower delays, lower cost, etc. In addition, new radio access technologies are developed to complement those of today. When using services in this environment with multiple radio access technologies, end users expect mobility between the different radio access technologies to be seamless. Network operators, in addition, look for efficient resource usage of different radio access technologies and more cost efficient ways to deploy networks. This requires increased flexibility in the radio access network architecture compared with today's architectures. In this paper a beyond 3G radio access network reference architecture supporting these requirements is presented. The architecture allows for various implementations and considers new deployment concepts such as multi-hop radio networks, moving networks and personal area networks.

Beyond 3G radio access network reference architecture

A system, method and node of downlinking transmissions to an unsynchronized UE in a telecommunications network. The method begins by a node in the network requesting synchronization of the UE with the network. A first transmission of data is sent from the node to the UE prior to synchronization of the UE. The UE then synchronizes with the network by the UE performing a Random Access procedure with the node, thereby triggering a time alignment command from the node to the UE to synchronize the UE with the network. A second transmission of data is then sent from the node to the UE after the UE is synchronized. A response feedback message is sent to the node from the UE. The message is a cumulative feedback message of the first transmission of data and the second transmission of data. Thus, data may be transmitted prior to synchronization of the UE.

Niklas J. Johansson

Papers

Multi-User ARQ

Multiuser diversity forwarding in multihop packet radio networks

Method and arrangement in wireless ad hoc or multihop networks

Beyond 3G radio access network reference architecture

System and method of downlinking data to an unsynchronized user equipment in a telecommunications network