Relays in cellular systems are sensitive to interference. A good relay design will mitigate received interference and will transmit in a way that avoids excess out-of-cell interference. This paper proposes a distributed relay beamforming algorithm for the two-hop interference channel, which models a relay-based cellular system. Interference pricing is used to provide the relays with information on how the interference impacts the end-to-end achievable rates. A new method is proposed to compute the interference prices via an approximation of the end-to-end achievable rate, which accounts for the relationship of the parameters in two hops. This alleviates the rate mismatch experienced by the naive application of single- hop interference pricing algorithms. Simulations show that the proposed algorithm outperforms single-hop interference pricing, zero forcing and maximal ratio transmission beamforming.

The transmission capacity of an ad-hoc network is the maximum density of active transmitters per unit area, given an outage constraint at each receiver for a fixed rate of transmission. Most prior work on finding the transmission capacity of ad-hoc networks has focused only on one-way communication where a source communicates with a destination and no data is sent from the destination to the source. In practice, however, two-way or bidirectional data transmission is required to support control functions like packet acknowledgements and channel feedback. This paper extends the concept of transmission capacity to two-way wireless ad-hoc networks by incorporating the concept of a two-way outage with different rate requirements in both directions. Tight upper and lower bounds on the two-way transmission capacity are derived for frequency division duplexing. The obtained bounds are used to derive the optimal solution for bidirectional bandwidth allocation that maximizes the two-way transmission capacity, which is shown to perform better than allocating bandwidth proportional to the desired rate in both directions. Using the proposed two-way transmission capacity framework, a lower bound on the two-way transmission capacity with transmit beamforming using limited feedback is derived as a function of bandwidth and the number of bits allocated for feedback.

Obtaining the most from multiple-antenna relay systems requires adapting the number of substreams based on channel conditions. In this paper, we develop algorithms for selecting dynamically the number of data streams and the transmit antenna subsets at both the source and relay to minimize the vector symbol error rate at a fixed overall rate. An antenna selection mode of operation is defined by the number of transmit antennas and the substream-to-antenna mapping at the source and relay. We propose three suboptimal multimode algorithms that aim to select the best mode based on the knowledge of channel conditions. The algorithms are shown by Monte Carlo simulations to achieve the full diversity order and to provide considerable gains over the existing designs.

The transmission capacity of an ad-hoc network is the maximum density of active transmitters in a unit area, given an outage constraint at each receiver for a fixed rate of transmission. Most prior work on finding the transmission capacity of ad-hoc networks has focused only on one-way communication where a source communicates with destination and no data is sent from destination to the source. In practice, however, two-way or bidirectional data transmission is required to support control functions like packet acknowledgements and channel feedback. This paper develops the concept of transmission capacity for two-way wireless ad-hoc networks, by incorporating the concept of a two-way outage with different rate requirements in both directions. Upper and lower bounds on the two-way transmission capacity are derived for frequency division duplexing, under the assumption that the channel coefficients are independent on different carrier frequencies. The derived bounds are used to derive the optimal solution for bidirectional bandwidth allocation that maximizes the two-way transmission capacity, which is shown to perform better than allocating bandwidth proportional to the desired rate in both directions.

Wireless ad hoc networks require bidirectional data transmission to support two-way traffic and control functions like packet acknowledgement. Most prior work on the capacity of wireless ad hoc networks, however, has focused only on one-way communication. In this paper we develop the concept of transmission capacity of two-way communication in wireless ad hoc networks. The transmission capacity has been used extensively to analyze one-way ad hoc networks - in this paper we provide a generalization that incorporates the concept of a two- way outage. We derive an upper bound and an approximation for the two-way transmission capacity, which are shown to be relatively tight for small outage probability constraints. We also quantify how the two-way success requirement reduces network capacity. Our numerical and simulation results show that for certain two-way networks the capacity loss is considerable.

Increasingly wireless networks use multi-antenna nodes as in IEEE 802.11n and 802.16. The physical layer (PHY) in such systems may use the antennas to provide multiple streams of data (spatial multiplexing) or to increase the robustness of fewer streams. These physical layers also provide support for sending packets at different rates by changing the modulation and coding of transmissions. Rate adaptation is the problem of choosing the best transmission mode for the current channel and in these systems requires choosing both the level of spatial multiplexing and the modulation and coding. Hydra is an experimental wireless network node prototype in which both the MAC and PHY are highly programmable. Hydra's PHY is essentially the 802.11n PHY, and currently supports two antennas and the same modulations and codings as 802.11n. Because of limitations of our hardware platform, the actual rates are a factor of 10 smaller than 802.11n. The MAC is essentially the 802.11 MAC with extensions, including the ability to feedback channel state or rate information from the receiver. Hydra was designed to allow experimentation with real radios, PHYs, and network stacks over real-world channels and it is well suited to studying rate adaptation in multi-antenna systems. To allow controlled experimentation, we also have the ability to perform experiments over emulated channels using exactly the same MAC and PHY used for RF transmissions. We present rate control experiments based on transmission over both real and emulated channels. Our experiments include measurements for single antenna systems and two antenna systems using a single or multiple spatial streams. We study rate adaptation algorithms using both explicit and implicit feedback from the receiver. A novel aspect of our results is the first experimental study of adaptation between single and multiple spatial streams for 802.11n style systems.

This paper investigates the tradeoff between throughput and feedback delay of limited feedback beamforming in indoor wireless channels with a practical MIMO-OFDM prototype. Past descriptions of this tradeoff are largely based on simplified models of the wireless channel. Unfortunately, wireless channel models may not accurately represent the complexities of a real wireless channel. Furthermore, system impairments, including channel estimation error, only exacerbate the problem of modeling a real wireless system. One such analytical result predicts the performance of limited feedback beamforming as an exponential function of feedback delay. This analytical result has been confirmed through Monte Carlo simulation under Rayleigh fading channel models. Through rigorous measurements and experimentation this paper both evaluates the performance of limited feedback beamforming under feedback delay and confirms the accuracy of the analytical results.