The throughput performance of smart antenna systems (SASs) in WiFi networks is studied in this work. Considering a WiFi network whose access points (APs) support the switched-beam SAS, a beam switching strategy which is compatible with the legacy WiFi protocol is proposed to enhance more concurrent links by allowing APs directionally sensing and accessing the wireless channel. The throughput advantage is verified by the simulation results in random deployment scenarios. More importantly, the proposed beam switching strategy provides extra design factors to adjust or balance the downlink and uplink throughput per WiFi user to meet the requirements of various applications and scenarios.

The throughput performance of a heterogeneous dense small cell network (HetNet) with collocated WiFi and LTE interfaces is studied herein. From a geometrical statistical viewpoint, the average throughput for LTE with the function of further enhanced inter-cell interference coordination (FeICIC) is investigated in an attempt to find a proper operation setting for FeICIC to benefit users on cell edges. Furthermore, the optimal setting for WiFi offloading is also studied to maximize the worse user experience in such a HetNet. The proposed analysis method allows us to further study a proper strategy that provides fair services to users under the severe interferences in HetNet.

A cross-layer analysis framework is developed for the design and performance evaluations of cooperative automatic repeat request (ARQ) with opportunistic multi-point relaying (OMPR) in mobile networks. In particular, three exemplar OMPR functions and their combinations are studied for opportunistic distributed space-time coded (ODSTC) retransmissions in time-division duplex long-term evolution advanced systems. According to the complexities and coordination delays of the OMPR functions, the influences of channel fading on the spatial and temporal diversities offered by ODSTC-based ARQ and their impacts on the system throughput, transmission energy consumption, and relays' queuing delays can be evaluated effectively under this framework. Based on the analytical results, a rate adaptation strategy is also proposed to maximize the system throughput subject to constraints on queuing delays at relay stations (RSs) and packet error rates at mobile stations (MSs). Analysis and simulation results show that in typical vehicular speeds, using maximally two active RSs for ODSTC is a very robust and effective choice to provide near best system throughput with less energy consumptions and lower protocol complexities. The throughput gains over a system without using RS retransmissions range from 16 ~ 80% for cell-edge MSs, with only 25 ~ 45% of the transmission energy consumption.

A joint beam training and rate adaptation mechanism for downlink transmissions is presented for multiple input multiple output systems with switched beam antennas. The proposed method only depends on the beam patterns and the acknowledgement (ACK) replied by the receiver, and is compatible with the legacy WiFi standards. This feature makes not only the complexity of the beam training scheme extremely low, but also the feasibility relatively high. Due to the practical limitations of antenna designs and manufacturing, beam patterns may not have distinctive directivity for certain directions. Therefore, beam pattern scaling is utilized to make the resultant beam patterns match well with the ideal user directions and channel conditions.Simulation results with real switched beam patterns show that the precision of the training scheme is very high.

The paper studies effective protocols and corresponding performance of automatic retransmission request (ARQ) via decode-and-forward (DF) opportunistic distributed space-time coding (ODSTC). Analysis shows that the diversity via ARQ can be drastically increased by allowing non-active relays to overhear the DSTC signal sent by the active relays. Nevertheless, simple schemes without overhearing may also achieve the same diversity when the link quality between the source-relay or the relay-destination is good, hence greatly alleviating the need for a complex protocol with overhearing. Moreover, using more relays opportunistically does provide a higher coding gain if the source-relay channel quality is extremely good, otherwise using two or three relays gives almost the same performance as if using all of them.

The paper investigates effective protocols and the corresponding performance for cooperative automatic retransmission request (ARQ) that uses the decode-and-forward (DF) opportunistic distributed space-time coding (ODSTC). According to the analysis, the delay-limited throughput enjoys significant enhancement even using one or two active relays only for ARQs than using direct retransmissions. Besides, allowing the non-active relays to overhear the DSTC signal sent by active relays during ARQs also leads to a better throughput via ODSTC. Nevertheless, in the high or the low SNR regimes, simple schemes without overhearing may provide almost the same performance offered by a scheme with overhearing despite the much inferior diversities. These features may be further explored to enhance the overall system throughput for multiple access systems using only limited system resources.