Rodolfo Torrea-Duran

Cross-Layer Optimization of Radio Sleep Intervals to Increase Thin Client Energy Efficiency

Thin client computing trades local processing for network bandwidth consumption by offloading application logic to remote servers. User input and display updates are exchanged between client and server through a thin client protocol. This thin client protocol traffic can lead to a significantly higher power consumption of the radio interface of the wireless device. In this contribution, we present a cross-layer algorithm that exploits thin client protocol layer information to determine intervals where no traffic from the server is expected. During these intervals, the wireless network interface card (WNIC) is instructed to enter the energy conserving sleep mode. Using this algorithm for a remote text editor, WNIC energy consumption reductions of 21-52% can be achieved.

Energy efficient transmission techniques for LTE

The goal of the European ICT Project EARTH is to enhance energy efficiency of current LTE cellular networks by 50 percent, enabling operators to provide more extensive coverage, namely to less densely populated areas, while ensuring high levels of efficiency, hence, reducing the carbon footprint. In order to achieve this objective, different strategies, at different levels, are being addressed. Energy efficient transmission techniques for LTE, proposed within the framework of EARTH, are addressed and discussed, with an emphasis on beamforming and MIMO. It is concluded that the strategic objective of EARTH is achievable. Combined MIMO mode selection, resource allocation, scheduling and precoding strategies will allow an energy consumption reduction of more than 50 percent in macro-cells. A reduction in MIMO power consumption of 91.7 percent (factor of 12) can be achieved in pico-cells.

Neighbor-friendly autonomous power control in wireless heterogeneous networks

The widespread deployment of base stations constitutes a promising solution to cope with the ever-increasing wireless data rate demands. However, it also increases the interference levels, especially at the cell-edge. Most interference management techniques assume coordination between base stations, which involves undesired overhead and delays. To solve this problem, we propose a neighbor-friendly autonomous algorithm for power control in wireless heterogeneous networks that protects victim users from neighboring cells through a penalty factor in the power allocation level. We refer to this algorithm as neighbor-friendly iterative waterfilling (NF-IWF). In addition, we propose a low-complexity closed-form version that fixes the penalty factor by assuming a linear approximation of the victim user data rate. In high interference conditions, it can achieve a victim user data rate increase by a factor of 3.5 compared to IWF, 15 compared to soft frequency reuse (SFR), and 60 compared to equal power allocation (EPA) with a marginal decrease of the primary user data rate.

Characterization of power consumption in thin clients due to protocol data transmission over IEEE 802.11

In thin client computing, applications are executed on a network server instead of on the user terminal. Since the amount of processing at the terminal is reduced, thin clients are potentially energy efficient devices. However, a network connection between client and server is required for the transmission of user input and display updates. The energy needed for this intense network communication might undo or even exceed the power savings achieved by the reduction in client-side processing. In this paper, we present experimental results on power efficiency of the wireless platform on the thin client in case of thin client traffic. The discussion is focused on VNC-RFB, a widespread thin client protocol, over an IEEE 802.11 link in three typical user scenarios. The results indicate that a cross-layer approach between application and wireless link layer could potentially lead to important power savings.

Double relay communication protocol for bandwidth management in cellular systems

The continuously growing demand for wireless connectivity has turned bandwidth into a scarce resource that should be carefully managed. A common solution is to assign disjoint portions of the bandwidth to different users, but the portion size decreases as the number of users grows. An alternative solution is to introduce spatial diversity through coordinated base stations, but such systems are very sensitive to timing and frequency synchronization offsets. To tackle these problems, we use principles of network coding for bandwidth management in a double relay cellular system of two base stations and two users. We propose a three-time-slot transmission strategy and a MMSE reception strategy. It avoids the need of tight frequency or timing synchronization through a simple communication protocol without using additional bandwidth or infrastructure. By finding a balance between spatial diversity and transmission time, our approach achieves the system capacity and fairness in all SNR conditions.

Reduction of HARQ memory in low mobility LTE systems

Hybrid ARQ (HARQ) combines Automatic Repeat Request (ARQ) and Forward Error Correction (FEC) to exploit information from erroneous packets after retransmissions. Due to its superior reliability, HARQ became a crucial component of several important 3G and 4G systems such as Long Term Evolution (LTE) and LTE-Advanced. Although the performance advantage is very attractive, implementing HARQ is challenging with emerging high throughput communication systems. Knowing that LTE and LTE-A systems would provide Gbps or hundreds of Mbps, a straightforward implementation would require around 10 Mbit memory for a HARQ buffer. With cost and power-limited wireless terminals, the available memory size to support HARQ requires an efficient implementation. In this paper we propose a novel method to exploit the time coherence of the wireless channel in order to reduce the memory storage for HARQ. This method has been evaluated with a full LTE MIMO simulation chain. Compared to state-of-the-art solutions, we show a promising memory compression factor close to 4, whereas performance degradation is marginal.

Exploiting frequency correlation in LTE to reduce HARQ memory

Hybrid ARQ (HARQ) combines Automatic Repeat Request (ARQ) and Forward Error Correction (FEC) to exploit information from erroneous packets after retransmissions. Due to its superior reliability, HARQ became a crucial component of several important 3G and 4G systems. Although the performance advantage is very attractive, implementing HARQ is challenging with emerging high throughput communication systems such as LTE and LTE-A. Specifically, a large amount of data needs to be stored whenever there is a retransmission. Knowing that LTE/LTE-A systems would provide Gbps or hundreds of Mbps, a straightforward implementation would require around 14 Mbit memory as HARQ buffer. With cost and power limited wireless terminals, the available memory size to support HARQ is a strict and challenging constraint. Hence, it is essential to find efficient techniques to minimize the memory footprint for storing erroneous packets with marginal or, preferably, no degradation. In the context of LTE systems, we propose a novel method to reduce the memory footprint for HARQ systems. This method has been evaluated with a fully fledged practical LTE simulation chain with MIMO transmissions. Compared to state-of-the-art solutions, we show a promising memory compression factor that is close to 5, whereas communication performance degradation is marginal.

Energy efficient PRACH detector algorithm in SDR for LTE femtocells

Mobile telecommunication networks are increasingly contributing to global energy consumption with the largest amount of energy consumed at the base station (BS) and at the core network side. Currently, large (macro) base stations can satisfy the user requirements; however the trend of deploying heterogeneous networks with both large and small base stations will be required to cover the user throughput demands. This will increase the already large contribution of mobile networks to global energy consumption. Traditionally, energy efficiency (EE) techniques have been proposed at the BS to reduce the energy consumption during downlink (DL) data transmission and idle periods. Nevertheless, during uplink transmissions, the random access procedure plays an important role for energy consumption since it has be to executed continuously, also when little data is being transmitted in the cell. The objective of this paper is to explore the energy consumption of the random access detector and to evaluate some techniques to improve its EE in a software defined radio (SDR). We show that through the use of a single antenna chain, and the generation of the random access preambles from one root sequence, we can decrease the energy consumption depending on the SNR at the detector by 82% without degrading the probability of detecting all the users. Part of this work is done for the Energy Aware Radio and network technologies (EARTH) project.

Topology-Aware Space-Time Network Coding in Cellular Networks

Space-time network coding (STNC) is a time-division multiple access (TDMA)-based scheme that combines network coding and space-time coding by allowing relay nodes to combine the information received from different source nodes during the transmission phase and to forward the combined signal to a destination node in the relaying phase. However, STNC schemes require all the relay nodes to overhear the signals transmitted from all the source nodes in the network. They also require a large number of time-slots to achieve full diversity in a multipoint-to-multipoint transmission. Both conditions are particularly challenging for large cellular networks where, assuming a downlink transmission, base stations (BSs) and users only overhear a subset of all the BSs. In this paper, we exploit basic knowledge of the network topology in order to reduce the number of time-slots by allowing simultaneous transmissions from those BSs that do not overhear each other. Our results show that these topology-aware schemes are able to increase the spectral efficiency per time-slot and bit error rate with unequal transmit power and channel conditions.

Cross-layer reduction of wireless network card idle time to optimize energy consumption of pull thin client protocols

Thin client computing trades local processing for network bandwidth consumption by offloading application logic to remote servers. User input and display updates are exchanged between client and server through a thin client protocol. On wireless devices, the thin client protocol traffic can lead to a significantly higher power consumption of the radio interface. In this article, a cross-layer framework is presented that transitions the wireless network interface card (WNIC) to the energy-conserving sleep mode when no traffic from the server is expected. The approach is validated for different wireless channel conditions, such as path loss and available bandwidth, as well as for different network roundtrip time values. Using this cross-layer algorithm for sample scenario with a remote text editor, and through experiments based on actual user traces, a reduction of the WNIC energy consumption of up to 36.82% is obtained, without degrading the applications reactivity.