Murat Torlak

Device Power Saving and Latency Optimization in LTE-A Networks Through DRX Configuration

Discontinuous reception (DRX) saves battery power of user equipment (UE) usually at the expense of potential increase in latency in the Long Term Evolution (LTE) networks. Therefore, an optimization is needed to find the best tradeoff between latency and power saving. In this paper, we first develop an analytical model to estimate power saving achieved and latency incurred by DRX mechanism for active and background mobile traffic. A tradeoff scheme is then formulated to maintain a balance between these two performance parameters based on operators preference for power saving and latency requirement of traffic. The analytical model is validated using system level simulation results obtained from OPNET Modeler. The results show that the proposed tradeoff scheme is efficient in keeping a balance between power saving and latency. The results also indicate that DRX short cycles are very effective in reducing latency for active traffic, while shorter inactivity timer is desirable for background traffic to enhance power saving. We also proposed a mechanism to switch DRX configuration based on traffic running at UE, using UE assistance procedure recently adopted by 3GPP in Release 11. DRX configuration switching increases the power saving significantly without any noticeable increase in latency of active traffic.

Blind estimation of FIR channels in CDMA systems with aperiodic spreading sequences

CDMA systems commonly use aperiodic spreading codes to distribute a signal spectrum uniformly over the channel bandwidth and differentiate neighboring cell sites. CDMA receivers often suffer from interference due to multipath fading. Blind signal estimation schemes cannot be used because they require periodic spreading sequences. RAKE receivers are often used, but they cannot fully exploit the rich structure of CDMA signals to minimize interference. This paper presents an iterative technique to estimate multipath parameters which can serve as a preprocessing step in a receiver to increase signal-to-interference ratio. We investigate the performance of the proposed method using computer simulations. Preliminary simulation results show an average of 10 dB gain on channel parameter estimation.

A Comparison of Energy Detectability Models for Spectrum Sensing

Energy detection has been adopted as an alternative spectrum sensing method for cognitive radios due to its low computational complexity and not requiring a priori information of the signal to be detected. In this work, we analyze the accuracy of different energy detector models in approximating the exact solution both in AWGN and Rayleigh channels. Our simulations show that all of these models have almost the same performance in Rayleigh channels. Hence, we conclude that any one of them can be used to represent the energy detectors performance instead of the complicated exact solution in Rayleigh fading channels. Using Edells model, we derive the relationship between the number of samples and the signal-to-noise ratio (SNR) for Rayleigh channels similar to the results obtained for AWGN channels. We also verify our analytical results by simulations.

Optimization of Interference Alignment Beamforming Vectors

The K user interference alignment scheme proposed by Cadambe and Jafar (CJ) achieves K/2 degrees of freedom for networks with frequency selective channels. In this paper, we provide two new algorithms that optimize the precoding subspaces which maximize the data rate performance of the CJ scheme while maintaining the achievable degrees of freedom. One design is obtained as a global solution of a constrained convex (concave) optimization problem that maximizes the sum rate. The other design provides a low complexity closed-form solution to a constrained maximization problem with a suboptimal sum rate objective function. We also show that both designs can be combined with Shen, Host-Madsen and Vidal orthonormalization that achieves further gains in sum rate by optimizing the precoding vectors generated by the proposed algorithms.

Fast estimation of weight vectors to optimize multi-transmitter broadcast channel capacity

We improve capacity of broadcast channels by employing multiple transmitters and exploiting the spatial diversity among the users. We derive fast algorithms to compute orthogonal, near-optimal, and optimal weight vectors for broadcasting message signals to two and three users. The key innovation is that we decouple the weight vectors in the measure of channel capacity to (1) simplify the optimization problem to a search for the maxima of a smooth multidimensional function and (2) derive closed-form expressions for the orthogonal and near-optimal algorithms.

Optimizing DRX configuration to improve battery power saving and latency of active mobile applications over LTE-A network

In LTE networks, mobile applications and their higher data rate requirements are the latest phenomenon causing a tremendous need in power saving for mobile devices. Discontinuous reception (DRX) is one of the key power saving mechanisms in LTE. Since DRX saves battery power of user equipment usually at the expense of potential increase in latency, an optimization is needed to find the best tradeoff between latency and power saving. In this paper, we first develop an analytical model to estimate power saving achieved and latency incurred by DRX operation. Then, a tradeoff scheme is formulated to maintain a balance between these two performance parameters based on operators preference for power saving. We validate analytical model using system level simulation results obtained from OPNET Modeler. The results show that the proposed tradeoff scheme is highly efficient in keeping a balance between power saving and latency. We have shown that proposed scheme can achieve a significant delay improvement with a small decrease in power saving. The results also indicate that short DRX cycles are very effective in reducing latency for active traffic.

Vector channels for smart antennas. Measurements, statistical modeling, and directional properties in outdoor environments

In wireless communications, smart antenna systems that employ antenna arrays coupled with adaptive signal-processing techniques at the basestation improve capacity, coverage, and trunking efficiency. However, design and performance analysis of smart antenna systems strongly depend on channel propagation characteristics of signals present at the antenna array, the so-called vector channels. Here, variation of narrow-band vector channels (spatial signatures) due to a moving terminal is studied in typical suburban settings. Vector channel measurements are taken using a real-time smart antenna system with a uniform circular array at the basestation and a mobile transmitter at several locations. Two different wireless scenarios, namely, pedestrian and car mobile, are implemented to emulate the random movement of the mobile user. In each scenario, the mobile transmitter locations are chosen so that there exists line-of-sight (LOS), nonline-of-sight (NOLOS), or both LOS and NOLOS (mixed) propagation to the basestation. We find that in all cases, the Beta distribution can be used to empirically represent the spatial signature correlations and that large spatial diversity exists in NOLOS cases compared to LOS cases. Also, direction-of-arrivals mostly do not change much with movement in a suburban environment.

A PDA-based Research Platform for Cochlear Implants

Currently researchers interested in developing new signal processing algorithms for commercially available cochlear implants must rely on coding these algorithms in low-level assembly language. We propose a personal digital assistant (PDA) based research platform for developing and testing in real-time new signal processing strategies for cochlear implants. Software development can be done either in C or in LabVIEW. The C implementation can be further optimized using Intels primitive routines. In this paper, we report on the real-time implementation of a 16-channel noise-band vocoder algorithm, which is a similar algorithm used in commercially available implant processors. We further report on EEG recordings on the PDA acquired through a compact-flash data acquisition card.

A Comparison of Energy Detectability Models for Cognitive Radios in Fading Environments

In this work, we first analyze the accuracy of different energy detector models in approximating the exact solution in AWGN. These models motivate us to develop approximation analysis to address energy detection for fading channels. Our analysis develops approximation that has almost the same performance as the exact solution in Rayleigh channels. Our new model is simple enough to derive the relationship between the required number of samples (N) and the signal-to-noise ratio for a single Rayleigh channel similar to the one obtained for AWGN channels. We also define a fading margin for link budget calculations that relates N in fading channels to AWGN channels. Furthermore, we analyze the impact of multiple antennas for cognitive radios considering two receiver diversity schemes and quantify the improvement in performance regarding this margin. All the analytical results derived in this paper are verified by simulations. Finally, we have implemented and verified energy detection models in our multiple antenna wireless testbed.

Performance of MC-CDMA systems using antenna arrays

In this paper, we consider the combination of antenna array and multi-carrier code division multiple access (MC-CDMA) in the presence of carrier and synchronization offsets. MC-CDMA systems are regarded as suitable modulation techniques for third and fourth generation cellular systems to handle unprecedented demands for wireless communication. On the other hand antenna arrays have been shown to improve the coverage quality and system capacity by suppressing co-channel interference. In this paper, the proposed scheme will be evaluated using two different pulse shaping filters. While one of the pulse shaping filters allows the low complexity receiver implementation, the other requires costly match filtering. We investigate the performance of two cases via computer simulations.