In this paper, we propose a new preamble design for channel estimation in filter bank multicarrier (FBMC) systems using offset quadrature amplitude modulation (OQAM). The proposed preamble structure is formed by duplicating a preamble unit (PU) defined as a 3×3 symbol matrix in a time-frequency lattice. The center element of the PU could be a real symbol "1" surrounded appropriately by imaginary symbols "j" and "-j" such that the mean-squared error (MSE) of channel estimation is minimized for the center subcarrier. The center element could also be an imaginary symbol "j" with surrounding symbols set as "1" or "-1" in a similar way for MSE minimization. The proposed preamble design can be regarded as an improved version of the extended interference approximation method with real or imaginary symbols (E-IAM-C) for channel estimation in FBMC/OQAM systems. With the estimated channel gains of all center subcarriers, we can calculate the remaining subcarriers' channel gains simply by linear interpolation. Theoretical analysis and simulation results show that the proposed preamble design for channel estimation outperforms E-IAM-C in terms of the MSE, bit error rate, and computational complexity.

We present an all-digital bit transistor characterization scheme for CMOS 6T SRAM array. The scheme employs an on-chip operational amplifier feedback loop to measure the individual threshold voltage (VTH) of 6T SRAM bit cell transistors (holding PMOS, pull-down NMOS, and access NMOS) in SRAM cell array environment. The measured voltage is converted to frequency with dual VCO and counter based digital read-out to facilitate data extraction, processing, and statistical analysis. A 512Kb test chip is implemented in 55nm 1P10M Standard Performance (SP) CMOS technology. Monte Carlo simulations indicate that the accuracy of the VTHmeasurement scheme is about 2-7mV at TT corner across temperature range from 85°C to -45°C, and post-layout simulations show the resolution of the digital read-out scheme isTHdistributions agree well with Monte Carlo simulation results.

IEEE 802.11-based devices employ rate adaptation algorithms to dynamically switch data rates to accommodate the fluctuating wireless channel conditions. Many studies observed that when there are other stations transmitting in the network, existing rate adaptation performance degrades significantly due to its inability to differentiate losses between wireless noise and contention collisions. In this paper, we first conduct a systematic evaluation on the effectiveness of various rate adaptation protocols, which try to address this issue by exploiting optional RTS frames to isolate the wireless losses from collision losses. We observe that these existing schemes do not perform well in many background traffic scenarios and can mislead the rate adaptation algorithms to persist on using similar data rate combinations regardless of background traffic level, thus resulting in performance penalty in certain scenarios. The fundamental challenge is to dynamically adjust the rate selection decision objectives with respect to different background traffic levels, as well as fluctuating wireless conditions. In light of such observations, we design a new Background traffic-aWAre RatE adaptation algorithm (BEWARE) that addresses the above challenge. BEWARE uses a mathematical model to calculate on the fly the expected packet transmission time based on current wireless channel and background traffic conditions. We implement BEWARE design in a Linux-based driver, and the test-bed experiment results show that BEWARE outperforms other rate adaptation algorithms by up to 250% in various indoor and outdoor scenarios.

IEEE 802.11-based devices employ rate adaptation algorithms to dynamically switch data rates to accommodate the fluctuating wireless channel conditions. Many studies observed that, when there are other stations transmitting in the network, existing rate adaptation performance degrades significantly due to the inability of differentiating losses between wireless noise and contention collisions. Previous studies proposed to exploit optional RTS frames to isolate the wireless losses from collision losses, and thus improve rate adaptation performance. In this paper, we conduct a systematic evaluation on the effectiveness of various existing rate adaptation algorithms and related proposals for loss differentiations, with multiple stations transmitting background traffic in the network. Our main contributions are two-fold. Firstly, we observe that existing RTS-based loss differentiation schemes do not perform well in all background traffic scenarios. In addition, our study reveals that RTSbased loss differentiation schemes can mislead the rate adaptation algorithms to persist on using similar data rate combinations regardless of background traffic level, thus result in performance penalty in certain scenarios. The fundamental challenge is that a good rate adaptation algorithm must dynamically adjust the rate selection decision objectives with respect to different background traffic levels. Secondly, we design a new Background traffic aware rate adaptation algorithm (BEWARE) that addresses the above challenge. BEWARE uses a mathematical model to calculate on-the-fly the expected packet transmission time based on current wireless channel and background traffic conditions. Our simulation results show that BEWARE outperforms other rate adaptation algorithms without RTS loss differentiation by up to 250% and with RTS by up to 25% in throughput.

Recent advance in IEEE 802.11 based standard has pushed the wireless bandwidth up to 600Mbps while keeping the same wireless medium access control (MAC) schemes for full backward compatibility. However, it has been shown that the inefficient protocol overhead casts a theoretical throughput upper limit and delay lower limit for the IEEE 802.11 based protocols, even the wireless data rate goes to infinitely high. Such limits are important to understand the bottleneck of the current technology and develop insight for protocol performance improvements. This paper uses a queuing system approach to extend the discussions of IEEE 802.11 protocol throughput and delay limits to the situation that arbitrary non-saturated background traffic is present in the network. We derive analytical models to quantify the performance limits for distributed coordination function (DCF) of legacy 802.11a/b/g and enhanced distributed coordination access (EDCA) of IEEE 802.11e. We find such limits are functions of the underlying MAC layer backoff parameters and algorithms, and are highly dependent on the load that background traffic injects into the network. Depending on the rate of arrival traffic, the packet delay limit may become unbounded such that no delay sensitive services can be operated under such condition. Moreover, we also discuss the effects of different frame aggregation schemes on the performance limits when data rate is infinite. The developed model and analysis provide a comprehensive understanding of the performance limitations for IEEE 802.11 MAC, and are useful in gauging the expected QoS for the purposes such as admission control

The IEEE 802.11g standard has been proposed to enhance the data rate of wireless LAN connections up to 54 Mbps, while ensuring backward compatibility with legacy 802.11b devices at the same time. However, in a hybrid 802.11b/g network, the throughput of 802.11g devices is compromised because of not only the overhead to interoperate with 802.11b devices, but also the unbalanced medium contentions between devices with different versions of the standard. We propose a hybrid Markov-chain based model to quantify this throughput reduction effect in 802.11b/g mixed scenarios. The analytical model is further verified with simulations and field measurements under different station numbers, data rates, and data packet sizes. In addition, a simple frame-bursting technique is shown to balance the throughput between 802.11b and 802.11g stations.