Recently, practical subframe-level schemes, such as frame combining and partial packet recovery, have been proposed for combating wireless transmission errors. These approaches depend heavily on the bit error behavior of wireless data transmissions, which is overlooked in the literature. We study the characteristics of subframe bit errors and their location distribution by conducting extensive experiments on several IEEE 802.11 WLAN testbeds. Our measurement results identify three bit error patterns: slope-line, saw-line, and finger. Among these three patterns, we have verified that the slope-line and saw-line are present in different physical environments and across various hardware platforms. However, the finger pattern does not appear on some platforms. We discuss our current hypotheses for the reasons behind these bit error patterns and how identifying these patterns may help improve the robustness of WLAN transmissions. We believe that identifiable bit error patterns can potentially introduce new opportunities in channel coding, network coding, forward error correction (FEC), and frame combining.

We consider beam selection using a fixed beamforming network (FBN) at a base station withMarray antennas. In our setting, a Butler matrix is deployed at the RF stage to formMbeams, and then the best beam is selected for transmission. We introduce some properties of the noncentral chi-square distribution and prove the resulting properties of the beam selection gain verifying that beam selection is superior to antenna selection in Rician channels with anyK-factors. Furthermore, we find asymptotically tight stochastic bounds of the beam selection gain, which yield approximate closed form expressions of the expected selection gain and the ergodic capacity. Beam selection has the order of growth of the ergodic capacity Θ(logM) regardless of user location in contrast to Θ(log(logM)) for antenna selection.

We describe a channel sounding system designed for small outdoor cell (~400 meter radius) wireless channel characterization. We report on a program of path loss measurements using a 2×2 MIMO configuration and data reduction in the frequency range of 5.7-5.75 GHz. The results are based on over 10,000 independent complex frequency responses collected in residential neighborhoods in northern New Jersey. We characterize and report on key path loss parameters such as path loss exponent and shadowing for low base station antenna heights. Based on our data analysis, we propose a statistical model for path loss in neighborhood area networks in suburban environments.

In a protocol assisted switched diversity (PASD) system, the time separated repetition blocks are accumulated in order to increase decoding reliability. In this paper, we consider transmit antenna selection (TAS) protocols in a PASD system. To minimize system complexity we assume the feedback information is limited to the SNR of the current pair of antennas. We propose three TAS protocols : the random TAS (R-TAS) protocol which selects antennas randomly, the threshold TAS (T-TAS) protocol which keeps the current selection if the predetermined SNR threshold level requirement is met and the minimum number of transmission TAS (MNT-TAS) protocol which determines the selection of a new antenna set or keeping the current set based on the evaluation of expectation of the number of necessary remaining transmissions. We analyze the average number of transmissions of three proposed protocols in a Rayleigh fading channel and compare numerical evaluations to simulation results under different system conditions.

Multiple co-channel WLAN BSSes (i.e., WLAN cells) overlapping in coverage are generally considered undesirable because members of the OBSSes compete for channel access, which typically increases the contention level of wireless medium access and reduces overall system performance. In this paper, we propose to use channel access throttling (CAT) for managing Wireless LAN radio resources for overlapping BSSes (OBSSes). CAT provides an access point (AP) of each BSS with a mechanism to control channel access parameters of its member stations on the fly. By coordinating the CAT operations of the OBSS APs, we can enable privileged channel access to an individual BSS at a particular time, for example, by assigning high priority access parameters to member stations associated with the BSS. By controlling how much each BSS may be given the privileged channel access, we can also achieve a proportional partitioning of channel capacity among OBSSes. We present evaluation results obtained from both simulations and experiments using testbed built with commercial off-the-shelf (COTS) WLAN hardware and open-source device driver. Our results show that with CAT, not only can we proportionally partition channel capacity among the OBSSes, but also improve channel utilization efficiency and increase overall capacity.

In IEEE 802.11 Wireless LAN (WLAN) systems, techniques such as acknowledgement, retransmission, and transmission rate adaptation, are frame-level mechanisms designed for combating transmission errors. Recently sub-frame level mechanisms such as frame combining have been proposed by the research community. In this paper, we present results obtained from our bit error study for identifying sub-frame error patterns because we believe that identifiable bit error patterns can potentially introduce new opportunities in channel coding, network coding, forward error correction (FEC), and frame combining mechanisms. We have constructed a number of IEEE 802.11 wireless LAN testbeds and conducted extensive experiments to study the characteristics of bit errors and their location distribution. Conventional wisdom dictates that bit error probability is the result of channel condition and ought to follow corresponding distribution. However our measurement results identify three repeatable bit error patterns that are not induced by channel conditions. We have verified that such error patterns are present in WLAN transmissions in different physical environments and across different wireless LAN hardware platforms. We also discuss our current hypotheses for the reasons behind these bit error probability patterns and how identifying these patterns may help improving WLAN transmission robustness.

The de facto QoS channel access method for the IEEE 802.11 Wireless LANs is the Enhanced Distributed Channel Access (EDCA) mechanism, which differentiates transmission treatments for data frames belonging to different traffic categories with four different levels of channel access priority. In this paper, we propose extending EDCA with Channel Access Throttling (CAT) for more flexible and efficient QoS support. By assigning different member stations different channel access parameters, CAT differentiates channel access priorities not between traffic categories but between member stations. Then by dynamically changing the channel access parameters of each member station based on a pre-computed schedule, CAT enables EDCA WLANs the benefits of scheduled access QoS. We also present evaluation results of CAT obtained from both simulations and experiments conducted using off-the-shelf WLAN hardware and open-source device driver. Our results show that CAT can proportionally partition channel capacity, significantly improve performance of multimedia applications, effectively achieve performance protection for admitted flows, and increase per cell VoIP call capacity by up to 41%.

For a multiple-antenna system, we find a simple and accurate expression for the asymptotic distribution of the antenna selection gain when the transmitter selects the transmit antenna with the strongest channel. We use this to estimate the underlying channel capacity distributions and obtain the approximate ergodic capacity. This estimate is compared with upper and lower bounds. This analysis demonstrates that unlike multiple-input/multiple-output (MIMO) systems, the channel for antenna selection systems hardens at a slower rate, and thus a significant multiuser scheduling gain can exist-Theta (1/logm) for channel selection as opposed to Theta (1/radicm) for MIMO, where m is the number of transmit antennas.

We consider a wireless transmission scenario, when a base station is endowed with a fixed beamforming network, where M antennas are employed at the base station to point beams to predetermined azimuthal angles. In our setting, a Butler matrix is deployed at the RF stage to form M beams, and then the best beam is selected for transmission. We derive the distribution of the beam selection gain for this scenario under a Rician channel assumption as a function of both the azimuthal location of the remote unit and the Rician A'-factor. Using some key properties of the noncentral chi-square distribution, we prove that beam selection outperforms antenna selection.

In this paper, we analyze the performance of a communication system that employs Protocol Assisted Switched Diversity (PASD) antennas under Rayleigh fading channel conditions. The PASD system accumulates time-displaced blocks, each with the same information but using different antennas, and then combines their symbols using maximal ratio combining (MRC). In the PASD system, quality of service (QoS) is ensured by comparing the available signal-to-noise ratio (SNR) at the output of the combiner against a preset threshold value. Our analysis shows that this method reduces the number of transmissions significantly as the channel deteriorates as compared with ARQ without memory while it can satisfy the symbol error rate (SER) requirement for most of blocks. It is also shown that deploying more than one antenna is important to stabilize the system when the channel becomes static.

For a multiple antenna system, we compute the asymptotic distribution of antenna selection gain when the transmitter selects the transmit antenna with the strongest channel. We use this to asymptotically estimate the underlying channel capacity distributions, and demonstrate that unlike multiple- input/multiple-output (MIMO) systems,the channel for antenna selection systems hardens at a slower rate, and thus a significant multiuser scheduling gain can exist. Additionally, even without this scheduling gain, it is demonstrated that transmit antenna selection systems outperform open loop MIMO systems at low signal-to-interference-plus-noise ratio (SINR) regimes, particularly for small number of receive antennas. This may have some implications on wireless system design, because most of the users in modern wireless systems have low SINRs.

The popularity of wireless LANs for Ethernet-speed home and business broadband connectivity has propelled their expansion into public applications such as "hot spots", "hot zones", and more recently, city-wide Internet access systems. As these outdoor WLAN-evolved 4G "nanocellular" networks grow, unconventional backhaul methods will need to be employed to bolster throughput as more users connect and begin to exchange richer multimedia content. In this paper we examine use of mesh-connected free-space optical links as a backhaul supplement to wireless, metallic and fiber media for such Neighborhood Area and city wide wireless networks.

This paper deals with the measurement and modeling of multipath delay on fixed wireless paths at 1.9 GHz in suburban environments. The primary focus is on the delay profile, which is the normalized plot of received power versus delay in response to an RT "impulse." We describe measurement campaigns in the western suburbs of Chicago, IL, and in suburban north-central New Jersey. Our analysis of the data suggests to us that, for directive terminal antennas, the delay profile can be modeled as having a "spike-plus-exponential" shape, i.e., a strong return ("spike") at the lowest delay, plus a set of returns whose mean powers decay exponentially with delay. This delay profile can be characterized by just two parameters (both variable over the terrain), namely, the ratio (K/sub 0/) of the average powers in the "spike" and "exponential" components and the decay time constant (/spl tau//sub 0/) of the "exponential" component. No such simple structure appears to apply for delay profiles using omnidirectional antennas. For a directive antenna with a 32/spl deg/ beamwidth, we find that: (1) the statistical correlation between the profile parameters K/sub 0/ and /spl tau//sub 0/ is negligible; (2) these parameters are relatively insensitive to antenna height and path length; and (3) over each measured region (Illinois and New Jersey), K/sub 0/ and /spl tau//sub 0/ have median values close to 8 dB and just below 0.2 /spl mu/s, respectively. Moreover, we have found simple probability distributions that accurately portray the variability of K/sub 0/ and /spl tau//sub 0/ over the terrain.