Mobile Radio Environment Research Articles

Implications of proactive datagram caching on TCP performance in wireless/mobile communications

The operation of TCP in wireless–mobile environments is discussed in this paper. After briefly presenting previous efforts to ameliorate TCP performance in the considered environments, we propose a new mechanism for tackling the slow-down problems caused by handovers. Our mechanism is based on stochastic datagram relocation. Traffic destined to the mobile terminal is tunneled to and cached into adjacent cells according to the output of a path prediction algorithm. To reduce the associated overhead, only percentages of inbound traffic are copied to the cell's neighborhood on the basis of estimated probabilities. The time scheduling for datagram relocation is also taken into account. Simulations of the proposed architecture show substantial performance improvements for TCP traffic.

A Generalised approach for evaluation of outage performance in micro- and pico-cellular networks

Outage analysis is one of the primary objectives in the design and operation of the current generations of cellular mobile communications systems, in order to increase spectrum efficiency and meet the quality of service and the grade of service demands. A general and unified semi-analytical approach is presented for the direct evaluation of the outage probability in the presence of L mutually independent co-channel interferers for micro- and pico-cellular mobile radio environments that follow Rice or Nakagami models. Outage probability is evaluated in a nested mode via the Laguerre numerical integration technique, avoiding the calculation of complex functions. The proposed formulation can be efficiently applied to practical wireless applications with arbitrary statistical characteristics for the modelling parameters, in both the case of an interference-limited environment (no minimum power constraint) and the existence of a minimum power constraint. Comments, comparison with other existing techniques and useful curves are also presented.

Impact of fast fading compensations on mobile radio system performance

This paper explores what benefits can result from the compensation of both log-normal shadowing and Rayleigh/Rice-fading fluctuations in a mobile radio environment. The focus is on those power-control policies that exclusively make use of local power-level measurements in order to improve the performance, measured in terms of signal-to-cochannel interference ratio, of time-division multiple-access cellular radio systems; the aim is in fact to demonstrate under what circumstances the power compensation can be advantageously exploited. The corresponding analysis accounts for the errors due to the delay constraints that time-division and transmission techniques such as interleaving inevitably force onto the control loops. The numerical results quantitatively outline the maximum benefits on system performance that derive from the introduction of fast fading compensation for the class of power-control policies investigated . The remarkable influence of delay errors on the effectiveness of the fast fading control procedures are clearly revealed.

Errata to "A UAPO solution for the field diffracted by building corners in wireless radio environments"

A uniform asymptotic physical optics (UAPO) solution is proposed to determine the field diffracted by building edges in mobile radio environments. A Gaussian rough lossy surface model is applied to describe the wedge surfaces. The diffraction coefficients are obtained by using a physical optics (PO) approximation of the surface current densities induced by an incident electromagnetic field on the illuminated face and by performing a uniform asymptotic evaluation of the resulting radiation integral. The resulting analytical expression is given in terms of the transition function of the uniform theory of diffraction (UTD) and is easy to handle and to implement in a computer code.

Algorithms for supporting disconnected write operations for wireless Web access in mobile client-server environments

In a wireless mobile client-server environment, a mobile user may voluntarily disconnect itself from the Web server to save its battery life and avoid high communication prices. To allow Web pages to be updated while the mobile user is disconnected from the Web server, updates can be staged in the mobile host and propagated back to the Web server upon reconnection. In this paper, we analyze algorithms for supporting disconnected write operations for wireless Web access and develop a performance model to identify the optimal length of the disconnection period under which the cost of update propagation is minimized. The analysis result is particularly applicable to Web applications which allow wireless mobile users to modify Web contents while on the move. We show how the result can be applied to real-time Web applications such that the mobile user can determine the longest disconnection period such that it can still propagate updates to the server before the deadline so that a minimum communication cost is incurred.

Diversity Combining and Equalization with TCM for Wireless Communications

This paper presents an equalization structure in which antenna diversity, adaptive decision feedback equalization (DFE), interleaving and trellis-coded modulation (TCM) can be effectively combined to combat both ISI and cochannel interference in cellular mobile radio environments. The feedback filter of the DFE can use either tentative or final decision symbols of the TCM Viterbi decoding to cancel tail ISI with the square root Kalman algorithm. A performance bound on the average pairwise error probability for TCM under perfect interleaving and equalization is obtained by analysis. Some simulation results which illustrate the potential of the proposed system will also be given. In particular, a performance comparison between the proposed method and uncoded QPSK modulation will be undertaken.

Secure multicast in wireless networks of mobile hosts: protocols and issues

Multicast services and wireless interconnection networks are among the emerging technologies of the last decade. A significant amount of research has been separately performed in the areas of secure multicast and wireless interconnection networks. In this paper we investigate the issues of designing secure multicast services in wireless mobile environments for dynamic groups and propose protocols for key management for a variety of scenarios. Our solution decouples mobility management from group dynamics management, by taking into account the level of trust in the support stations. In particular, we show that protocol efficiency on the mobile host side can be traded-off with the level of trust in the support stations.

Syndrome: a light‐weight approach to improving TCP performance in mobile wireless networks

AbstractIt is well known that the performance of TCP deteriorates in a mobile wireless environment. This is due to the fact that although the majority of packet losses are results of transmission errors over the wireless links, TCP senders still take packet loss as an indication of congestion, and adjust their congestion windows according to the additive increase and multiplicative decrease (AIMD) algorithm. As a result, the throughput attained by TCP connections in the wireless environment is much less than it should be. The key problem that leads to the performance degradation is that TCP senders are unable to distinguish whether packet loss is a result of congestion in the wireline network or transmission errors on the wireless links.In this paper, we propose a light‐weight approach, called syndrome, to improving TCP performance in mobile wireless environments. In syndrome, the BS simply counts, for each TCP connection, the number of packets that it relays to the destination host so far, and attaches this number in the TCP header. Based on the combination of the TCP sequence number and the BS‐attached number and a solid theoretical base, the destination host will be able to tell where (on the wireline or wireless networks) packet loss (if any) occurs, and notify TCP senders (via explicit loss notification, ELN) to take appropriate actions. If packet loss is a result of transmission errors on the wireless link, the sender does not have to reduce its congestion window.Syndrome is grounded on a rigorous, analytic foundation, does not require the base station to buffer packets or keep an enormous amount of states, and can be easily incorporated into the current protocol stack as a software patch. Through simulation studies in ns‐2 (UCB, LBNL, VINT network simulator, http://www‐mash.cs.berkeley.edu/ns/), we also show that syndrome significantly improves the TCP performance in wireless environments and the performance gain is comparable to the heavy‐weight SNOOP approach (either with local retransmission or with ELN) that requires the base station to buffer, in the worst case, a window worth of packets or states. Copyright © 2001 John Wiley & Sons, Ltd.

Scalable, error-resilient, and high-performance video communications in mobile wireless environments

A novel mobile communications system is proposed, which provides not only effective but also efficient video access for mobile users when communicating over low-bandwidth error-prone wireless links. The middleware implemented by a mobile proxy server at the mobile support station is designed for the seamless integration of mobile users with video servers, so the specific details of the underlying protocols and source/channel coding techniques are hidden to both the video server and mobile client. Based on the concept of application-level framing, the application (video codec in our case) plays a significant role in network communications that most of functionalities of the system are implemented as part of the application. As such, at the application layer, adaptive source- and channel-coding techniques are developed to jointly provide the user with the highest possible video quality. For efficient source coding, our high-performance low-complexity video-coding algorithm called a 3-D significance-linked connected component analysis (3D-SLCCA) is chosen. Due to its high robustness against channel-error propagation, 3D-SLCCA is well suited for wireless environments. For error-resilient channel coding, a multilayer transmission error-control mechanism is developed. Since there is no additional requirement imposed on either the mobile client or the video server, mobile users interact with the server in exactly the same way as stationary users. Extensive computer experiments demonstrate the effectiveness of the proposed system.

Design and implementation of IMT-2000 system test plant for wideband-CDMA modulator/demodulator

In this paper, we describe the design, implementation and testing of the IMT-2000 (International Mobile Telecommunication-2000) system test plant for W-CDMA (wideband-code division multiple access) base station modulator and demodulator based on cdma2000 RTT (Radio Transmission Technology). The performance of the implemented base station is measured and compared with theoretical performance bound. The system test plant equipped with this base station provides wireless services, such as high quality speech of 9.6 kbps, real-time video of 384 kbps and IP (Internet protocol) based data services of 144 kbps in a mobile radio environment.

Finite-length decision-feedback equalisation for optimum combining diversity in digital cellular mobile radio

The paper analyses the performance of optimum space combining diversity with finite-length decision-feedback equalisation operating in a mobile radio environment with frequency-selective fading and quadrature amplitude modulation transmission. Selection diversity, another very common diversity method, is also involved in the analysis. By analytic techniques and simulation, it is shown that for the same number of taps, generally, the improvement is greater with combining diversity structure. However, numerical results also indicate that combining diversity may be inferior to selection diversity in some cases where the channels are severely distorted and combining diversity may lead to noticeable amplitude dispersion which cannot be compensated for easily by using a finitely long postcombining equaliser. Estimates of average errors and outage probabilities as functions of length of equalisation are provided for different diversity structures. The required length of equalisation to approach the optimum performance (achievable with an infinitely long equaliser) for different values of relative delay spread is examined carefully. Admitting perfect elimination of intersymbol interference postcursors, considerably superior performance of decision feedback equalisation can be obtained in terms of a fairly small number of taps to achieve this best performance, in addition to a smaller minimum mean square error than linear equalisation if the channel suffers severe time-dispersive multipath fading, particularly in the combining diversity case.

TCP in a wireless world

Mobile wireless is one of the more challenging environments for the Internet protocols, and for TCP in particular. One approach to supporting the wireless environment is the so-called "walled garden". Here the transport protocol used within the mobile wireless environment is not TCP, but is instead a transport protocol that has been specifically adapted to mobile wireless. In this model, Internet applications interact with an application gateway to reach the wireless world, and the application gateway uses a wireless transport protocol and potentially a modified version of the application data to interact with the wireless device. The most common implementation of this approach is to extend a Web client into the mobile wireless device, using some form of proxy server at the boundary of the wireless network and the Internet. This is the approach adopted by the Wireless Access Protocol (WAP) Forum. An alternative is to allow mobile wireless devices to function as any other Internet-connected device. This approach requires some form of end-to-end direct IP continuity and an associated end-to-end TCP functionality, where the TCP path straddles both wired and wireless segments. Ensuring the efficient operation of TCP in this environment becomes integral to the development of the environment itself; the problem is no longer one of adjusting TCP to match the requirements of the wireless environment, but one of providing seamless interworking between the wired and wireless worlds.

Predictive mobility support for QoS provisioning in mobile wireless environments

With the proliferation of wireless network technologies, mobile users are expected to demand the same quality of service (QoS) available to fixed users. This paper presents a predictive and adaptive scheme to support timed-QoS guarantees in pico- and micro-cellular environments. The proposed scheme integrates the mobility model into the service model to achieve efficient network resource utilization and avoid severe network congestion. The mobility model uses a probabilistic approach to determine the most likely cluster to be visited by the mobile unit. The admission control is invoked when a new call arrives or an existing call performs a handoff to verify the feasibility of supporting the call. The performance of the proposed schemes is compared to the shadow cluster scheme. The performance of the proposed scheme under different traffic patterns is also presented.

An efficient multicast routing protocol in wireless mobile networks

Providing multicast service to mobile hosts in wireless mobile networking environments is difficult due to frequent changes of mobile host location and group membership. If a conventional multicast routing protocol is used in wireless mobile networks, several problems may be experienced since existing multicast routing protocols assume static hosts when they construct the multicast delivery tree. To overcome the problems, several multicast routing protocols for mobile hosts have been proposed. Although the protocols solve several problems inherent in multicast routing proposals for static hosts, they still have problems such as non-optimal delivery path, datagram duplication, overheads resulting from frequent reconstruction of a multicast tree, etc. In this paper, we summarize these problems of multicast routing protocols and propose an efficient multicast routing protocol based on IEFT mobile IP in wireless mobile networks. The proposed protocol introduces a multicast agent, where a mobile host receives a tunneled multicast datagram from a multicast agent located in a network close to it or directly from the multicast router in the current network. While receiving a tunneled multicast datagram from a remote multicast agent, the local multicast agent may start multicast join process, which makes the multicast delivery route optimal. The proposed protocol reduces data delivery path length and decreases the amount of duplicate copies of multicast datagrams. We examined and compared the performance of the proposed protocol and existing protocols by simulation under various environments and we got an improved performance over the existing proposals.

Coherence bandwidth characterization in an urban microcell at 62.4 GHz

Results of experiments made at 62.4 GHz in an urban mobile radio environment to characterize the coherence bandwidth are presented. The correlation coefficients between signal envelopes separated in frequency are measured and expressed as functions of distance from the base station. Due to the high variation of this coefficient with distance and in order to provide accurate estimates of the coherence bandwidth in the microcell, the correlation coefficient has to be measured over large sections. Two methods to calculate the coherence bandwidth are presented and compared. It is shown that estimating this parameter from the frequency correlation function obtained at each position may yield incorrect results. The coherence bandwidths for correlation levels of 0.5, 0.7, and 0.9 are given. A ray-tracing tool has been used to assist in interpreting experimental results.

Simple and Efficient Techniques to Implement a Self-Reconfigurable ARQSystem in a Slowly Varying Mobile Radio Environment

Four simple algorithms to track the slowly varying channel state conditions in a mobile radio environment are examined. These indirect channel state estimators can be used for link adaptation in two-way radio communications. As an application background, the performance of stop-and-wait and selective-repeat automatic repeat-request schemes with adaptive packet length are investigated. Numerical results reveal that the proposed sliding observation interval algorithm with weighted success and error errors can provide a reliable channel state information within only a few block transmissions, and require the shortest observation interval among the four schemes.

Design and implementation of a wideband CDMA demodulator for an IMT-2000 system

This paper describes the design, implementation and testing of wideband code division multiple access (CDMA) base station demodulator for the International Mobile Telecommunication-2000 (IMT-2000) system test plant based on cdma2000 radio transmission technology (RTT). The performance of the implemented base station demodulator is measured and compared with the theoretical performance bound. The system test plant equipped with this demodulator provides wireless services, such as high quality speech (9.6 kbps), real-time video (384 kbps) and Internet protocol (IP) based data services (144 kbps) in a mobile radio environment.

Modern approaches in modeling of mobile radio systems propagation environment

In this article a review of popular propagation models for wireless communication channels is given. Macrocell, microcell, and indoor prediction methods are considered separately. Advantages and disadvantages of these models are discussed. Also, some practical improvements of the existing models as well as some new models are given.

Local and Global Signal Variability Statistics in a Mobile Urban Environment

Signal variability in mobile radio communication environments has been extensively studied. It is always assumed that, for small areas, the variability is Rayleigh-distributed, although experimental results have proven that this is not always the case. Other distributions present more accurated results. For large areas, the Suzuki distribution has been proposed based on the Rayleigh assumption. This work presents a review of the most commonly used distributions for the local and global statistics of the received signal variability and the theoretical foundation for a generalized distribution for large areas based on the assumption of a Weibull distribution for short-term variability. All distributions are presented in a linear and logarithmic form to facilitate comparison with experimental data. This new proposed distribution gives better accuracy than the Suzuki distribution.

Performance analysis of session oriented data communications for mobile computing in cellular systems

Future cellular communication systems must seamlessly support services for a wide range of user needs, including voice, data, video and multidmedia. It is envisioned that mobile users may do much computer processing in an offdline mode but must occasionally connect to a network in order to exchange data andsor files. For this purpose a communication session is initiated. During the session the user has access to network resources, although this access may be shared with others. Owing to the mobile environment, the user's connection to the network during a session may be severed. Since the mobile user can act semidautonomously, such disconnections can be transparent. That is, the mobile user can continue to function in an offdline mode while the system will begin transparent automatic reconnection attempts to reestablish a link to the network. Only after a fixed lgivenr number of such attempts to reconnect have failed, is the session deemed to have failed. The issue is complicated by the hostile mobile radio environment and by user mobility. We consider sessiondoriented communications and develop a tractable analytical model for traffic performance based on multiddimensional birth–death processes. The approach allows consideration of various platform types, such as pedestrians, automobiles, and buses, which may have very different mobility characteristics. Performance characteristics, such as: blocking, forced session termination, carried traffic, the average time per suspension, and the average number of suspensions per session, are calculated.