Soft Handoff Research Articles

무선망에서 소프트 핸드오프 알고리즘을 위한 새로운 대역폭 예약 기법

무선망에서 소프트 핸드오프 알고리즘을 위한 새로운 대역폭 예약 기법

Coverage expansion and capacity improvement from soft handoff for CDMA cellular systems

Previous research examining coverage expansion due to soft handoff in code division multiple access (CDMA) cellular networks did not address the relationship between the coverage expansion and the amount of carried traffic in the cell. In this paper, we first present a method to estimate reverse-link multicell coverage using a duration-outage approach. Using this, we then proceed to quantitatively analyze reverse-link coverage expansion from soft handoff as a function of the carried traffic in the cell under different propagation environments. Our results show that for modest values of carried traffic, the coverage expansion is almost constant. However, when the amount of carried traffic is large, the coverage expansion can increase dramatically. Next, we examine the dependence of the soft-handoff coverage expansion on key characteristics of the large-scale shadowing environment. Results are presented that quantify the coverage-expansion dependence on the large-scale shadowing's correlation distance (d/sub c/), variability (/spl sigma//sub Z/), path-loss exponent (K/sub 2/), and the correlation between the two-cell large-scale shadowing environments (C/sub Z/). The results indicate that realistic variations in these propagation conditions have a significant, and sometimes dramatic, effect on the coverage expansion provided by soft handoff. Finally, we analyze the capacity improvement from soft handoff and the tradeoff between capacity improvement versus coverage expansion. Understanding and managing the capacity improvement versus coverage expansion tradeoff is critical for achieving optimal CDMA cell-network performance.

Comparison of Downlink Power Allocation Mechanisms in Soft Handoff for the WCDMA System with Heterogeneous Cell Structures

Handoff in heterogeneous cellular networks is one of the hot topics for wireless networks beyond the third generation. We observe that a power exhausting issue may occur in a code division multiple access (CDMA) system with mixed-sized cells. During soft handoff in the downlink transmission, a number of base stations transmit signals to a user simultaneously. Usually, a microcell has a more stringent limitation on the total available power than a macrocell. Thus, ignoring the impact of various cell sizes, the traditional downlink power allocation techniques for soft handoff may easily consume excessive power to serve soft handoff users, while leaving insufficient power for serving other regular users.To resolve such an power exhausting issue in CDMA systems, we investigate different downlink power allocation techniques used in soft handoff subject to the impact of mixed-sized cells. For the single-site power allocation technique we consider the site selection diversity transmission (SSDT) technique, while for the multi-site power allocation we study the link proportional power allocation (LPPA), the quality balancing power allocation (QBPA), and the equal power allocation (EPA) techniques. We find that the multi-site LPPA technique can more efficiently allocate power to both handoff and non-handoff users than others. In an example with the ratio of the micrcocell radius/macrocell radius equal to 1/3, it is demonstrated that LPPA can improve the capacity over EPA, QBPA, and SSDT by 125, 30, and 5%, respectively. By taking account of measurement errors in the same case, the capacity improvements of LPPA over EPA, QBPA, and SSDT become 180, 41, and 23%, respectively.

A capacity acquisition protocol for channel reservation in CDMA networks

In this paper, a capacity acquisition protocol is proposed for channel reservation in CDMA networks. Under this protocol, a cell is virtually divided into three regions (i.e., inner region, forced handoff region, and active handoff region). A new call in the active handoff region works in soft handoff mode upon its admission, while a new call in the inner and forced handoff regions works in single mode. However, in the forced handoff region, calls working in single mode can be forced into soft handoff mode, when extra capacity is needed by soft handoff calls. As a result, no explicit channel reservation is required before a call enters soft handoff. By adjusting the size of the forced handoff region, the capacity acquisition can adapt to traffic load and guarantee a desired call dropping probability. To evaluate the capacity acquisition protocol, an analytical model is derived and is also validated through computer simulations. Numeric results illustrate that the capacity acquisition protocol significantly reduces the call dropping probability.

SIP-based vertical handoff between WWANs and WLANs

Future-generation wireless networks have been envisioned as the integration of various wireless access networks, including both wireless wide area networks and wireless local area networks. In such a heterogeneous network environment, seamless mobility support is the basis of providing uninterrupted wireless services to mobile users roaming between various wireless access networks. Because of transparency to lower-layer characteristics, ease of deployment, and greater scalability, the application-layer-based session initiation protocol has been considered the right candidate for handling mobility in heterogeneous wireless networks. However, SIP entails application-layer transport and processing of messages, which may introduce considerable delay. As a case study of the performance of mobility management protocols in the heterogeneous wireless networks, we analyze the delay associated with vertical handoff using SIP in the WLAN-UMTS internetwork. Analytical results show that WLAN-to-UMTS handoff incurs unacceptable delay for supporting real-time multimedia services, and is mainly due to transmission of SIP signaling messages over erroneous and bandwidth-limited wireless links. On the other hand, UMTS-to-WLAN handoff experiences much less delay, mainly contributed by the processing delay of signaling messages at the WLAN gateways and servers. While the former case requires the deployment of soft handoff techniques to reduce the delay, faster servers and more efficient host configuration mechanisms can do the job in the latter case.

Adaptive and predictive downlink resource management in next-generation CDMA networks

Guard channels have been proposed to minimize handoff call dropping when mobile hosts move from one cell to another. Code-division multiple-access (CDMA) systems are power- and interference-limited. Therefore, guard capacity in CDMA networks is soft, that is, a given capacity corresponds to variable number of connections. Thus, it is essential to adjust the guard capacity in response to changes in traffic conditions and user mobility. We propose two schemes for managing downlink CDMA radio resources: guard capacity adaptation based on dropping (GAD), and guard capacity adaptation based on prediction and dropping (GAPD). In both schemes, the guard capacity of a cell is dynamically adjusted so as to maintain the handoff dropping rate at a target level. In the second scheme, there is an additional, frequent adjustment component where guard capacity is adjusted based on soft handoff prediction. We show through extensive simulations that GAD and GAPD control the handoff dropping rate effectively under varying traffic conditions and system parameters. We also find that GAPD is more robust than GAD to temporal traffic variations and changes in control parameters.

Prioritized Call Admission Control of Multiservice Hierarchical Wireless Networks

Call admission control of multiservice hierarchical wireless networks supporting soft handoff is studied, where users with different average call holding times are assigned to different layers, i.e., microcells in the lower layer are used to carry users with shorter call holding time, whereas macrocells in the upper layer are for users with longer call holding time. Further, to give handoff calls priority over new calls, handoff queues are provided for handoff calls that can not obtain the required channel immediately. According to whether handoff queues are provided in microcells and/or macrocells, four different call admission control schemes are proposed and studied. We derive the mathematical model of the considered system with multi- dimensional Markov process, and find the steady state probability distribution iteratively and thus the performance measures of interest: new call blocking probability, forced termination probability, and mean waiting time in handoff queue. Analytical results show that providing handoff queues in both microcells and macrocells can achieve the best blocking performance at the expense of mean waiting time in handoff queue.

Soft Handoff Analysis of Hierarchical CDMA Cellular Systems

A new soft handoff analysis for hierarchical code-division multiple-access (CDMA) cellular systems is presented. Hierarchical cellular architectures have been proposed to increase cellular system capacity and flexibility. In order to extend such architectures to CDMA-based systems, the performance of soft handoff in hierarchical architectures must be considered. We first develop an analytical method for studying the interference in hierarchical CDMA cellular systems. We then apply the obtained results to a soft handoff analysis model to study the performance of soft handoff in hierarchical architectures. It is observed that dynamic handoff parameter assignment, where parameters are dynamically adjusted according to given interference conditions, offers a more efficient handoff mechanism than fixed handoff parameter assignment.

Achieving all the time, everywhere access in next-generation mobile networks

The anytime, anywhere access view of nomadic computing is evolving towards all the time, everywhere views of pervasive computing. The all the time access requires mobile devices to be always connected, even if connectivity may be compromised due to Access Point overload and to transient signal degradations. The everywhere access requires mobile devices to use heterogeneous Access Points (ranging from Bluetooth and 802.11 to 2.5G and 3G cellulars), leading to high variability of the connection status. A mobility management solution that leverages connection availability, while enabling applications to be aware of the connection status, is thus needed. This paper proposes a Last Second Soft Handoff scheme that leverages the availability of connection. The proposed scheme has been integrated in a mobility management architecture, which provides connection awareness via an API, named NCSOCKS. Implementation issues are discussed and experimental results are provided.

Soft handoff and uplink capacity in a two-tier CDMA system

This paper examines the effect of soft handoff on the uplink user capacity of a CDMA system consisting of a single macrocell in which a single hotspot microcell is embedded. The users of these two base stations operate over the same frequency band. In the soft handoff scenario studied here, both macrocell and microcell base stations serve each system user and the two received copies of a desired user's signal are summed using maximal ratio combining. Exact and approximate analytical methods are developed to compute uplink user capacity. Simulation results demonstrate a 20% increase in user capacity compared to hard handoff. In addition, simple, approximate methods are presented for estimating soft handoff capacity and are shown to be quite accurate.

On the uplink SIR of a mobile in soft handoff with applications to BLER estimation

We focus on the uplink and derive the joint probability density function of the signal to interference (+noise) ratios (SIRs) at the base stations (BSs) for the general case of n BSs supporting a mobile in soft handoff, each using maximal ratio combining with m-fold diversity. We then calculate the correlations between the SIRs for some example cases. The insights gained here lead to a fresh approach to the estimation of the block error rate (BLER) at the output of the frame selector (FS), which is the usual measure of quality of service in UMTS. For low specified BLER (e.g., 0.1%), the method of counting cyclic redundancy code errors requires an impractically large number of data blocks for an accurate estimate of BLER. We propose to estimate the BLER at the FS output based on measurements performed by the individual BSs. The initial setting of the target SIR in the outer loop power control can then be selected in the same way in order to expedite BLER convergence to the desired setting.

Measurement-based band allocation in multiband CDMA

Multiband code division multiple access (sometimes called multicarrier CDMA) is a promising approach to increasing the capacity of CDMA networks, while maintaining compatibility with existing systems. This paper investigates a family of algorithms for allocating new calls to bands based on measured path gains, or alternatively, on estimates of the users' positions. By separating strong and weak users into separate bands, this approach reduces the other-cell interference on the uplink. This is shown to reduce the number of calls in soft handoff, which reduces the hardware requirements at the base stations. Under a range of conditions, it also provides significantly lower outage than alternative algorithms. An additional benefit of this approach is a reduction in the dynamic range required for uplink power control.

Performance Evaluation of Hierarchical Cellular CDMA Networks With Soft Handoff Queueing

We consider hierarchical cellular code-division multiple-access networks supporting soft handoff, where users with different mobility are assigned to different layers, i.e., microcells in the lower layer are used to carry slow users, whereas macrocells in the upper layer are for fast users, and handoff queues are provided for handoff calls that cannot obtain the required channel immediately, so that forced termination probability can be reduced. According to whether handoff queues are provided in microcells and/or macrocells, four different call admission control schemes are proposed and studied. We derive the mathematical model of the considered system with multidimensional birth-death process and utilize Gauss-Seidel iterative method to find the steady-state probability distribution and thus the performance measures of interest: new call blocking probability, handoff failure probability, and forced termination probability. Analytical results show that providing handoff queues in both microcells and macrocells can achieve largest performance improvement. Furthermore, handoff queue size greater than a threshold is shown to have little effect on performance measures of interest. Last but not least, the studied two-tier system is compared with a one-tier counterpart. It is shown that the two-tier system performs better in terms of average number of handoffs per fast call.

Sensitivity Analysis of Handoff Algorithms on CDMA Forward Link

We analyze the performance of different handoff algorithms on the forward link or downlink of a code-division multiple-access (CDMA) cellular system. Unlike the reverse link, soft handoff on the forward link requires additional resources such as CDMA codes and transmit power and also causes additional interference. If handoff requests can be processed and completed instantaneously, transmission from the base station with the best link to the user would achieve a significant fraction of the macrodiversity gain without utilizing additional resources. However, in practical systems, there is a nonzero handoff completion delay and soft handoff provides the required robustness to delays, although it comes at the expense of additional network resources. Thus, there is a tradeoff between the extent of soft handoff required and the handoff execution delay. We compare the performance of hard and soft handoff schemes and study their sensitivity to the delay in the execution of the handoff. Outage probability and the total average power required are used as performance metrics. We present an analytical framework to study this tradeoff and also discuss simulation results with field data. The results provide insights on the conditions under which soft handoff can be eliminated and on the effect of relevant handoff thresholds on the performance.

Studying the Effect of Delay Diversity on a DS-CDMA Downlink

In this paper, we study the error rate performance of a direct sequence CDMA downlink similar to the CDMA2000 standard. While analytical computation of the error rate is possible in DS-CDMA systems under certain simplifying assumptions, computer simulations are perhaps the only means to quantify the error rate for realistic channel models. Since the delay spread of the multipath channel varies with distance, we investigate the error performance as a function of the distance from the base station. The computer simulations reveal certain interesting results which to our knowledge have not explicitly appeared in open literature.We compare the RAKE error rate performance under situations of no handoff, soft handoff, independent fading, correlated fading, chip-spaced sampling, sub-chip sampling, and pilot channel cancellation at the mobile receiver. These results indicate that narrow-band DS-CDMA downlink has certain severe shortcomings, and that it may be vital to ensure line of sight propagation for the “near” users, and exploit base-station (macro) diversity for the “far” users (in soft handoff) to ensure adequate error rate performance.

Analysis of multiservice cellular networks with asymmetrical traffic and handoff queue

The next generation mobile communication system should be able to accommodate various services, including voice and Internet dial-up. As is well known, voice service has symmetrical, i.e. equal uplink and downlink bandwidth requirements, whereas Internet dial-up traffic usually has asymmetrical, i.e. unequal uplink and downlink bandwidth requirements. Frequency division duplex (FDD) usually allocates the same bandwidth to both uplink and downlink, whereas time division duplex (TDD) can change the number of time slots allocated to uplink and downlink to allocate different bandwidth to uplink and downlink. Thus, for scenarios with asymmetrical traffic, TDD is a better solution. To provide better quality of service during handoff, make-before-break soft handoff is preferable to break-before-make hard handoff. Furthermore, as forced termination of an ongoing call is less tolerable than blocking of a new call, handoff calls are usually given priority over new calls. We consider cellular multiservice TDD networks supporting both symmetrical and asymmetrical traffic with soft handoff and handoff queueing. First, we derive the mathematical model of the considered system with multi-dimensional birth–death process. Second, since the asymmetrical traffic may lead to the waste of uplink or downlink bandwidth, the effects of the traffic mix and the number of uplink slots on the system performance are studied. Third, the comparison of TDD and FDD with the presence of asymmetrical traffic is performed. Fourth, as the handoff region area increases, more calls will enter into or originate at that region. Thus, we study the effect of handoff region area on performance. Fifth, as a handoff call that cannot obtain the required channel upon arrival will be rejected if the handoff queue is full, the effect of handoff queue size on the performance is studied.

A Network-Layer Soft Handoff Approach for Mobile Wireless IP-Based Systems

Handoff is the process during which a mobile node (MN) needs to change its connectivity point to the wireless internetwork from one access node (AN) to another during an ongoing communication. If MNs are allowed to have two or more simultaneous connections to the internetwork through different ANs, then the handoff is said to be soft; otherwise, it is said to be hard. Traditionally, during forward-link soft handoff, multiple identical copies of each packet are simultaneously transmitted to the MN through the associated ANs. At the MN's physical-layer, the received signals are combined on a bit-by-bit basis resulting in improving the bit-error rate. However, this approach requires tight synchronization of the ANs involved in the soft handoff. In addition, as shown in the literature, the capacity often decreases due to the increase of the number of channels used by MNs during soft handoff. In this paper, we propose, analyze, simulate, and implement a soft handoff scheme called soft handoff over IP (SHIP) for forward-link that 1) overcomes the need for synchronization and 2) increases the capacity of the network. Through both analytic and simulation studies, we show that SHIP achieves significant performance improvements. We derive analytic expressions of the power-capacity relationship for two-dimensional (2-D) and one-dimensional (1-D) cell models. By comparing our scheme with the hard handoff, we empirically show that the capacity increases by about 30% and 20%, respectively, for the 2-D and 1-D cell models. Further, the simulation results show that SHIP saves up to 30% of the total power consumed by the ANs.

QoS-Controlling Soft Handoff Based on Simple Step Control and a Fuzzy Inference System With the Gradient Descent Method

Quality of service (QoS) is an important criterion considered in mobile communication systems since it directly affects system users in terms of both call blocking and call quality. This paper aims to control the QoS in code-division multiple-access mobile communication systems by proposing two new practical methods applied to the conventional soft handoff (SHO) process based on a simple step-control method and a human-oriented information method of fuzzy inference system with classical mathematics of the gradient descent method. The output parameters selected to be controlled by adapting SHO thresholds are the trunk-resource efficiency (TRE), the forward link's average energy per bit to the interference plus noise power spectral density ratio (E/sub b//I/sub 0/), and the outage probability (P/sub out/). TRE is concerned with call blocking while E/sub b//I/sub 0/ is an indicator of the call quality. The number of the remaining channels of serving base station (CH/sub rm/), the moving average number of pilots in active set of any mobile station (no/sub BS/), and E/sub b//I/sub 0/ is or are selected to be one, two, or three of the control plant inputs, depending on the method applied. QoS-controlling SHO (TRE- and E/sub b//I/sub 0/-controlling SHO) can improve the system performance. However, which method that will be selected for use is according to the environment and the requirement of the system operator and/or users.

A novel soft handoff technique using STTD for MC‐CDMA in a frequency selective fading channel

AbstractAntennas are commonly located in close proximity to each other and there is high correlation between them. In such cases, there exists correlated fading among antennas. The correlated fading degrades the performance of this system. This paper presents an effective soft handoff technique using space time transmitter diversity (STTD) with two base stations for multi‐carrier (MC)‐CDMA to achieve high diversity gain in a wireless broadband channel. The proposed system is a dual step diversity scheme in frequency selective fading channel environments combining the frequency diversity effect achieved by the use of multi‐carriers and the antenna diversity gain afforded by STTD. This is an effective diversity scheme, which improves the signal quality at the receiver by simple processing across two transmit antennas. No feedback from the receiver to the transmitter is required in this system and the computation complexity is low. Copyright © 2004 AEI

On the Impact of the Soft Handoff Threshold and the Maximum Size of the Active Group on Resource Allocation and Outage Probability in the UMTS System

We study the effect of two control parameters of the universal mobile telecommunication system handoff algorithm on resource allocation and the outage probability of the forward link. We thereby provide information on the effect of the parameters on the probability distributions of the number of base stations (BSs) supporting a mobile station (MS), the number of MSs supported by a single BS (per carrier), and the maximal cell size, subject to a given probability of downlink outage on the cell boundary. Denoting the set of BSs supporting an MS in soft handoff as the active group, the first parameter n/sub A/ is the maximum allowed size of the active group. The second, denoted /spl tau/, is the upper limit on the difference between the local mean received power from the dominant BS and that from any other member of the active group (in decibels), where the term "dominant" refers to the BS with the lowest path loss to the MS. We assume that the MS is equipped with a Rake receiver capable of performing "maximal ratio combining" of the signals it receives from the transmitting BSs. We present general analytical derivations along with results derived for specific situations through simulations and numerical integration.