Abstract: A prototype microcellular wireless asynchronous transfer mode network (WATMnet) capable of providing integrated multimedia communication services to mobile terminals is described in this paper. The experimental system's hardware consists of laptop computers (NEC Versa-M) with WATMnet interface cards, multiple VME/i960 processor-based WATMnet base stations, and a mobility-enhanced local-area ATM switch. The prototype wireless network interface cards operate at peak bit-rates up to 8 Mb/s, using low-power 2.4 GHz industrial, scientific, and medical (ISM)-band modems. Wireless network protocols at the portable terminal and base station interfaces support available bit rate (ABR), variable bit rate (VBR), and constant bit rate (CBR) transport services compatible with ATM using a dynamic time-division multiple-access/time-division duplex (TDMA/TDD) MAC protocol for channel sharing and data link control (DLC) protocol for error recovery. A custom wireless control protocol is also implemented between the portable and base units for support of radio link related functions such as user registration and handoff. All network entities including the portable, base and switch use a mobility-enhanced version of ATM ("Q.2931+") signaling for switched virtual circuit (SVC) connection control functions, including handoff. In the first stage of the prototype, the application-level API is TCP/UP over ATM ABR service class using AAL5. Early experiments with the WATMnet prototype have been conducted to validate major protocol and software aspects, including DLC, wireless control, and mobility signaling for handoff, Selected network-based multimedia/video applications requiring moderate bit-rates (/spl sim/0.5-1 Mb/s) in the ABR mode have been successfully demonstrated on the laptop PC.

Abstract: A method and apparatus for performing inter-system hard handoff between communication systems (S1, S2) or inter-frequency hard handoff within a CDMA communication system is disclosed. The purpose of this invention is to reduce the probability of dropped calls during intersystem hard handoff. In the event that a hard handoff attempt is unsuccessful, the mobile station (18) will return to the original system (S1) with information which the communication system of the present invention uses to assist in the performance of future handoff attempts. Alternatively, with no handoff attempt made, the mobile station (18) monitors the destination system (S2), then returns to the original system (S1) with information used to assist in subsequent handoff attempts. The information returned from monitoring a CDMA system consists of results of a search for one or more pilots given at offsets in a specific list provided to the mobile station (18) by the base station (B1-B5) or a set of offsets based upon a predetermined search algorithm.

Abstract: Network protocols in cellular wireless data networks must update routes as a mobile host moves between cells. These routing updates combined with some associated state changes are called handoffs. Most current handoff schemes in wireless networks result in data loss or large variations in packet delivery times. Unfortunately, many applications, such as real-time multimedia applications and reliable transport protocols, adapt to long term estimates of end-to-end delay and loss. Violations and rapid fluctuations of these estimates caused by handoff processing often result in degraded performance. For example, loss during handoff adversely affects TCP performance [4], and high packet loss and variable delays result in poor real-time multimedia performance. In this paper, we describe a multicast-based protocol that eliminates data loss and incurs negligible delays during a handoff. The basic technique of the algorithm is to anticipate a handoff using wireless network information in the form of received signal strengths and to multicast data destined for the mobile host to nearby base stations in advance. This routing, combined with intelligent buffering techniques at the base stations, enables very rapid routing updates and eliminates data loss without the use of explicit data forwarding. We have implemented this protocol using IP Multicast and Mobile IP-like routing. In our implementation, handoffs typically take between 8 and 15 ms to complete and result in no data loss.

Abstract: In the planning of modern cellular mobile communication systems, the impact of customer behavior has to be carefully taken into account. Two models dealing with the call retrial phenomenon are presented. The first model considers a base station with a finite customer population and repeated attempts. A Markov chain modeling is proposed, and an efficient recursive solution of the state probabilities is presented. The second model focuses on the use of the guard channel concept to prioritize the handover traffic. Again, the retrial phenomenon plays an important role. The influence of the repeated attempt effect on the quality of service experienced by the mobile customers is discussed by means of numerical results.

Abstract: For use with a wireless computer network having a plurality of access points, a mobile station adapted for communicating with the network via a current access point and having a scanning circuit for locating a new access point, the scanning circuit requiring a scanning period for time to locate the new access point, a method of operation of the mobile station and a wireless computer network infrastructure. The mobile station comprises: (1) a detection circuit that generates a ready-to scan signal indicating that the mobile station is about to activate the scanning circuit and (2) a suspension circuit, coupled to the detection circuit, that receives the ready-to-scan signal and generates, in response thereto, a data suspend signal for transmission to the current access point, the data suspend signal causing the current access point to suspend transmission of data to the mobile station, thereby preventing loss of the data during the scanning period.

Abstract: The approximate position of a mobile station in a cell can be predicted by measuring the signal strength between the mobile station and the base station of the cell in which it is located and the base stations of the neighboring cells. After a series of instantaneous signal strength measurements have been collected, the velocity and direction of the mobile unit can be determined. Based on the velocity and direction of the mobile unit, future locations of the mobile unit can be predicted including the projected signal strength between the mobile station and the base stations of the cell in which it is located and neighboring cells. Analyzing the projected signal strength values, the time when the mobile unit will require handover to a neighboring cell can be determined and if desired, resources in a neighboring cell can be allocated in anticipation of the mobile unit being handed over to that cell. New signal strength measurements are periodically collected and new projections are made to increase the accuracy of the estimate of when handover will occur and to what neighboring cell.

Abstract: Hierarchical cellular networks with subscribers of varying mobility are considered. Microcells are used to address the high-intensity traffic of mainly slow mobility areas, and macrocells are overlaid over the microcells to cater mainly to high-mobility lower density traffic. The two tiers of microcells and macrocells provide a secondary resource for new traffic as well as handoffs for mobile subscribers of different mobility classes. Furthermore, resources in alternate layers are monitored to assign the appropriate resource types when they become available. We develop an analytical model to evaluate the performance of such systems, and quantify the gain obtained by providing overflow to alternate resources as well as the advantages in resource reassignment according to the speed classification.

Abstract: A telecommunications system and method for performing a handover between the fixed (wireline) network and a mobile network during a call placed to or from a dual mode device, without any interruption in the voice or data connection. Therefore, for calls initiated in the fixed network, once the subscriber leaves the coverage area for the fixed mode of the dual mode device, the call continues as normal by transferring the call to the mobile network. Similarly, for calls initiated in the mobile network, once the subscriber moves back into the fixed mode coverage area, the call can be transferred to the fixed network in order to provide a lower rate to the subscriber, without any service interruption.

Abstract: A method and apparatus which permits handoffs while the mobile station is in the system access state. This is achieved by providing for channel assignment messages to be sent over the paging channel of a plurality of base stations, which increases the probability of one of the messages getting through. In addition, this assures the mobile station will be able handoff to a different base station and have a traffic channel allocated to it on the new base station without delay. In addition, a method and apparatus which permits the mobile station to be directly assigned into a soft handoff state upon traffic channel assignment.

Abstract: This paper proposes algorithms for handoff, location, and connection management in a wireless asynchronous transfer mode (ATM) local-area network (LAN). Fast handoffs while maintaining cell sequence and quality-of-service (QoS) guarantees are achieved by distributing switching functionality to base stations, and using a networking scheme based on provisioned virtual trees. A new distributed location management scheme using a minimal registration procedure and broadcasts on wired links is proposed for this LAN. The detailed signaling procedures that support the algorithms for mobility and connection management are described. Finally, an implementation of these procedures and an analysis of the measured data is presented. Measurements of service times obtained from this implementation indicate that over 100 calls/s. can be handled by each node in 50-node network with a high-percentage of mobiles (75%) relative to fixed endpoints. This is comparable to current wired ATM switch call handling throughputs, in spite of the fact that these nodes perform additional handoff and location management functions. The data also indicates handoff latency times of 1.3 ms. This validates our proposal for maintaining cell sequence while performing handoffs.

Abstract: In cellular networks, blocking occurs when a base station has no free channel to allocate to a mobile user. One distinguishes between two kinds of blocking, the first is called new call blocking and refers to blocking of new calls, the second is called handoff blocking and refers to blocking of ongoing calls due to the mobility of the users. In this paper, we first provide explicit analytic expressions for the two kinds of blocking probabilities in two asymptotic regimes, i.e., for very slow mobile users and for very fast mobile users, and show the fundamental differences between these blocking probabilities. Next, an approximation is introduced in order to capture the system behavior for moderate mobility. The approximation is based on the idea of isolating a set of cells and having a simplifying assumption regarding the handoff traffic into this set of cells, while keeping the exact behavior of the traffic between cells in the set. It is shown that a group of 3 cells is enough to capture the difference between the blocking probabilities of handoff call attempts and new call attempts.

Abstract: In a communications network, a network user communicates using a remote unit (125) with another user (30) via at least one base station (B1A). The communications network has a first base station (B1A) having a first coverage area and a second base station (B2A) having a second coverage area. In the situation where communication is not established between the second base station (B2A) and the remote unit (125), in order to provide communication between the remote unit (125) and the first base station (B1A) when the remote unit (125) is in the first coverage and simultaneously in the second coverage area, the first base station (B1A) produces a first active communication signal as intended for the remote unit (125). The first base station transmits the first active communication signal from a first antenna (130). The base station delays the first active communication signal to produce a first delayed active communication signal and transmits it from a second antenna (135) wherein the second antenna (135) is oriented with respect to the first antenna (130) such that the first active communication signal and the first delayed active communication signal fade independently as perceived by the remote unit. The first base station (B1A) may measure a round trip delay of the first active communication signal in order to identify that the remote unit (125) is located within the second coverage area.

Abstract: The present invention realizes a rapid and efficient cell search and small-size instrument for an asynchronous DS-CDMA cellular system. This cell search detects the correlation between the received signal and the short code of the control channel, and matched filter 22 detects the maximum electric power correlation peak location. Next, using correlators 28-1 to 28-n which are parallelly set in a plurality for RAKE processing with plurality, identifies the long code that is set in the system with the detected long code timing. After the long code is synchronized, a multipath signal is received using 28-1 to 28-n, and the data is judged by RAKE processing. When peripheral cell search is executed, after long code timing is detected by using matched filter 22, the long code of the candidate peripheral cell is designated using the same matched filter. Handover is safely realized by receiving the signal from the connected base station by correlators 28-1 to 28-n, and the base station signal through handover by 22.

Abstract: In a communications network, a network user communicates through a remote unit (30) with another user via at least one base station (100). The communications network includes a first mobile switching center (MSC-I) which controls communications through a first set of base stations including a first base station (100). The remote unit (30) stores a list of active base stations which has an entry corresponding to each base station with which active communication is established. The first base station (100) has an entry on the list of active base stations. The first base station (100) measures a round trip delay of an active communication signal between the first base station (100) and the remote unit (30). A handoff of the active communication signal is initiated if the round trip delay of the active communication exceeds a threshold if the first base station (100) is designated as a reference base station. Alternatively, the remote unit (30) also stores a list of candidate base stations comprising an entry corresponding to each base station through which active communication may be possible but is not established. A handoff of the active communication signal is initiated if the list of candidate base stations comprises an entry corresponding to a triggering pilot signal.

Abstract: A method and apparatus are provided for controlling mobile and base stations (14 and 16) during satellite (12) based telecommunications to perform scheduled handovers between two communications channels (26 and 27). The base station (16) determines when a handover will be necessary. Once determined, the base station (16) generates a handover scheduling command (node #4) which includes a scheduled handover time representing a time in the future at which the handover will occur. The handover scheduling command is transmitted over the first channel to the mobile station (14). Upon receipt of the handover scheduling command, the mobile station (14) performs steps necessary to establish a second communications link over a second channel (27), prior to the scheduled handover time. At the scheduled handover time (node #21), the mobile and base stations (14 and 16) have established the second communications link on the second channel (27). To establish the second channel the mobile station calculations of the second channels frequency, timing offset and power level (node #11). By using a scheduled handover process, the mobile and base stations (14 and 16) avoid the production of interference and breaks within a conversation transmitted therebetween.

Abstract: In a charging system for cellular communications, real-time prices for new connections are offered to the customer. The real-time prices take into account both prevailing and historic traffic patterns, together with the probability that the customer will move from their current cell into a neighbouring cell. Cells of the network are monitored in clusters. If a customer is located in one particular cell, the price offered to that customer will take into account the probability of handover to a neighbouring cell during the connection, and may also take into account the probability of incoming traffic from neighbouring cells during the connection. To enable this predictive pricing, the system stores, and may also update, historic data.

Abstract: To mitigate the adverse effects of the load imbalance phenomenon prevalent in spread spectrum, multi-carrier wireless communication systems, the system beneficially employs the use of handoff as a means for balancing the call traffic (commonly referred to as 'load') among a plurality of carriers within the communication system. The multi-carrier wireless communication system monitors (214) a plurality of metrics corresponding to the loading of each of the plurality of carriers in the communication system and, based upon the metrics, will handoff (220) call traffic to and between the plurality of carriers, thereby mitigating the adverse effects associated with the load imbalance, in accordance with the invention.

Abstract: In the handover of a radio extension of an ATM network there are used markers that are located at fixed locations in the ATM cell stream in order to indicate the ending of an uplink and downlink cell stream, so that the switching of cell streams can be carried out in a synchronised fashion, and cells are not lost and their relative order does not change. If a downlink data transmission via the old access point succeeds, the old access point attaches to the last forwarded information field a notice of closing the traffic, in which case the mobile terminal transfers the information of successful transmission to the new access point. In another case, the old access point sends the unforwarded cells to the new access point and terminates the transaction with the same marker that generally indicates the end of a downlink cell stream.

Abstract: In a communications network, a network user communicates through a remote unit via at least one base station. The communications network includes a first mobile switching center for controlling communications through a first set of base stations and a second mobile switching center for controlling communications through a second set of base stations. The network also includes a service providing base station controlled by the first mobile switching center and providing service to a first transition coverage area using a first pseudorandom noise code. The first transition coverage area defines a boundary between a first system controlled by the first mobile switching center and a second system controlled by the second mobile switching center. The network further includes a passage providing base station controlled by the second mobile switching center for providing service to the first transition area using a second pseudorandom noise code offset in time by a first amount from the first pseudorandom noise code. The passage providing base station provides service to a remote unit only if the remote unit is entering the first transition coverage area while exiting the second system or is about to exit the first transition coverage area while entering the second system.

Abstract: The design of handoff algorithms for cellular communication systems based on mobile signal strength measurements is considered. The design problem is posed as an optimization to obtain the best tradeoff between the expected number of service failures and expected number of handoffs, where a service failure is defined to be the event that the signal strength falls below a level required for satisfactory service to the subscriber. Based on dynamic programming arguments, an optimal solution is obtained, which, though impractical, can be used as a benchmark in the comparison of suboptimal schemes. A simple locally optimal handoff algorithm is derived from the optimal solution. Like the standard hysteresis algorithm, the locally optimal algorithm is characterized by a single threshold. A systematic method for the comparison of various handoff algorithms that are akin to the receiver operating characteristic (ROC) curves of radar detection is presented. Simulation results show that the locally optimal algorithm outperforms the hysteresis algorithm, especially in situations where accurate prediction of signal strength is possible. A straightforward technique for adapting the locally optimal algorithm to changing environments is suggested. That natural adaptability is the algorithm's principle advantage over current approaches.

Abstract: In a communications network, a network user communicates using a remote unit (125) with another user (30) via at least one base station (B1A). The network is comprised of first (MSC-I) and second (MSC-II) mobile switching control stations respectively controlling communications through a first set of base stations (B1A-B1E) including a first base station (B1A) and through a second set of base stations (B2A-B2E) including a second base station (B2A). To direct communications between the remote unit (125) and the first (B1A) and second base stations (B2A) the first base station (B1A) measures a round trip delay of an active communication signal between the first base station (B1A) and the remote unit (125). The remote unit (125) measures a first phase offset of a pilot signal received from a first candidate base station (B1A) and reports it to the first mobile switching center (MSC-I) via the first base station (B1A). The first mobile switching center (MSC-I) calculates a candidate round trip delay between the remote unit (125) and the first candidate base station (B1A) based on the first phase offset and the round trip delay of the active communication signal. An active communication control unit accesses a measurement directed hard handoff table to determine a location of the remote unit based on the round trip delay corresponding to the first active communication signal and the candidate round trip delay.

Abstract: A method for increasing the success rate for soft handoffs, particularly under rapidly fluctuating fading conditions, using traffic channels of active set base stations. In one embodiment of the present invention, increased success rate for soft handoffs is achieved using a candidate base station, in addition to the active set base stations, to transmit a handoff command message to the mobile-telephone. Specifically, the handoff command message is transmitted by the active set base stations over the traffic channels assigned to mobile-telephone for communicating with the active set base stations, and by the candidate base station over the traffic channel assigned to the mobile-telephone for communicating with a primary base station. Additionally, a pilot signal may be transmitted by the candidate base station to enable the mobile-telephone to coherently demodulate the handoff direction message transmitted by the candidate base station.

Abstract: The invention relates to a handover method in a mobile communication system where the serving base station (BTS1) measures the level and/or quality of the uplink signal of a mobile station (MS). The base station controller (BSC) commands neighbouring base stations (BTS2, BTS3) to measure the level and/or quality of the uplink signal of the mobile station (MS), when the serving base station (BTS1) has measured an uplink signal level and/or quality lower than a predetermined triggering value. The base station controller (BSC) performs a handover to the neighbouring base station (BTS2, BTS3) that has measured the most suitable uplink signal level and/or quality, when the serving base station (BTS1) has measured an uplink signal level and/or quality below a predetermined threshold value for handover.

Abstract: During a 'make before you break' handoff performed within a CDMA communications system, duplicate downlink communications (50) are generated and routed through different base stations (16) for delivery to the mobile station (18). At the mobile station, the duplicate signals are received and the frame sequence numbers of the substantially simultaneously received frames (58) therein are compared (60). If the sequence numbers do not match, the duplicate signals are identified as not being synchronously received by the mobile station. In response thereto, the mobile station signals (64) the communications network on the uplink with a timing adjustment message, and appropriate timing modifications (66) are made with respect to the transmission of the duplicate signals in order to provide for substantially synchronous reception. Diversity combination and decoding (68) are then performed on the received signals. Alternatively, the mobile station buffers (72) the earlier arriving signal and waits for the later arrival of the sequence number matching frame of the duplicate signal before engaging in diversity combining and decoding of the received signals.

Abstract: A method is disclosed of analyzing lists of neighboring cells in a cellular telecommunications system comprising a plurality of active mobile stations and a static network, the static network having a first cell and a plurality of cells neighboring the first cell, each of the cells having a base station. The method is applicable to GSM and GSM-like systems. In the case of a GSM system, the analyzing method may comprise the steps of: extracting from the static network the GSM MEAS RES produced by the mobile stations in said first cell and producing a reporting list including, for each position in the GSM BA(SACCH) list, the number of times that any of the base station identifiers has been reported for that position; extracting from the static network GSM HANDO CMD messages for handovers from said first cell, and producing a handover list of the GSM BCCHs and corresponding GSM BSICs in the extracted handover messages; correlating the reporting list and the handover list with respect to the BCCHs; and analyzing the correlated lists to determine whether any of the control channels is affected by bad frequency planning.

Abstract: In order to locate the position of a mobile station of a mobile radio system, the distance of the mobile station from a base transceiver station is determined, or the distances of the mobile station from at least two base transceiver stations are determined and the position is found by triangulation. In a GSM-type system, there is a predetermined known response delay between a particular signal received by the mobile station from the base transceiver station and a particular response transmitted from the mobile station to the base transceiver station; the distances can therefore be determined from the response delay and a measured period between transmission of the particular signal and reception of the particular response. In a GSM-type system, the mobile station transmits messages indicating the signal strengths of the base transceiver stations it is receiving, and on that basis the system allocates one of the base transceiver stations to the mobile station; in order to measure the distance between the mobile station and at least one other base transceiver station, the messages are modified in order to force a handover from one base transceiver station to another. In a CDMA-type system, the "soft hand-off" feature can be used to determine the distance to two or more BTSs at the same time.

Abstract: Intelligent allocation of wireless resources can help avoid the problems of dropped calls in a mobile communications network. A service for allocating resources is designed to develop profiles of subscribers' mobility. That is, profiles are developed for routes, traveled by subscribers at certain days and times. For example, some subscribers commute to and from work at relatively fixed times. By compiling this information in the profile and tracking the movement of the subscriber, the mobile network can anticipate the subscriber's movement into a next cell. Thus, wireless resources in the next cell can be reserved prior to the anticipated handover of an ongoing call involving the subscriber. Thus, uninterrupted service can be maintained and dropped calls can be avoided.

Abstract: Efficient handoff algorithms cost-effectively enhance the capacity and Quality of Service (QoS) of cellular systems. This research presents novel approaches for the design of high performance handoff algorithms that exploit attractive features of several existing algorithms, provide adaptation to dynamic cellular environment, and allow systematic tradeoffs among different system characteristics. A comprehensive foundation of handoff and related issues of cellular communications is given. The tools of artificial intelligence utilized in this research, neural networks and fuzzy logic, are introduced. The scope of existing simulation models for macrocellular and microcellular handoff algorithms is enhanced by incorporating several important features. New simulation models suitable for performance evaluation of soft handoff algorithms and overlay handoff algorithms are developed. Four basic approaches for the development of high performance algorithms are proposed and are based on fuzzy logic, neural networks, unified handoff candidate selection, and pattern classification. The fuzzy logic based approach allows an organized tuning of the handoff parameters to provide a balanced tradeoff among different system characteristics. The neural network based approach suggests neural encoding of the fuzzy logic systems to simultaneously achieve the goals of high performance and reduced complexity. The unified candidacy based approach recommends the use of a unified handoff candidate selection criterion to select the best handoff candidate under given constraints. The pattern classification based approach exploits the capability of fuzzy logic and neural networks to obtain an efficient architecture of an adaptive handoff algorithm. New algorithms suitable for microcellular systems, overlay systems, and systems employing soft handoff are described. A basic adaptive algorithm suitable for a microcellular environment is proposed. Adaptation to traffic, interference, and mobility has been superimposed on the basic generic algorithm to develop another microcellular algorithm. An adaptive overlay handoff algorithm that allows a systematic balance among the design parameters of an overlay system is proposed. Important considerations for soft handoff are discussed, and adaptation mechanisms for new soft handoff algorithms are developed.

Abstract: In cellular communication systems overlapping coverage areas of nearby base stations provide some users with access to more than one base. This can be used to improve teletraffic performance. Mobile users who are distant from base stations are helped most by this because they are more likely to be in communication range of other nearby bases. Reuse partitioning, on the other hand, tends to be most helpful to users that are close to base stations, because they can use channels from more partitions. This paper considers the combined use of overlapping coverage and reuse partitioning so that all users gain some advantage. We develop an analytical model for such systems. Theoretical traffic performance characteristics are obtained and compared with those for fixed channel assignment schemes. Priority for handoff calls is considered. Simulation results validate the analysis.