Performance study for a microcell hot spot embedded in CDMAmacrocell systems

The code-division multiple-access (CDMA) system can provide more capacity than other systems, and the hierarchical layer of cells is required for system design. However, the problem is whether the same radio frequency (RF) channels used in a CDMA overlayed/underlayed macrocell and microcell structure also obtain a high capacity as in the homogeneous structure. We investigate the interference of uplink and downlink from both the microcell and macrocell under a hierarchical structure. A downlink power control scheme and two power control methods for the uplink are also considered. Performance measures such as blocking probability, C/I, capacity, and service hole area are also obtained by computer simulation. Besides, some extra efforts for a microcell are also noted, such as more power need to be transmitted by a microcell base station (BS) if the same RF channels are used in the hierarchical structure. The capacities of macrocell and microcell in the overlaying/underlaying structure are limited by the uplink and downlink, respectively. With downlink power control, the microcellular capacity can be increased. However, the combination of downlink power control for the microcell and C/I uplink power control for the macrocell causes the overall system capacity to significantly increase     

Capacity analysis for underlaying CDMA microcell/macrocell systems

The CDMA system can provide more capacity than the conventional AMPS system and the hierarchical layer of cells is required for system design in the future. However, the problem is whether the same RF channels used in a CDMA underlaying macrocell/microcell structure also obtain high capacity as in the homogeneous structure. This paper investigates the interference of uplink and downlink from both the microcell and macrocell in a hierarchical structure. Downlink power control is also considered. The results show that the capacity of microcell in a hierarchical structure is 23 per cent less than in homogeneous cells. The capacity of macrocell in a hierarchical structure decreases dramatically in proportion to the number of microcells. The capacities of the microcell and macrocell are limited in downlink, and uplink, respectively. In addition, more efforts for microcell should be made, such as more power is transmitted by microcell basestation if the same RF channel is used in a hierarchical structure. The results suggest that different RF channels are used in a two‐tier cellular environment. Copyright © 2001 John Wiley & Sons, Ltd.     

Radio resource management technique for multilayered cell systemwith different bandwidths

An intelligent radio resource management technique is proposed for multi-layered cellular systems to achieve flexible and high capacity wireless communication systems. In the proposed system, terminals with lower mobility are connected to the wideband microcell systems to support a variety of services, including high bit rate data transmission; terminals with higher mobility are connected to the narrowband macrocell systems to reduce intercell hand-off frequency. To flexibly cope with temporal and spatial variations of the ratio of the microcell and macrocell terminals, system bandwidths for microcell and macrocell systems are adaptively changed to minimise the sum of the call blocking rate of both systems. Computer simulations confirm that the proposed system can improve the blocking probability under any traffic condition     

Repacking on demand for two-tier wireless local loop

This paper proposes a radio channel assignment scheme called repacking on demand (RoD) for two-tier wireless local loop (WLL) networks. A two-tier WLL overlays a macrocell with several microcells. When a new call arrives at a two-tier WLL with RoD, if no idle channel is available in both the microcell and the macrocell, repacking is performed (i.e., a call in the macrocell is moved to its corresponding microcell), and then the reclaimed macrocell channel is used to serve the new call. An analytic model is proposed to compute the call blocking probability of the two-tier WLL with repacking. This analytic model is validated against simulation experiments. We prove that the blocking probability is not affected by the call holding time distributions, but is only dependent on the mean of the call holding times. Compared with some previous proposed schemes, RoD has low blocking probability and significantly reduces repacking rate.     

Performance analysis of microcellization for supporting twomobility classes in cellular wireless networks

We study the call blocking performance obtained by microcellizing a macrocell network. Each macrocell is partitioned into microcells, and some of the channels originally allocated to the macrocell are assigned to the microlayer cells according to a reuse pattern. The arriving calls are classified as fast or slow; fast calls are always assigned only to macrocell channels, whereas for slow calls a microcell channel is first sought. Slow calls may be allowed to overflow to the macrolayer, but may be repacked to vacated microcell channels. Calls can change their mobility class during a conversation. We develop an approximate analysis for computing the slow and fast call blocking probabilities in such a system. We adopt the technique of analyzing an isolated macrocell with the Poisson arrival assumption and then iterating on the stationary analysis of the isolated macrocell to obtain stationary results for the multicell system. Simple, but accurate approximations are developed for analyzing the isolated macrocell and its associated microcells. The analyses based on the approximate isolated cell model are validated against simulations of a multicell model     

Error rate performance of noncoherent detection of duobinary coded MSK and TFM in mobile radio communication systems

Two continuous phase constant envelope modulation schemes are considered for use in digital mobile radio communication systems. These two schemes, duobinary coded minimum shift keying (MSK) and tamed frequency modulation (TFM), use partial response signaling to achieve efficient power spectrum. Therefore, they are suitable candidates for the application of digital data transmission via mobile radio where spectrum efficiency is an important consideration. The mobile communication channel is characterized by fast Rayleigh fading and cochannel interference resulting from the reuse of the channels. The error rate performance of duobinary coded MSK and TFM has been studied under these environments with noncoherent detection. A closed form expression for the probability of error of duobinary coded MSK with discriminator detection has been derived and evaluated for different cases of fast and slow fading and cochannel interference. The probability of error of duobinary coded MSK and TFM with differential detection has been calculated by numerical integrations for different cases of slow and fast fading and cochannel interference.     

Capacity analysis of macro/microcellular CDMA with power ratiocontrol and tilted antenna

The reverse link capacity is obtained analytically for macro/microcellular code-division multiple-access (CDMA) systems operating in the same frequency band. The focuses are on the ratio of required receive power of the macrocell base station (BS) to that of the microcell BS and the tilt angle of the microcell antenna to increase the system capacity. The microcell-to-macrocell interference is derived in closed form by geometric approximation, and the macrocell-to-microcell interference is calculated on the divided regions of macrocell. The optimal tilt angle is obtained by defining the minimum interference tradeoff factor that maximizes the system capacity. It is shown that the system capacity increases remarkably with power ratio control in macro/microcellular environments. Also, the properly chosen antenna tilt angle adds more capacity, and enables the microcell users to save on the transmit power     

Performance analysis of microcell/macrocell with reuse partitioning in TDMA‐based cellular systems

System capacity and grade of service (GoS) are both important for the rapid growth of cellular communication services. In this paper, we propose a two‐tier TDMA‐based cellular system with macrocell overlaid on microcell clusters by implementing fixed channel assignment (FCA) scheme and fixed reuse partitioning (FRP) scheme in microcell layer and macrocell layer, respectively, named FCA–FRP overlay scheme. Improvement can be achieved in both system capacity and GoS. Theoretical analysis based on the overlay scheme without overflow and with overflow is first presented. It shows that the simulation results are agreed with the analytical results. Then, simulation results, obtained from the overlay scheme with and without overflow, show that the performance in terms of the call blocking probability, the call dropping probability and system capacity of such a system can be greatly improved compared with a conventional one‐tier cellular system deployed with FCA or FRP scheme. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance of optimum transmitter power control in cellular radiosystems

Most cellular radio systems provide for the use of transmitter power control to reduce cochannel interference for a given channel allocation. Efficient interference management aims at achieving acceptable carrier-to-interference ratios in all active communication links in the system. Such schemes for the control of cochannel interference are investigated. The effect of adjacent channel interference is neglected. As a performance measure, the interference (outage) probability is used, i.e., the probability that a randomly chosen link is subject to excessive interference. In order to derive upper performance bounds for transmitter power control schemes, algorithms that are optimum in the sense that the interference probability is minimized are suggested. Numerical results indicate that these upper bounds exceed the performance of conventional systems by an order of magnitude regarding interference suppression and by a factor of 3 to 4 regarding the system capacity. The structure of the optimum algorithm shows that efficient power control and dynamic channel assignment algorithms are closely related     

Capacity analysis of spectrally overlaid macro/microcellular CDMA systems supporting multiple types of traffic

The reverse link capacity of a spectrally overlaid macrocell/microcell cellular code-division multiple-access system supporting various types of traffic is analyzed. Several narrowband subsystems are overlaid with a wideband subsystem in macrocells, while in a microcell, a single narrowband subsystem is operated with the same spectrum as one of the macrocell narrowband subsystems. Using a typical propagation model, the reverse link signal power and interference are characterized as the relative user signal power and the cross-tier interference factors between the macrocell and the microcell, considering various system parameters. The reverse link capacity of the overlay system is then analyzed. Results show that the dominant parameters affecting the system performance are the spectral overlay ratio and the distance between the microcell and macrocell base stations. In particular, when the distance equals half of the macrocell radius, optimum performance can be achieved by minimizing the cross-tier interference factors. These results can be applied to network planning for future wireless communication services.     

A movable safety zone scheme in urban fiber-optic microcellularsystems

We consider an urban fiber-optic microcellular system in which a cigar-shaped cell consists of several microzones with their own antenna sites connected to a central station through optical fibers. To increase the efficiency of radio resources and reduce unnecessary handoffs between microzones, we propose a movable safety zone scheme. A safety zone is a virtually guarded area that does not permit cochannel interference. Outside the safety zone, cochannels can be reused. The safety zone can move under the condition that its user does not meet cochannel interference as he moves to an adjacent microzone. Considering user mobility characteristics in the cigar-shaped cell, we analyze and evaluate the proposed system in terms of intracell and intercell handoff rates, blocking probability, intracell call-dropping probability, and channel reuse parameter. The proposed system can handle a traffic capacity of about 12 Erlangs for seven traffic channels under a call blocking probability of 1% and generates a negligible number of intracell handoffs compared with those of intercell handoffs     

A cellular mobile telephone system with load sharing-an enhancementof directed retry

A load sharing facility, which enables subscribers in a mobile telephone system to look for free radio channels in more than one cell, is investigated with respect to blocking probability and channel utilization. Results show that a substantial improvement, compared to other channel allocation techniques, can be achieved. The positive effects are especially notable for an asymmetric traffic situation. The improvement is accomplished at the expense of an increased risk of cochannel interference between mobiles     

Cochannel Interference in High-Capacity Mobile Radio Systems

The probability of cochannel interference is evaluated for a mobile radio system operating in Rayleigh fading and log-normal shadowing environment. All cochannel cells surrounding the base station are taken into consideration in this evaluation. The results obtained are used to calculate the reuse distance and the cluster size (number of cells in a group). The blocking probability, the number of channels in each cell, the protection ratio, and the standard deviation are taken as parameters in this evaluation. The results show that shadowing has a severe effect on the interference level and on the channel reuse distance ratio.     

Channel management in microcell/macrocell cellular radio systems

In this paper, we study spectrum management in a two-tier microcell/macrocell cellular system. Two issues are studied: micro-macro cell selection and frequency spectrum partitioning between microcells and macrocells. To keep the handoff rate in a two-tier cellular system at an acceptable level, low mobility users (with speed υ0 ) should undergo handoffs at microcell boundaries, and high mobility users (with speed υ>V₀) should undergo handoffs at macrocell boundaries. The mobile determines user mobility from microcell sojourn times and uses it for channel assignment at call origination and handoff. The probability of erroneous assignment of a mobile to a microcell or macrocell is shown to be significantly lower than previous approaches. We investigate the optimal velocity threshold, V₀, and propose that it may be dynamically adjusted according to traffic load. Finally, we propose a systematic way for finding an optimal partition of frequency spectrum between microcells and macrocells. This partitioning is based on the traffic load and velocity distribution of mobiles in the system     

Improved cochannel interference cancellation for MFSK/FH-SSMAsystems

There has been much interest in the application of frequency hopping-spread spectrum multiple access (FH-SSMA) to wireless systems because of its frequency diversity and resistance to the near-far problem. In FH-SSMA systems, cochannel interference (CCI) is the dominant factor limiting the user capacity. Various techniques used to resolve the CCI problem have been extensively investigated. We propose several multiuser detection schemes based on canceling CCI to increase user capacity in a multilevel frequency shift keying (MFSK)/FH-SSMA system. The performance of systems with the proposed interference cancellers over a Rayleigh fading channel are analyzed and compared with those of the conventional and other interference cancellers. Our results show that the proposed schemes have the best performance in both nonfading and fading channels     

High reuse efficiency of radio resources in urban microcellularsystems

A new microcell configuration scheme in urban areas is proposed in order to increase the reuse efficiency of radio resources. Multiple directional-beam antennas lined up along a street form a cigar-shaped cell. Antennas are located at intersection and are directed along the intersecting streets. Two antennas facing each other form a microzone and, therefore, a cigar-shaped cell consists of multiple microzones in a line. Each microzone is protected against cochannel interference from neighboring microzones by confinement of transmitted signals to a microzone using down-tilting of antenna beams. Therefore, a radio channel which is occupied in one microzone can be used in adjacent microzones. An increase in system capacity is achieved by the high reuse efficiency of radio resources. The proposed system increases system capacity up to 560% with a call blocking probability of 1% when compared with a conventional system. In addition, a macrodiversity scheme using two antennas in a microzone improves the call quality on the reverse link     

Ordered dynamic channel assignment scheme with reassignment inhighway microcells

An ordered dynamic channel assignment with reassignment (ODCAR) scheme is proposed, and its performance is studied in a highway microcellular radio environment. Channels are assigned in an ordered basis in conjunction with a minimax algorithm under cochannel interference constraints, to provide high capacity and to alleviate worst case channel congestion in each microcell. Simulation results show significant performance improvements in terms of channel utilization and probability of call failure, at the expense of an increase in complexity and call switching requirements     

Analytical methods to calculate the performance of a cellularmobile radio communication system with hybrid channel assignment

Two approximate techniques are presented to evaluate the performance of large-scale mobile radio systems using a hybrid channel assignment scheme and a cellular structure, The two approximate analyses give the steady-state probability distributions of the system which are used to obtain expressions for the blocking probabilities. In the first method, the blocking probability is obtained by finding the interarrival time probability distribution function of one composite interrupted Poisson process (IPP) stream consisting of several IPP streams overflowing from the cell of interest and its cochannel interference cells. The second method is proposed to solve the blocking probability of the system by regarding each call as a GI/M/m (m) model. Analytical results are compared with simulation results, and good agreement is observed for both channel assignments (hybrid and fixed). The methods presented are applicable to the design of hybrid channel assignment schemes and dynamic channel assignment schemes     

Effects of correlated shadowing signals on channel reuse in mobileradio systems

The authors present a study to evaluate the cochannel interference probability for the desired and interference signals which are correlated due to shadowing. The effects of correlation on the normalized reuse distance are investigated. A generalized expression for the cochannel interference probability is derived by combining uncorrelated (fast) Rayleigh fading and correlated (slow) log-normal signals. The cochannel interference probability for sectorized cell layouts is compared with the lower bound of cochannel interference probability for omnidirectional antenna systems. It is found that omnidirectional cell layouts cause the highest level of cochannel interference probability. An omnidirectional system requires a higher cluster size than a directional system to maintain acceptable cochannel interference