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Microcell Hot Spot and Smart Antennas Evaluation in WCDMA Macrocell System
Carlos H. M. Lima, Francisco R. P. Cavalcanti, Vicente A. de Sousa, Emanuel B. Rodrigues
TL;DR
The paper asks how microcell and smart-antenna strategies can manage hot-spot traffic in a WCDMA macrocell system. It uses system-level simulations to compare these alternatives and their combination through SIR performance. The results identify different best-performing choices depending on whether overall area performance or user location and serving antenna are emphasized.
Problem
The study addresses how to handle additional hot-spot traffic in a WCDMA macrocellular system and determine which antenna strategy suits each scenario.
Method
System-level reverse-link simulations compare a nearby microcell antenna, macrocell smart antennas, and their combined deployment using SIR evaluations.
Results
The macrocell-only spatial matched filter with an 8-element array gives the best overall target-sector performance, while combined HCS MF8 is best for macrocell-connected users when location and connection are distinguished.
Takeaways & Limitations
The MAS FB alternative is identified as the most suitable solution when complexity and performance trade-offs are considered.
Abstract
from arXiv · showhide
The purpose of this contribution is to analyze a hierarchical cell structure with a hot spot embedded in a WCDMA macrocellular system. Particular attention is paid to the impact of introducing smart antennas in this context. A system-level simulation approach is employed, in which we consider two main solutions to address hot-spot traffic: the placement of a microcell antenna or the adoption of smart antenna technology at the macrocell. Relative gains in signal-to-interference ratio (SIR) are then evaluated. Distinct scenarios are defined based on the proportion of users of different service classes. An evaluation of which solution is best suited to each scenario is then conducted.
I. INTRODUCTION
The paper studies hot-spot traffic in a WCDMA macrocell system using system-level reverse-link simulations. It compares microcell, macrocell smart-antenna, and combined strategies across service settings.
- The study examines a macrocell cluster containing a hot spot within a WCDMA system.
- Two primary approaches address hot-spot traffic: placing a nearby microcell antenna or using smart antennas at the closest macrocell site.
- A combined microcell and macrocell smart-antenna strategy is also evaluated for possible synergistic effects.
- The simulator models multiple three-sector macrocells, one microcell sector covering the hot spot, and shared-band uplink interference from two macrocell layers.
- The analysis uses speech and data services with uniformly distributed users and increased density in the hot-spot area.
B. The Propagation Model
The propagation model combines distance-dependent path loss, correlated shadowing, antenna gains, power control, and uplink interference measurements. Smart antennas modify the antenna-gain term for users connected to the target macrocell.
- Received power follows path loss proportional to distance^-n with spatially correlated log-normal shadowing, while fast fading is averaged through diversity.
- Macrocell and microcell links use distinct propagation conditions: n=2 in the microcell LOS switching area and n=4 outside it.
- The transmitted power expression incorporates required base-station received power, reference antenna gain, separation distance, path-loss exponent, and shadowing.
- The simulator uses signal-strength criteria for admission and power control, with blocking when required transmission power exceeds a threshold based on the highest-power service class.
- Interference is measured at the central target sector after power control, including contributions from mobiles connected to other base stations.
- Smart-antenna deployment modifies the interference model by replacing the reference antenna gain with a smart-antenna gain for the target-sector link.
C. Traffic Model and Power Allocation
The traffic and power-allocation model represents speech and high-data-rate users, assigns class-dependent power according to QoS and activity, and establishes the microcell boundary at equal required transmission power.
- The two service classes are speech at 15 Kbps and HDR data at 144 Kbps, modeled with variable processing gain for multirate transmission.
- The simulator independently models user activity at each base station through an on-off process.
- The QoS measure is SIR expressed as Eb/Io, with required values of 5 dB for SPC users and 7 dB for HDR users.
- Power allocation between service classes depends on processing gain, required SIR, and activity factor to preserve prescribed service quality.
- The microcell boundary is defined where macrocell and microcell users require equal transmission power, using average path loss and the geometry parameters D and r.
D. Definition of Test Scenarios
Two test scenarios vary service composition while distributing users across the cellular network. The speech scenario contains only SPC users, whereas the urban scenario mixes SPC and HDR users.
- The speech scenario contains only SPC users uniformly distributed throughout the cellular network.
- The urban scenario contains 70% SPC users and 30% HDR users uniformly distributed across the system.
- The scenarios concentrate the same amount of users in a macrocell.
III. ANTENNA STRATEGIES
The paper evaluates microcell and smart-antenna strategies for alleviating hot-spot traffic in a WCDMA macrocell system. Smart antennas use either switched fixed beams or spatial matched filtering.
- Microcell antennas and smart antennas are the two strategies assessed for alleviating Hot Spot traffic.
- The microcell antenna is positioned on the microcell site border at distance D from the central macrocell base station.
- Two smart-antenna algorithms are considered: switched fixed beam and spatial matched filter.
- Switched fixed beams select among pre-established narrow beams, but gain decreases when users lie between beams.
- Spatial matched filtering steers a beam toward the desired user's direction without accounting for interferer positions or performing null-beamforming.
IV. RESULTS FOR MICROCELL ANTENNA IN HCS
The HCS simulations compare microcell performance across service mixes, hot-spot positions, and user densities. Microcell deployment generally improves SIR, but benefits depend on location and traffic conditions.
- The system performance metric is the postdespreading SIR of users inside the Target Sector.
- The microcell required power is 10.10 dB higher than the macrocell’s.
- Approximately 8 dB of SIR 10th percentile degradation occurs when the load is 15 users per sector and the scenario changes from speech to urban.
- Microcell deployment gives users inside the Hot Spot higher gains through higher received power, while users outside benefit from reduced interference.
- At D = 2R, HDR users near the macrocell sector border achieve better link quality through the microcell than through the macrocell.
- Higher microcell-to-macrocell user superficial density lowers the SIR 10th percentile relative gain between HDR and SPC users.
ANTENNAS STRATEGIES AT MACROCELL SITE
Macrocell smart-antenna performance is evaluated with arrays of four and eight elements, focusing on interference reduction. In urban HDR conditions, smart antennas substantially improve overall SIR performance.
- The smart-antenna evaluation uses macrocell arrays with 4 and 8 elements and explores their interference-reduction property.
- Overall HDR SIR performance increases considerably after smart-antenna approaches are incorporated in the urban scenario.
- The MF alternative with 8 antenna elements provides the best performance gains among the evaluated antenna alternatives.
MACROCELL SMART ANTENNAS STRATEGIES
The paper assesses a combined microcell and macrocell smart-antenna strategy. The combination redistributes performance: macrocell-connected users gain SIR, while microcell-connected users experience degradation.
- The combined alternative joins microcell and macrocell smart-antenna strategies to assess their system behavior together.
- In the urban HDR target sector, combining both solutions degrades users connected to the microcell antenna compared with macrocell smart antennas alone.
- The combined solution increases SIR for users connected to the macrocell antenna relative to smart antennas at the macrocell alone.
- Macrocell-only smart antennas provide SIR gains for users outside the Hot Spot in both speech and urban scenarios.
- Users inside the Hot Spot are not influenced by added smart-antenna functionality in the combined alternative.
VII. SUMMARY OF RESULTS
The study compares microcell, macrocell smart-antenna, and combined strategies for hot-spot traffic in hierarchical WCDMA systems. Results vary with hot-spot density, user location, service demand, and antenna configuration, with MAS MF8 and MAS FB offering distinct advantages.
- SIR increase allows more users or higher-data-rate services when interference remains below the specified threshold.
- The combined microcell and macrocell smart-antenna solution did not provide the best overall performance because microcell-connected users lacked macrocell interference cancellation.
- MAS MF8 achieved maximum SIR for low and moderate hot-spot user densities, USD = 100 and 200.
- MAS FB was the most flexible architecture, satisfying minimum QoS requirements across varied system-demand conditions.
- MAS FB8 outperformed MAS MF8 at USD = 300 with 5 users/sector when the hot spot lay between two fixed-beam intersections.MAS MF8 lacked sufficient spatial resolution for this difficult hot-spot placement, whereas the advantage disappeared when the hot spot aligned with a fixed-beam main lobe.
- The MAS FB alternative offered the most suitable complexity–performance trade-off for the hot-spot problem.
VIII. CONCLUSIONS
The conclusions show that hot-spot placement and smart-antenna sophistication materially affect SIR and system performance. Macrocell-only smart antennas help users outside the hot spot, while combined deployment redistributes benefits between macrocell- and microcell-connected users.
- Hot spots near the macrocell sector border produced better performance and higher capacity overall.Without a microcell antenna, moving from D = R to D = 2R improved the SIR 10th percentile by approximately 1 dB at 15 users/sector.
- Macrocell-only smart antennas produced SIR gains for users outside the hot spot.
- Combined smart-antenna and microcell deployment degraded microcell-connected users’ overall performance while increasing macrocell-connected users’ SIR relative to macrocell-only smart antennas.
- More sophisticated antenna strategies improved system performance in normal conditions, but their selection requires a complexity–implementation-cost trade-off.