UMTS and HSDPA are the preferred services in many markets. However, challenges with interference and isolation among the RF cells are reducing bandwidth and coverage in dense areas and inside buildings.
This article proposes a supplemental UMTS/ HSDPA network architecture that cost-effectively improves service by increasing isolation to reduce interference, while increasing data speed bandwidth and coverage. The architecture improves network efficiency while reducing mobile operator production costs.
UMTS and HSDPA are noise sensitive systems. The data speed delivered is directly related to SNR. Every cell in a UMTS/HSDPA network use the same frequency, and signals from different cells overlap, collide and interfere with one another.
This interference is due to a lack of cell isolation. This phenomenon reduces data speeds and decreases network capacity since it causes inter-cell interference in the overlapping areas (Figure 1 below ).
|Figure 1: Shown is the frequency overlap in traditional UMTS/HSDPA networks.|
The overlap issue impacts mobile operators' ability to deliver service in urban core areas and inside buildings, where broadband users rely on the service most.
Subscribers opt for UMTS/HSDPA to gain the benefits of high-speed broadband. Unfortunately, the lack of isolation limits network bandwidth in densely populated areas.
Signals at UMTS/HSDPA frequencies attenuate quickly due to their high frequency, making it a challenge to penetrate buildings. In fact, buildings present several key challenges in terms of coverage from traditional macro networks:
High penetration losses . As signals pass through building walls, they attenuate very quickly, making it nearly impossible to provide adequate service within the interior areas of many buildings (Figure 2 below ).
|Figure 2: Signal attenuation limits penetration of macro network signals inside buildings.|
High power load per user . Due to poor signal quality, user devices must operate at maximum power to maximize connectivity, thereby reducing mobile battery life.
A drain in overall UMTS network Capacity . Dense user-communities inside buildings exhaust much of the macro network's power and capacity for a given area, limiting the ability of mobile operators to serve others in the cell site vicinity.
Lack of single cell dominance and large soft handover (SHO) zones at the edges of cells . Most urban areas are covered by more than one UMTS cell, so that user devices hunt from cell to cell.
This limits HSDPA performance, degrades network capacity and limits the business case for mobile operators. When covering indoor users from the macro layer, user devices may “see” more than one serving cells inside a building (Figure 3 below ).
|Figure 3: Two adjacent cells are providing high signal levels inside the building, but the lack of isolation will give relatively slow data service.|
As a result, user connection speeds won't likely exceed 360Kbit/s, despite a strong ambient signal level. The most effective solution for in-building coverage is an indoor distributed antenna system (DAS). An in-building DAS establishes one signal source that is far stronger than any coming from the macro network, and it can cover every area of a building (including underground facilities) with equal signal strength.
However, the cost of outfitting every building with an in-building DAS can be prohibitive. This is why service providers are exploring the use of micro cells, which can penetrate buildings if they are in close proximity.
However, while micro cells raise the signal used to cover outdoor or in-building areas, they don't solve the isolation problem because of overlapping areas between cells.
In the overlapping areas, the data speed on HSDPA is reduced, and UMTS mobiles will load the network with soft handover (SHO) taking up resources in more cells for the same call. The key culprits here are SHO loading, which cannibalizes UMTS capacity, and SNR degradation, which reduces data speeds.
The key to using micro cells without interference issues is to improve isolation between the cells. By limiting the area in which cells overlap and providing maximum isolation between cells, we significantly improve SNR, minimize interference in the network, maximize network utilization, and improve data performance on both uplink and downlink paths.
These problems can be solved with better RF network design and with remote radio heads that support simulcast and adjustable digital delay features. To isolate cells, mount the RF nodes back-to-back on the same mast (Figure 4 below ). This creates a minimum of 50dB isolation between cells—far more than the 7dB found in traditional macro deployments.
|Figure 4: To isolate cells, mount the RF nodes back-to-back on the same mast.|
With the isolation provided by this back-to-back antenna configuration, it is possible to deliver near 100 percent HSDPA coverage using the following advanced performance features:
Simulcast . Broadcasting simultaneous signals through each of the back-to-back antennas, offset in time;
Adjustable delay . Adding one chip of delay to one of the back-to-back antennas;
Rake receiver in the mobile and base station . A rake receiver that can receive as many as three signals from the same source, as long as there is more than one chip delay between signals.
By deploying cells facing inward and simulcasting, mobile operators can significantly improve outdoor and indoor coverage with minimal SHO load, maximum HSDPA performance (increasing it from approximately 15 percent utilization to 98 percent utilization), higher capacity and higher revenues. The impact on in-building deployments is shown in Figure 5 below .
|Figure 5: Traditional cellular architecture degrades due to distance and shadowing.|
Not all remote RF nodes can support the architecture outlined above. RF nodes with the following features offer several advantages for mobile operators:
Adjustable digital delay to individual remotes;
Digital distribution to eliminate signal degradation over distance and up to 26dB of loss;
Small footprint means limited hardware impact on-site for easy zoning and fast deployment;
Remote antennas support centralized base station hotels, which reduce CAPEX and OPEX;
High-quality signals for voice QoS and high data speed;
Flexible topology: SMF star or daisy chain;
Ability to use MMW radio links when fiber is difficult or impossible;
Support for GSM, DCS, UMTS, HSPA, WiMAX and LTE technology;
BTS interface supporting digitized RF and OBSAI/CPRI.
Wireless service providers must find ways to optimize high performance UMTS/HSDPA broadband service to fully maximize their investments and deliver the QoS that users demand.
Success will require new architecture thinking that goes beyond traditional macro cell and micro cell coverage. This architecture using remote RF nodes provides high isolation between cells, limits cell overlap, delivers the highest data rates and optimizes network utilization to deliver best-in-class performance.
Tony Le Febvre is director of Product Management, Outdoor Wireless Products, and Morten Tolstrup is Technical Director at ADC Network Solutions.