Taking Aim At Costly “Dead Zone” Problems

Feb. 26, 2003
Deployment Strategies For In-building Distributed Wireless Systems
Comsearch, headquartered in Ashburn, Virginia, with offices in the European and Asia Pacific regions, provides total solution global offerings for wireless engineering design, acquisition, installation, and maintenance services. Over a quarter century of experience in developing solutions for mobile and broadband wireless applications, Comsearch has expanded its expertise into distributed wireless In-building design, real estate telecommunications services and private land mobile radio. Its experienced engineers, software products and information databases have addressed the specific challenges of network design and spectrum management for the evolving global wireless industry. Copyright © 2002 Comsearch. All rights reserved. You may not photocopy, fax, electronically transmit, download to a database, or otherwise copy, reproduce, distribute, disseminate, transfer, sell, publish, send or circulate this article, or any portion thereof, to any person without the express written permission of Comsearch.Taking Aim at Costly “Dead Zone” Problems In today’s wireless environment, it is common for wireless users to experience a dropped signal while in the middle of an important phone call. Attempting to make calls while in shopping malls, airports, tunnels, or other public structures, users often find their calls will not go through due to lack of coverage in a particular area. Wireless carriers and service providers know that dropped calls mean lost revenue, and that “dead zones” can be deadly to their business. The solution to this problem is to eliminate costly dead zone problems and ensure constant coverage for your wireless users. That’s where a distributed wireless antenna system comes in to play. In-building Distributed Wireless Systems (DWS) While Distributed Wireless Antenna Systems (DWS) can be more challenging to deploy than a Macro system, they are a great way to provide additional coverage and capacity to your subscriber base while at the same time helping to eliminate lost subscriber-based revenue due to wireless network “dead-zones.” Although dropped calls are the main driving force behind the need for the installation of a Distributed Wireless System (DWS), an additional incentive for installation is that these systems are not only cost effective, but also extremely successful at eliminating dropped calls and dead zones. Given that, it is important to note that the implementation procedures are more stringent when deploying a DWS than with a Macro-site design. This is due in part to various construction issues involved with deploying an in-building DWS. However, the benefits to carriers, service providers, and more importantly, customers, are immense, and far outweigh the stringent implementation procedures required. For a high caliber in-building DWS to be successful, several key issues must be addressed and specific tasks must be completed prior to deployment. For example, antenna sites used in the DWS must seamlessly integrate into the current Macro system. In addition, interference and capacity direction must also be addressed in order to ensure a successful DWS is deployed. Both of these key elements are often times overlooked when optimizing these systems. DWS and Integration with TDMA and CDMA based Systems Most Distributed Wireless Systems are deployed to serve a need based on shortcomings in the existing Macro system. These shortcomings are based on key system problems including coverage dead zones, system interference, and capacity limitation. Coverage dead zones are selfexplanatory. There simply is not enough RF energy to meet your system’s link budget in the intended service area. Interference concerns, however, are a bit more involved. In a TDMA based system, interference can stem from adjacent channel or co-channel interference. It is usually the latter, and is very similar to interference problems that RF engineers face when working on networks on a Macro-level. In a CDMA based system, interference within a DWS is usually based on lack of a dominant pilot (PN) serving the coverage area, which often is due to multiple PNs (or sectors) serving the same area. This is called pilot pollution, and is often faced by RF engineers on the Macro-level. Additionally, capacity issues can be a major obstacle that must be overcome, especially in areas that are highly congested, such as airports, convention centers, and shopping malls. Traditionally, carriers have felt that constructing additional sites in close proximity to the desired coverage area can easily provide a quick fix for these types of dead zones. However, while these sites do dramatically increase the coverage and capacity to these areas, the carrier cannot guarantee there will be coverage/capacity in the depths of building structures. Carriers are finding that the Macrosite approach is also more costly and takes a significant amount of time as zoning issues alone can stall Macro site development for years. Expanding coverage to areas where it is non-existent today in order to decrease subscriber churn rate is another underlying factor for installing a DWS. Deployment of a DWS provides added coverage and capacity to areas of poor coverage via the DWS infrastructure. While the infrastructure design is similar between vendors (i.e., Mikom, LGC Wireless, Foxcomm, etc.), there are two primary methods that provide the carrier’s signal into the building or other structure. Deployment Methodology In the first method, a repeater is used for the front end of the system. This is because a repeater is less costly than a full Base Transmission System (BTS) and can be easily placed into/onto the building. After the first fiber hub is installed, the rest of the infrastructure is the same, as seen from the repeater or BTS. However, a repeater cannot provide carriers with the additional capacity needed in the building. This is where the second method comes into play that involves using a Base Transmission System as the front end to the DWS. While this is not the most cost-effective method, it is the most robust solution for deploying a DWS. Using a BTS provides the flexibility to allocate capacity as needed versus taking it from another site. This method also allows engineers to conduct frequency/PN planning on a cluster basis with the rest of the Macro system, allowing the DWS to be seen as its own separate and independent site within the carrier’s system. Additionally, lower-cost microcell BTSs will soon be available from various vendors that will make the equipment footprint area more manageable. It is clear that Distributed Wireless Systems provide a major advantage in meeting in-building coverage goals. However, implementing these systems can often be intimidating to the inexperienced engineer. There are pitfalls that an experienced RF engineer may fall into due in part to their lack of experience with these types of systems. When implementing a successful DWS, strict attention must be given to the following key areas: a) Receiving existing building drawings prior to the initial walk through of the building b) Access to perform system testing prior to system design c) Performing optimization/integration testing after the DWS is deployed Taking Aim at Costly “Dead Zone” Problems In order for the site acquisition specialist and the RF engineer to fully understand the “lay of the land” prior to arriving at the building or structure, it is crucial that they have access to sitedrawings prior to arriving at the building or structure. All too often problems such as antenna layout, building layout, or equipment room locations are encountered at the site and can be eliminated when preliminary studies of the site drawings are performed prior to arriving at the building. This enables a speedy site walk, and more importantly, enables the RF engineer to decide ahead of time where to perform the path loss measurements and existing signal strength measurements. Another potential roadblock that may be encountered during the design phase of the System is gaining access to the building or structure in order to conduct system testing of the coverage area. Proper access to the building or site cannot be stressed enough as the determination of antenna locations must be decided upon prior to the actual testing and validated by doing a site-walk through the building or structure. Access to antenna and equipment locations so that the RF Engineer, site acquisition specialist, and construction manager can validate these locations and determine their viability is crucial to the overall success of the system. There are numerous instances where assumptions are made in equipment location or antenna cable/fiber runs only to have them fail from a constructability standpoint. Having access to the building helps all parties determine if the design assumptions and ideas can be implemented. Part of the validation process for the design is the actual testing of these locations by performing pathloss test measurements. These tests help the RF engineer determine the RF environment of the building or structure. Part of this validation process should include testing of the existing signal level within the building or structure. This is a crucial step that must be performed to ensure that antenna locations can be verified and logically placed in order to overcome strong signals that may penetrate the system from outside sources. Overcoming the outside system must be achieved if you use a dedicated BTS to ensure the capacity for the building or structure is being provided by the DWS system. Once the design is complete, and the system has been constructed, the next crucial step involves testing of the actual DWS to ensure that the System integrates seamlessly into the Macro-system. A common pitfall encountered in testing Distributed Wireless Systems is to simply walk around and monitor mobile phone receive power levels. While this may be acceptable at ground level, it becomes critically misleading on higher floors or at higher elevations on the structure where penetration from the outside Macro-system increases to higher levels. On the higher floors of a building or at higher levels on a structure, it is important to measure DWS interference from the outside system and to properly test the interference from the DWS. However, experience has shown that due to their relatively low power levels, a DWS rarely interferes with the existing Macro system. A critical element that must be addressed for proper operation of the System is to take the necessary steps to make sure that the Macro system does not interfere into the DWS. This can negatively impact the DWS in two ways. First, it can create significant downlink interference into your system, and not enable the DWS to carry the traffic of the building, which is one of its primary coverage requirements. Secondly, if the measurements of the existing signal levels are performed completely and accurately, then the antennas should be placed in optimal positions to overcome outside interference. Finally, the last step in the process is to monitor performance statistics to verify that the capacity is distributed where it is most needed. Fortunately these systems are so adaptive that capacity can be re-allocated as necessary. Summary The benefits you will experience when deploying a DWS system are easily realized. You stay competitive and provide your customers with what they want—a dominant, capacity-plentiful, wireless service—anytime, anywhere. In addition, you will expand coverage to areas where it is nonexistent today while at the same time increase the capacity of your network. However, there are pitfalls that need to be addressed to ensure these systems are designed and executed quickly, efficiently, and cost effectively. Taking the time to address these issues when deploying a DWS will prove very beneficial to you and your customers. Help eliminate lost subscriber-based revenue, reduce your churn rate, and take dead aim at your wireless network “dead zone” problems by deploying a Distributed Wireless antenna system today.

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