DAS: A Review of Planning, Analysis and Design – Part 1
The technical buzz in the wireless industry today is mostly “small cells”. Perhaps one can say small cells are similar to Distributed Antenna Systems in some sense. After all, a small cell is a distributed “micro-cell” as is a “pico-cell”. I speculate that small cells will be the future because of spectrum demands; tower sites will be too high and thus a small cell “drop-in” will be appealing to the wireless carriers. The idea is for a small cell “shoebox” to contain integrated 3G/4G/Wi-Fi, but most manufacturers are still in the product development stage for that topology. This summary review is a basic design guide for DAS coverage enhancement, in a multipart series on in-building wireless enhancement.
There are numerous challenges both from a business and technical perspective when designing and implementing indoor coverage solutions. This report does not address the business case for the mobile operator but it does provide a cursory review of key planning, analysis and design milestones. The author can assist you with a basic cost structure for a DAS as well as analysis and design in order to purchase a commercial off the shelf (COTS) system.
To the end, the design is only as good as the installation because a well-designed DAS that has significant hardware impairments due to poor trade work or faulty equipment can result in costly remedies and poor network performance. Comparatively, a well-designed and high performing technical solution needs to consider future wireless carrier trends and technologies and the associated tradeoff between design and cost for this future proofing.
GENERAL PLANNING ISSUES
The interior of many office buildings is light construction with mainly drywall over metal stud with some wood construction as well as drop tile ceilings. Most if not all of the floor material is steel and concrete which attenuates wireless vertically on the order of 30 dB (shielding effectiveness). IN these types of buildings, wireless service can degrade significantly in both coverage and as more employees occupy a facility, in capacity. Moreover, as in any wireless network there is a tradeoff in coverage versus capacity and as up to 8000 employees at one corporate facility, a well-designed DAS is pertinent to high performing wireless services.
Before purchasing an in-building system, a well-structured plan must exist to ensure the DAS is both technically and economically feasible as well as designed and implemented for a successful project. Although a sales or key account manager will be the main point of contact for a firm undertaking a DAS project, it is the Carrier RF Planning team that must approve the technical solution. The firm should review the initial plans and design with each carrier independently and control the funding and DAS maintenance to assure multiple commercial wireless services (LTE, Cellular, PCS) are offered to the desired coverage area. The mobile wireless operator will require a well-structured design and implementation plan to evaluate the business case installation process for the DAS. To that end, the enterprise should be mindful of the political and cultural issues surrounding a DAS. However, the wireless carrier must approve the design and performance of the DAS according to FCC rules. The initial system design should consider the buildings’ communication and IT spaces, aesthetics constraints determined by the building owner/architect as well as construction schedules.
General Process for DAS Planning, Analysis and Design
- Planning and consultation with DAS Expert
- Site Survey and performance analysis
- Design and documentation (wireless service provider(s) approval)
- Wireless Service Provider coordination and interface requirements – ensure the DAS ‘type’ will accept the carrier’s infrastructure (i.e. BTS-to-DAS head end)
- Supervise installation – DAS components especially coax is susceptible to trade work damage
- Test of coaxial system (RL and PIM)
- Final specialty installation of parts and electronics
- Integrate and test whole system – Head end installation work should be performed by a DAS Specialist (specialty work)
- Commission service – Commissioning should only be performed by a “DAS Specialist” (i.e. experienced RF Engineer)
- Training – Firm should consider internal capabilities to operate the DAS in the event of wireless carrier collaborations.
TYPICAL LARGE OFFICE CASE:
The figure below displays the coverage plot for a typical office building at the Corporation irradiated by 750 MHz electromagnetic energy. The coverage plot is colored coded from a very “hot” signal level of -40 dBm (bright red, “5 bars”) to -60 dBm (green, “5 bars”) to -80 dBm (blue, “4 bars”) and then a weak signal, but likely useable of -100 dBm (black). Signals emerging from the building will diminish significantly due to building losses and antenna EIRP and with optimization there will be a seamless handover to the macro cellular system once a user leaves the building to the outside.
Speaking of “handovers”, the DAS system is implemented for future capacity growth needs and therefore, a sector plan by the wireless carrier is required. Well defined handover zones are critical for GSM, UMTS and LTE systems to prevent “ping-pong” or even drop calls. Moreover, handover control parameters are crucial in the DAS final layout and optimization.
Signal “path” loss (Free space path loss) is dependent on frequency and distance between the transmitting antenna and the wireless device according to:
where f is the carrier frequency in MHz of the transmitter and d is the distance (meters) from radiating antenna to the receiving device.
For example, the FSL inside a typical office building (200 feet) for an LTE signal transmitting at 750 MHz is approximately 65.6 dB. The distance to nearest base station off campus with line of sight (LOS) is approximately 5000 feet so that FSL at 750 MHz is 93.5 dB. A typical LTE base station ERP per channel is 56 dBm and therefore the signal level just outside the office building will be on the order of -37 dBm. The building wall attenuation at LTE frequencies is on the order of 20 to 30 dB. Thus the interior signal level can be -37 dBm – 20 dB – 65.6 dB = -122.6 dBm which is below the noise floor of a 4G handset.
In addition to the theoretical modeling, an on-site RF survey is required by a DAS specialist. The DAS specialist should record spectral plots for various office buildings in a frequency range that includes all wireless carriers. Signals that fall below -100 dBm are considered “weak”. However, the spectrum analyzer measurement techniques are required to accurately discern the signal levels relative to the noise background.
The fundamental goal of a Distributed Antenna System (DAS) is to distribute a uniform dominant signal inside a building using indoor antennas to provide sufficient coverage and capacity to users. Also fundamentally, in facilities with a relatively large number of users (i.e 1000 to 10,000) requires a DAS with enough capacity, although not at one time.
Among the choices for the “type” of DAS, a passive system that simply repeats the outdoor coverage will not have enough capacity for a high density facility. Alternatively a dedicated active DAS with dedicated base transceiver stations (BTS) housed on-site and feeding an optical and coaxial distribution network can provide much higher capacity and meet coverage requirements if well-designed. A Commercial DAS Scheme recommended by the author is a neutral host (firm/enterprise managed), multi-band optical distribution system with coax-fed remote antenna units (dual “MIMO” antennas).
As mentioned previously, a DAS will not perform properly if the antenna distribution system has been materially compromised. The installer is required to practice “duty of care” in loading/unloading, handling, extending, connecting and terminating coaxial (as well as fiber) cables.
Not treated here, but of potential concern in some situations and jurisdictions, is the matter of radio frequency safety. In general, power density from most DAS installations is sufficiently low as to be acceptable under most codes and standards. However, the designer should always examine and provide for these possibilities.
In summary, DAS design is more than an RF coverage assessment – it includes planning, analysis, RF spectrum background noise levels and propagation characteristics and modeling of the building. DAS antenna layout is only effective after these steps so that excellent coverage results. Finally, before choosing a coverage enhancement solution, proper analysis of the macro and micro system design is required by cooperating with the wireless carrier. The design objectives for the DAS solution should balance cost with coverage. The cost balance results in a system that provides voice and moderately high data rate services. The DAS solution should be compatible with multiple carriers (operators) with 2G, 3G and LTE services including future multiple input multiple output (MIMO) needs for LTE and AWS bands.