Distributed Antenna Systems – No Replacement for Wireless Strategy


I received the following blog post from Stephen Olsen, Principal at Integra Systems. Steve has spent more than 20 years in the wireless industry in engineering, sales and business development. Steve’s wireless experience extends beyond health care to include public safety, cellular and 802.11.

In the past I’ve extended an invitation to a few select industry experts and thought leaders to post their writing. Steve is the first to take me up on my offer. Enjoy:

Over the last few years, MobileAccess and InnerWireless have generated considerable interest in broadband Distributed Antenna Systems (DAS) for the healthcare market. These systems can support a wide range of applications (WiFi, cell phones, mobile radios, pagers, WMTS) and frequency ranges (400/800 MHz up to 6 GHz).

The appeal to providers is the idea that a broadband DAS will remove all wireless headaches: no more cell phone complaints, WiFi will work better, no more dead spots for mobile radios, no more tricky RF interference problems, etc. Disappointment ensues when the DAS does not live up to its promise.

The fundamental problem is that expectations are not realistic. Broadband DASs supports some applications better than others. Namely, they work very effectively for cellular like services, but can have cost and scalability challenges with others, namely WiFi and WMTS. In some extreme cases, the equipment was barely installed when the healthcare provider realized that they wanted 802.11a, but the DAS as implemented did not support it; hardly scalable or future proof.

The better way is for the provider to figure out their wireless needs, craft a strategy, and determine how a DAS may or may not be able to help. Weigh the pros and cons, challenge the vendors, etc. If nothing else, you’ll know what to expect from the DAS in the future. I don’t think that there are any shortcuts here. You may find that a narrowband DAS approach from a company like ADC/LGC or Commscope/Andrew meets your needs less expensively.

WiFi and DAS
The primary benefit of running WiFi on a DAS is that the APs can be stored in wiring closets, where they are much easier to maintain than above the ceiling, and secure in a locked room. This is especially relevant in areas of infectious control and hard lid ceilings. AP vendors tend to downplay this benefit, but I have yet to meet an IT person who does not love this feature.

However, this benefit comes at a cost. Antenna density will be higher than that of traditional WiFi installations due to loss of antenna diversity, and RF signal losses over coaxial cable. Coax losses are especially prevalent at 802.11a frequencies (6.7 db/100 ft at 6 GHz/802.11a and 4.2 db/100ft at 2.4 GHz/802.11b/g). Vendors may rectify this by adding amplifiers in the plenum, but that at least partially defeats the purpose of having APs in wiring closets in the first place, since you still have electronics to maintain in the ceiling.

It’s essential to consider future technology directions, how will MIMO be supported for 802.11n given that there’s only one antenna element, and MIMO requires at least two? Although the provider should legitimately question the need for 802.11n’s capacity given the light load on existing a/b/g networks, one should proceed cautiously when differing from industry standard approaches. Be especially diligent when the technology is evolving very rapidly like that of WiFi.

A broadband DAS is bound to limit WiFi flexibility and scalability due to the nature of coaxial cable, which is more expensive, more difficult to run, and more difficult to terminate. From a cost, complexity, and time perspective, moving antennas on a coax based DAS system is an order of magnitude higher than that for standard WiFi implementations, where one is just moving a CAT5 drop. For example, some providers have had to redesign their WiFi networks in order to support RFLS more effectively by moving antennas out to the perimeter (patient rooms instead of just hallways). This is an expensive change with CAT5, but the costs could be astronomical with a coax based system.

If after thorough analysis, running WiFi on a DAS is the right solution, the provider should select a DAS technology with one antenna per AP. That way they’ll still benefit from more advanced WiFi functionality such a location services, radio resource management and rogue detection.

DAS and Cellular
Since DASs originated in the cellular industry, it’s no surprise that this is where they’re most effective. Standards are relatively stable (especially for cellular voice) and the RF design methodologies are pretty well known. Phones work very well with a -85dBm design goal (as opposed to -67 dBm for WiFi), and RF designs tend to be simpler than that of WiFi due to the simulcast nature of cellular RF on DAS.

Some care should be taken specifying your needs on the cellular data side where design goals may need to be more stringent for higher data throughput, and Carriers are acquiring new spectrum (Advanced Wireless Services in 1.7/2.1 GHz, the 700 MHz spectrum auction presently underway, etc.). Such care will better prepare your DAS for supporting future cellular technologies.

Providers should definitely engage the Carriers before procuring a DAS. Although these conversations can be difficult, the Carriers may be a source of funds, and they need to approve the use of equipment needed to drive the DAS (Base station or bi-directional amplifier). A DAS is really just a very large and sophisticated antenna that still requires a radio at the head end to drive it. The provider may need to budget for additional funds for radios for the traffic to actually run over the DAS in the form of base stations (BTS), bi-directional amplifiers (BDA), or Fiber-Fed Amplifiers (FFA). Nobody wants to spend $2.50 per square foot on a DAS only to find out that WiFi is the only application that works without additional funding.

Narrowband DAS
Occasionally broadband DAS providers disparage narrowband DAS providers saying that they can only implement a narrow range of applications. But the point is, narrowband DAS vendors ADC and Commscope implement the cellular like services very well and often more cost effectively. They must be doing something properly, since Commscope and ADC are the two largest DAS vendors by market share.

And although this is not their primary marketing message, MobileAccess has a solution here as well; a MobileAccess DAS in a ‘narrowband’ version will do an excellent job of cellular services and be substantially cheaper without WiFi or WMTS enabled.

Broadband systems do provide a framework for mitigating (not eliminating) interference related issues. With narrowband systems, the Healthcare provider will need to consider interference issues. However with the money saved, one will be able to afford to hire an outside expert to help troubleshoot and monitor interference problems.

So a perfectly valid and economical solution is a narrowband DAS for cellular services along with a standard discrete WiFi network. WiFi is a given in just about any hospital. What I like about narrowband DAS, is that although it does not cover as broad a range of services, it does address the cellular problem very well. And in my mind, the biggest impediment to having more DASs fix cellular coverage in hospitals is the cost.

What to Do
The first step is to inventory existing wireless applications. It can be difficult if there are many old undocumented systems, but you really should know everything that’s running in your hospital. The added benefit is this information will certainly help a professional helping troubleshoot interference issues.

The next step is a good old fashioned needs assessment and planning session. What applications and frequencies to you want to run in your hospital and why (WiFi, Cellular, Paging, VOIP, WMTS, RFLS, etc.) today and for the foreseeable future. Perhaps almost as important, which applications would you eliminate given insufficient funds. In other words, prioritize those applications while weighing their benefits. Education on what’s available is essential here also.

Readers can post questions and comments here. If you’d like to contact Steve directly, you may email him here.

Pictured above is an antenna element from MobileAccess shot at a past HIMSS.



  1. Alfred C Greiner

    Good stuff. Over here in the UAE most of the buildings have a DAS to support the GSM service providers. There are many new projects in the Dubai Medical City which could benefit from the information you have to offer.



  2. Excellent information. I am just finishing up a 3 year evaluation on what DAS system to use for our Hospital System to increase cellular coverage. We have several hospitals in the South Florida area that already have discrete WIFI networks throughout. I evaluated ADC/LGC, Innerwireless, Mobile Access, and PowerWave. We also looked at Off Air Repeaters versus base stations. The Off Air Repeaters were a lot less expensive than dedicated base stations. One repeater can handle 2G, 2.5G and 3G wireless where as we would need a separate base station for each type of wireless service from each cellular carrier. The reason we looked at base stations is because of the potential for Hurricanes to disrupt communications on cell towers.

    I basically came to the same conclusion you did. The concern for new wireless technology such as 802.11n, VoIP, and location based services may or may not work or scale on a broadband DAS solution.

    What do you think of a broadband DAS solution in a green field opportunity? We are planning the construction of a new hospital in two years. Would you still lean toward separate wireless networks for WiFi, Cellular, WMTS, Emergency Responders, etc…?

  3. Has anyone done an RFI test to see what a DAS would do to other hospital equipment?

    What if you had a day with a heavy load of cell phone traffic?

    My first thought is that there wouldn’t be any RFI issues, but you know what happens when you assume…

  4. Mark,

    Having the DAS in place actually greatly reduces potential interference with other devices in an indoor environment. All cellular devices vary their transmit power back to the outdoor BTS (base-station) based upon the power level being received at the BTS from the user. With no DAS in place the BTS is receiving a weak signal from the cellular device and therefore instructs the cellular device to keep raising it’s transmitter power until it can “hear” the phone well enough. In many non-DAS situations (e.g. – coverage is poor enough to warrant a DAS in the first place) this will cause the cellular device to increase it’s transmit power to high levels if not at maximum limits. If intereference issues are possible (i.e. – a particular hospital device happens to be suceptible to that particular cellular device) it is at these higher transmit powers that it has a much greater chance of occuring. With a DAS in place the phone will lower it’s transmit power output. Many times the difference is orders of magnitude (instead of 100mW it is transmitting at 10mW, 1mW or even less). If you wish to avoid potential interference make sure a DAS is in place.

    Kind Regards,
    Keith Ebel
    Proximity Wireless, Inc.

  5. Stefan Lavriv

    Hello there,
    I am RF Engineer, worked for many Carriers and DAS providers in Canada, USA, Australia and New Zealand.
    I’m designing DAS systems since 1998 and main issue missed here is power/Hz. All DAS Providers have that defined as Total Composite Power from remote unit. That is major limit for all designers and providers. As you add more channels, services, technologies, you affect everyone else. DAS Designer don’t think about future since they have task to make it as cheaper as possible, that exclude or limit any future planning. I worked with medical facilities and they refuse DAS and cellular phone service providers but finally they start installing DAS system since they started wearing and caring Blackberry devices. That made change big time. After that, DAS started caring all kind of services without too much of influence to existing equipment. Same logic as Cell phones in the airplane. There is no record of negative results caused by cell phones on Airplane electronic equipment, but we still have ban on cell phones in the plain. Same applies on Hospitals.

  6. Hello,
    I’m a university professor and RF Engineer and this information is timely. I am looking top introduce this to my course and have the students take advantage of the everchanging industry.

    Linwood Scott, MSEE
    LCSJ Communications

  7. Hi all
    I much appreciate to know if there is Diversity antenna in case of DAS ? in case of WIFI ?

  8. This is an awesome article, and I am glad to see it. Anyone considering a DAS for Cellular + WiFi needs to ‘read and heed’. I have been in the DAS business since the early 90′s and the issues with WiFi on DAS is more severe than indicated here. A high QoS WiFi system requires many more AP’s than antennas needed for celluar, and if you implement a combined system, then you end up with typically 30-50% fewer AP’s than you really need. The end result is what you expect from too few AP’s: poor WiFI coverage (especially at 5 GHz), low speeds, and frequent drops and high re-access attempts. We always strongly recommend separate DAS and WiFi systems, unless WiFi use is expected to well below average. Unfortunately, this eventually backfires, and you will eventualy get stuck with too little WiFi bandwidth and coverage.

  9. As a low voltage cable installation contractor, with limited experience with DAS; we recognize the business opportunities with these systems and are exploring selling and installing DAS. I have a two part question. Is there an average per square foot cost, for an installed system?
    And also,is there a consumer report, per se, on which systems perform the best?

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>