WiFi over DAS: When does it make sense to merge the 2 systems?
WiFi for In-Building systems have become almost universal in buildings, enabling convenient connection to the Internet for hundreds of applications, and this trend will continue as more work and play moves onto the Internet, Similarly, the use of Cellular services is ever-expanding, and the increase in applications is seemingly non-stop. As the applications merge, and as the technologies merge to some degree, it makes to complete sense to ask the question: “Why not merge a WiFi system and a Distributed Antenna System (DAS)”?
DAS products do exist to make this happen, and have been available for several years. These implementations typically fall into 2 types:
Coaxial piggyback architecture: This DAS architecture piggybacks the WLAN (WiFi) A/P (Access Point) signals onto the same spans of coax that carry the Cellular and Public Safety signals from the final amplifier (or optical to radio frequency converter) to the actual radiating antenna from which all signals are radiated.
Shared Cat 5/6 cabling architecture: This combined DAS + WiFi architecture places the Cellular & Public Safety signals on Cat5/6 cabling. Some of this can be shared cabling with Access Points for the WLAN, but some new cabling typically has to be added to each radiating location for added bands of Cellular service.
Typical Pro’s and Con’s of the Coaxial Piggyback Approach to WiFi over DAS
Pro: Keeps several Access Points (A/P’s) in one location for easier maintenance, rather than on/in ceiling
Pro: Shares a cabling resource that is wideband (for 1/2″ coax)
Con: Limits the number of AP’s to the number of coaxes leaving an area with DAS equipment
This is the critical disadvantage of this combined system type, and is discussed at length below.
Con: Limits A/P placement flexibility and increases frequency planning difficulties
Con: Has much higher loss for 5 GHz WiFi signals and will significantly restrict 5GHz coverage area
Con: 5 GHz 802.11a signals cannot typically be carried on larger coax cables due to maximum mode frequency limits
Con: Becomes costly quite or impossible for MIMO technology in the WLAN equipment
Typical Pro’s and Con’s of Shared Cat 5/6 Cabling Approach to WiFi over DAS
Pro: Keeps WLAN architecture in it’s traditional form
Pro: Shares some cabling infrastructure
Pro: Uses a cable type that may be more familiar to WLAN installers
Pro: Cellular is modular by bands. More in-ceiling modules can be added later for new bands.
Con: Usually only 2 Cellular bands can share a Cat5/6 cable with an A/P; extra bands require added Cat5/6 cable. This somewhat defeats the purpose as often 2 new cables have to be added for a full cellular solution that covers all of the present Cellular bands in use.
Con: Distance limits apply from the DAS hub to the in-ceiling modules that cause cost inefficiencies
Con: Best locations of Cellular modules often does not overlay the A/P locations, resulting in cost inefficiencies.
Con: If all of the present Cellular band modules are added, along with the needed additions of Cat5/6 cable, the overall system cost is typically 1.5-2 times higher than putting in a dedicated full spectrum DAS.
Capacity and design mismatches between WiFi and DAS causes system overlay mismatch
One of the key operational factors in WiFi system is the number of users expected, and this relates to WiFi system capacity to give the needed operational levels of speed and availability. In WiFi systems, there are several potential ‘choke points’ for capacity but the one capacity point that usually dictates the wisdom of combining a WLAN and a Distributed Antenna System is the fact that the number of A/P’s has to be somewhat proportional to the number of WiFi users.
This is due to a combination of: 802.11 technology constraints, available bandwidth and channel counts, and reasonably useful data speeds. When these 3 factors are balanced in situations with medium to high user density, one ends up with a maximum reasonable number of users per A/P. Beyond those maximum user numbers on a WLAN system results in slower data speeds, followed by dropped accesses and frequent re-accesses as the number of users increases beyond the maximum. There are newer A/P technologies that are helping to mitigate this to a degree, but some of these depend on clients that can use the different technologies. For general WiFi use, or at sites where the WLAN system owner does not fully control WiFi client technology, these new technologies will have limited benefit. So, one is always left with a key WiFi system capacity issue: designing to a maximum number of users per A/P, or, conversely, installing enough A/P’s to meet the user capacity demands
How does this effect a merged WiFi and DAS system? Part of the reason lies in the difference in which capacity is allocated in WiFi versus Cellular systems. While WiFi capacity is controlled at several points (backhaul speed, switch counts, and A/P’s), Cellular system capacity is controlled at the Base Stations. When designing a DAS, one typically designs for specific coverage levels that meet Cellular industry standards. The capacity in the cellular side is provided via the DAS drive sources, be it a repeater from a nearby tower for lower user counts, or a BTS sectors (or sectors) driving the DAS for high user counts.
When one designs of WiFi system with the A/P count properly proportioned to the number of WiFi users, and then performs a Cellular Distributed Antenna System design, there is a significant mismatch in the number of A/P’s needed for a medium to high WiFi usage site versus the number of radiating points needed for the Cellular DAS. As a general rule of thumb, a ratio of 1.5 A/P’s per Cellular radiating point is a good ratio for these sites. If one uses 5 GHz 802.11 standards, then the ratio is higher, perhaps a 2:1 ratio of A/P’s per Cellular DAS radiating points.
What this design mismatch between the 2 systems means for the system owner is that:
If one overlays Cellular radiating points on a WLAN with sufficient A/P’s for good capacity and future user growth, you will end up with more Cellular radiating points than needed, and this drives the Cellular DAS cost much higher than it should be. You waste money.
If the number of A/P’s is limited to the most cost effective count of Cellular DAS radiating points, then you will end up with too few A/P’s for a medium to high use environment. This results in inadequate WLAN capacity for a good user experience and high WLAN system availability.
So when does it make sense to combine WiFi and DAS, and when does it not?
The combination of circumstances with the design mismatch does not occur in some cases of WiFi and Cellular DAS overlay. These cases are when you have a fairly typical Cellular DAS design COMBINED WITH low to moderate WiFi user counts, and where no strong WiFi user growth is anticipated. Examples of such would be:
Factory areas where there a limited number of WiFi users and wide areas between users.
Service areas where there is no general public WiFI user access
Sites with controlled WiFi user counts and controlled WLAN clients and low likelihood of user growth
Situations where it is unwise to overlay Cellular DAS and WiFi systems are:
Public use spaces where users counts are uncontrolled. This is even more true when the WLAN client devices are uncontrolled.
Office spaces where WiFi growth is expected to be high or is already high.
Hospitals where there is a steady growth of WLAN applications.
Locations where 5GHz WiFi is needed.
Additional Factors to Consider with Overlaid WiFi and DAS:
Note that there whenever you overlay Cellular DAS and WiFi systems, there is a degree of design and operational flexibility that is lost. For example, if you need to add an A/P, then you may be constrained to where it can be added, or may have to add un-needed Cellular DAS parts at the same time.
Technology changes are rampant in both the Cellular world and the WLAN world. Having 2 systems too closely intertwined may bring grief when you want to upgrade only one of the 2 systems.