Our testing of WLAN systems at scale dramatically demonstrated the effects of interference on limiting WLAN capacity and performance. And that opens the question of what other means are available to control WLAN interference and hence increase performance.
We tested enterprise wireless LAN systems based on Cisco infrastructure deployed in two ways:
- in a conventional microcellular architecture and
- deployed using the InnerWireless Horizon Distributed Antenna System (DAS).
In both cases, the same Cisco access points and wireless LAN controller were used. The main difference is in the physical layer - how the wireless signals are distributed throughout the coverage area.
We discovered that controlling interference through carefully crafting antenna coverage doubled the WLAN capacity using the same APs.
Horizon is a Distributed Antenna System (DAS) that can accommodate a broad range of wireless services operating from 400 MHz to 6 GHz including paging, public safety, 2-way radio, cellular/PCS, WMTS and Wi-Fi. Horizon implements Wi-Fi, 802.11 a, b, g, or n wireless LANs, as layers of independent wireless service. This layered architecture is known as a layered DAS or L-DAS. The Horizon L-DAS provides pervasive coverage, and layering increases capacity in the same area by using multiple Wi-Fi access points (APs) operating on different channels.
The layering capability is unique when compared the conventional microcellular architecture for wireless LANs, where a single access point (AP) provides both coverage and capacity in an area. In the L-DAS approach the RF coverage is engineered ahead of time. The access points are located in a wiring closet (or other convenient location with other telecom gear) on each ﬂoor and multiple antennas are placed throughout the coverage area such that the wireless signal level for each channel used is spread consistently throughout the entire area.
In the classic microcellular approach, the access points are placed out in the coverage area and each one has its own antennas. The channel and signal level of each AP are set independently by the Wireless LAN controller and can change dynamically. Usually, the resulting coverage pattern is that adjacent access points operate on different channels. In an L-DAS, all channels are available in all locations. We compared the difference in performance for a system deployed in the conventional manner and a system deployed using InnerWireless Horizon Distributed Antenna System (DAS).
Horizon is made up of multi-coverage diversity antennas that are engineered to provide uniform signal-level RF coverage throughout a facility. Each coverage area uses a single coax cable with multiple RF radiating elements (antennas) to provide a typical coverage area of 6000 square feet, about three times the size of the coverage area expected from a discrete access point in a microcellular deployment. This coverage area is called a segment.
An access point is connected to the antenna segment using an access point combiner. Access points for each segment are collocated in a common location (e.g. a telecommunication closet or recessed ceiling cabinet) so that they are secure and easy to maintain. Multiple segments can be combined to provide coverage throughout an entire facility. In this architecture, a single channel of Wi-Fi covers the entire area. If more capacity is needed, more APs can be added to the antenna segment and tuned to a different channel. Each new channel provides an independent layer of wireless LAN service. At any speciﬁc location, all channels that are in use are available.
In the 2.4 GHz band there are 3 unique channels and the DAS distributes signals for each channel throughout the entire coverage area. More channels are available in the 5 GHz band depending on which version of 802.11 is being used. Horizon supports from one to six layers on a single DAS segment for wireless LAN coverage, - three channels of 2.4 GHz and up to three channels in the 5 GHz band.
A fully loaded L-DAS with three layers operating in the 2.4 GHz band would have three access points per 6000 square foot segment, one for each channel. A micro cellular deployment supporting the same applications and designed for the same capacity will usually require the same number of APs.
Our testing conﬁguration used 3 DAS segments with 3APs each for a total of nine APs. The Microcellular
system we tested used 9 APs to deliver coverage and capacity to the same area.
- The L-DAS exhibited lower interference between APs in the same system than the classic microcellular system.
- The L-DAS delivered more than double the data capacity of the discrete microcellular system in our tests.
- The clients on the L-DAS exhibited more uniform and predictable performance.
- The DAS system did not compromise the basic functionality of the Cisco Wireless LAN system. All of the expected features worked well on the DAS system.
- Roaming, voice support, QoS mechanisms, 802.11a/b/g and 802.11n with 20 or 40 MHz channels worked on the L-DAS with the same clients and software as the discrete microcellular system.