WLAN System Design Fundamentals Part 1: General Concepts
With much talk about small cells, a review of wireless LAN (WLAN) design is in order. This review is a basic design guide for WLAN coverage enhancement, the first of a multipart series on in-building wireless enhancement.
The Wi-Fi Alliance, the organization that owns the Wi-Fi (registered trademark) term specifically defines Wi-Fi as any WLAN product based on the Institute of Electrical and Electronics Engineers’ (IEEE) 802.11 standard. Some folks think WiFi technology is not part of a “small cell”. However, one can argue that unless a wireless technology is part of a tower’s local infrastructure, it is considered a small cell. A WLAN design includes “small cells” of wireless access points (WAP) much like a cellular DAS. Even if a carrier adds “WiFi offloading” the WLAN is detached from the tower and hence could be described as a small cell.
The demands on WLANs for functionality and scalability are growing due to the rapid proliferation of new network devices and applications. The number of devices and connections per user is steadily increasing. It is common for most users today to not only have a primary computing device but also at least one other smart device. Wireless operators have worked hard to accommodate the increased demand for data services over wireless networks. They have been forced to consider alternative offload strategies, including wirelessly connecting electronic devices (Wi-Fi). Unfortunately, the majority of smartphones being introduced into the marketplace only support Wi-Fi at 2.4 Gigahertz (GHz), which is rapidly increasing pressure on Wi-Fi designers and administrators to design products for the smallest segment of bandwidth available. Many devices now include the 5 GHz band (vis-à-vis 802.11n). Administrators and IT Managers are finding themselves faced with the challenge of providing ever-increasing levels of WLAN service in areas where simple coverage is the singular design goal. While there have been great advances made in the speed and ease of implementation of Wi-Fi networks, the basic nature of radio frequency (RF) is generally unchanged. Increasing the number of users who can access the WLAN in a small physical space remains a challenge. The steps and process for a successful high user density WLAN design that can be proven, implemented, and maintained.
The general concepts underlying medium-density Wi-Fi design remain true for many environments. But it is important to note that the content and solutions presented here will not fit every WLAN design scenario. Rather, the intent of the design guide is to explain the challenges in WLAN design and to offer successful strategies so that engineers and administrators understand them and are able to articulate the impact of design decisions.
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 WLAN installations is sufficiently low as to be acceptable under most codes and standards. However, the designer should always examine and provide for these possibilities.
WLAN design refers to any environment where client devices will be positioned in office environments where coverage expectations of a normal enterprise deployment, in this case a traditional, multi-floor, carpeted office are both ubiquitous and robust. For reference, a typical office environment has indoor propagation characteristics for signal attenuation as shown below. User density is a critical factor in the design. Aggregate available bandwidth is delivered per radio cell, and the number of users and their connection characteristics (such as speed, duty cycle, radio type, band, signal, and signal-to-noise ratio) occupying that cell determines the overall bandwidth available per user. A typical office environment may have APs deployed for 2,500 to 5,000 square feet with a signal of -65 dBm coverage and a maximum of 15 to 20 users per cell. That is a density of one user every 100 square foot (sq. ft.) and yields a minimum signal of -65 dBm.
Before a WLAN design is completed, a heat map survey and RF spectrum measurements are performed. A WLAN survey detector as well as a spectrum analyzer can reveal both existing WAPs and any interference that may be present in a facility. Graphical heat maps help visualize anticipated wireless LAN behavior for planning and design of a new or upgraded system. During the survey general information on the architectural characteristics of an office should be noted. A sample of existing AP signals is collected while spectrum readings are made to support the study and analysis.
Propagation of electromagnetic waves inside buildings is a very complicated issue. There are various models based on various principles with various requirements for input data complexity. In real situations, any piece of furniture, open doors and windows, moving people, reflections from outside the building and other effects influence the signal propagation. In a propagation study, several assumptions are made based on either typical WLAN equipment parameters including AP power output and average antenna gain. Predicted coverage levels on an individual multiple input multiple output (MIMO) WLAN channel may vary up to plus or minus several Decibels (dB) due to antenna gain variations in azimuth. The WLAN signal also changes due to attenuation of the walls and ceiling. Assuming a median attenuation for both frequency bands, wall losses are also estimated based on the survey and empirical data. Heavy wall losses are modeled as moderate cement member unit losses. Floor to floor penetration is modeled as the floor construction is assumed to be concrete poured on a metal pan. However, with some signals penetrating the floors, coverage will be enhanced with AP parameter optimization. Exterior wall and window losses are modeled as based on survey observations and other empirical data. It is also assumed that the 802.11n AP’s will be operated in narrow channel mode (i.e. 802.11b) so that multiple channels can be utilized for frequency division multiplexing of users. The design criteria are -65 dBm in coverage under a load of 15 users. A typical office may have APs deployed for 2,500 to 5,000 square feet with a signal of -65 dBm. However, to the end, typical AP layout goal is to assure contiguous coverage throughout all of the building.
In summary, WLAN design fundamentals include planning, analysis and design of the heat map data, RF spectrum background noise levels and propagation characteristics of the building. WLAN AP layout is only effective after these steps so that excellent coverage for 802.11n technologies will be available to all users in the building. A WLAN upgrade may be necessary at your facility to ensure business needs are met with technology- business drives technology!
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