Linda Hui, MD, Ruckus Wireless
As more Wi-Fi capable devices surface, the Wi-Fi environment is getting crowded, more bandwidth is required. More flexible mobile working environment has both advantages and disadvantages. The disadvantages arrive due to unstable performance and limited coverage. It is time for us to rethink high density Wi-Fi design.
First of all, we have to understand that Wi-Fi deployment in dense environment is different. Most Wi-Fi networks are designed for offices, public buildings, schools, warehouses, hospitals, etc. These sites typically require a wireless network that is optimized for good capacity with a fairly large coverage area for each access point (AP). Very dense venues such as stadiums, airports, railway stations exhibition halls have very different requirements.
The most challenging factor is the need to support a large group of closely packed users and devices which require a large number of APs that are in close proximity while minimizing radio-frequency (RF) interference.
Large and densely-populated venues often incur significantly higher installation costs, hence a more accurate estimation of APs is essential. Underestimating capacity can result in poor performance and coverage gaps while overestimation increases interference and ultimately reduces capacity.
There are a number of strategies to mitigate RF interference while providing high capacity:
• Increase the use of 5 GHz RF spectrum for less interference and more channels;
• Shrink AP cell sizes as much as possible;
• Increase AP count, but only to the extent of estimated capacity to avoid over-building network;
• Adopt high frequency re-use, directional antennas, structural separation and attenuation,
• Deploy non-traditional 2.4 GHz channel plans;
• Leverage RF simulation tools to optimize design and optimize configuration;
• Opt for adaptive algorithms to handle changing RF conditions without manual intervention.
“Large and densely-populated venues often incur significantly higher installation costs; hence a more accurate estimation of APs is essential”
Performance is Pivotal for Wi-Fi
• To keep tomorrow’s users happy; the WLAN must become robust multimedia hotspots to deliver high performance, predictable and secure coverage in high density environments. Thus, the following guidelines need to be considered: As density goes up, the amount of airtime dedicated to traffic management (scanning for APs, broadcasts, etc.) will go up. This is also true as the number of SSIDs broadcast increases. Thus, both cases decrease the amount of airtime available for each device.
• Avoid or reduce co-channel interference as much as possible.
• Consider mounting APs in Non-Line of Sight (NLoS) locations - this will help attenuate the signal, which reduces the cell size and potential for co-channel interference. High-density applications have more than enough APs to make individual coverage.
• Make sure there are no other sources of RF interference nearby (non-802.11 sources).
• Keep in mind the denser the population of devices, the higher the average latency and jitter - this can limit the types of applications that can be supported in highlydense situations.
• Remember the goal for capacity is to maximize the SNR per AP.
Overall Factors to Consider when Designing Very High Density Wi-Fi
Design process is closely related to performance requirements. Thus, high-density design should be the key performance metrics of applications, bandwidth and latency.
The usage of applications also matters. The applications can be used for general internet access, voice, video, food service, ticketing, etc. The requirements for these applications define the minimum device requirements for successful operation, which are vital for the calculation of the number of devices per AP and the number of required APs. Other performance indicators include minimum bandwidth required to satisfy supported applications, expected number of active WiFi devices at peak traffic time with maximum latency and jitter tolerated, physical density of users, AP hardware selection and whether encryption will be utilized and what type. Each of these indicators requires careful estimation to ensure performance.
How does 802.11ac Wave 2 with MUMIMO Improve Performance?
Wi-Fi has traditionally been a half-duplex technology, where AP radios only connected to one device at a time. 802.11ac Wave 2 with multi-user MIMO (MU-MIMO) is a huge step up that allows radios to serve clients in groups, using up to four downlink spatial streams simultaneously. The standard provides 160 MHz channel support, 256 QAM modulation and data rates up to 2.3 Gbps in the 5 GHz band. But this isn’t just about bumping up peak data rates. Wave 2 MU-MIMO radically improves spectrum utilization, which makes Wi-Fi performance better for everybody, including non-Wave 2 clients.
High-density Wi-Fi networks pose unique deployment and performance challenges. Any deployment that caters to have a large number of wireless devices needs to address a number of issues as fully and as early in the design process as possible:
• Performance requirements (key performance indicators)
• Supported applications
• Minimum bandwidth
• Minimum, average and maximum devices per AP
• Maximum latency tolerated
• Number and density of APs
• Client capabilities (802.11n vs. legacy)
• Clients per AP
• RF environment (multipath, attenuation, coverage and cell size)
• AP mounting and location
• Wave 2 Ready
Founded in 2004, Ruckus Wireless is a provider in the wireless infrastructure market, enabling carriers and enterprises to access high-bandwidth application and services.