Outdoor 802.11 Mesh Links RF Impacts Considerations

1. Outdoor 802.11 Mesh Links RF Impacts Considerations Portland, Oregon 12-15 July 2004 Tricci So <tso@nortelnetworks.com> Tricci So, Nortel Networks

2. Objectives • Discuss the RF impact considerations when deploying fixed topology wireless access Community Area Network in metropolitan area using 802.11 wireless links in mesh configuration. • Highlight the challenges when supporting multi-hop transport using 802.11 radios to form the mesh configuration. Tricci So, Nortel Networks

3. Key Areas of Discussions • Spectrum regulatory influences on the outdoor 802.11 mesh deployment • RF Performance prediction considerations for designing and deploying outdoor 802.11 mesh network Tricci So, Nortel Networks

4. Regulatory Influences on Outdoor Mesh Deployment • The spectrum regulatory body of each country (e.g. FCC) imposes constraints on signal power levels of various frequencies even on the license free band to avoid RF interference. The result impacts the 802.11 outdoor mesh deployment configuration, range and RF performance. • FCC part 15.247 limits the of EIRP (equivalent isotropically radiated power) that represents the total effective transmit power of the radio, including gains that the antenna provides. • The gain of antenna represents how well it increases effective signal power in a particular direction and is expressed in dBi (decibels relative to an isotropic radiator). Every 3 dBi doubles the power of an RF signal and therefore it extends the range of a 802.11 radio link. • FCC imposes much tighter restriction of EIRP on in-door configuration using omni-directional antennas, but it eases EIRP for fixed, pt2pt outdoor system that use higher gain directional antennas. For example, if the antenna gain is at least 6 dBi, the FCC allows operation up to 4 watts EIRP. Even though such relaxation on regulation creates the opportunity for the outdoor 802.11 deployment, it also introduces new consideration on the outdoor 802.11 system performance issues in term of link budget calculation, noise and receiver sensitivity, interference etc. Tricci So, Nortel Networks

5. System Performance Prediction Considerations Prior our work, empirical models to predict 802.11 outdoor performance are not available • Areas that require understanding for 802.11 outdoor performance, but not limited to the following, are: • range extension for LOS and NLOS • multi-path behavior • Interference • link budget • delay spread Tricci So, Nortel Networks

6. System Performance Prediction Considerations – Extended Range • Extended outdoor range in “mesh” configuration benefits from LOS which is difficult even through use of sector or directional antennas • To achieve optimal performance, the proper frequency band selection and the degree of minimizing the obstruction in the outdoor auto-configured mesh configuration are important considerations for community network deployment • The ability to avoid link blockage is heavily dependent on the size of the Fresnel zone – as frequency goes up, the zone gets smaller, thus better avoiding the blockage that degrades link performance. • In general, it is difficult to accurately predict the overall propagation loss for any wireless link over a long distance that does not have clear, unobstructed LOS – i.e. NO guarantee • To guarantee availability and reliability for the outdoor 802.11 mesh network deployment, the expected variations MUST be based on long-term statistical elements in a dynamic outdoor environment. 802.11s mesh standard needs to include RF behavior considerations when designing the mesh network architecture. Tricci So, Nortel Networks

7. System Performance Prediction Considerations – Multipath • Outdoor Multipath Behaviors • The effects of outdoor multipath can be quite different than the effect of indoor multipath as the RMS delay spread are quite different in the indoor and outdoor environments because of the differences in propagation environments, Tx power, cause of attenuation etc. • The two most major phenomena that need to be understood better in outdoor environment are: Delay Spread and Multipath Fading • Multipath Fading is the radio signal fades at certain places, and may also change with time. It is caused by separate radio waves interfering with each other. At the receiver, these waves arrive at different phase and they cancel each other if the phases are different by 180o Multipath Fading is typically combated using receiver diversity. For a mesh configuration, consideration for mesh point cost and size present challenges for diversity design. Tricci So, Nortel Networks

8. System Performance Prediction Considerations – Delay Spread • Delay Spread (i.e. multipath dispersion) increases as the cell radius (range) of the transit links increases • Delay spread is characterized as the time dispersive property of wireless channels. The first moment of the time-delay profile is the mean delay, and the square root of its central moment (about the mean) is defined as the delay spread of the channel. • In typical urban outdoor environment, root-mean-squared (RMS) delay spreads can be several microseconds dependent on the frequency band, the power and the distance, compared with nanoseconds in typical indoor WLAN environments. As a result, ISI can sometimes span several data symbols. Because of the dispersion, symbols can collide and result in data loss. • Most off-the-shelf adaptive equalizer cannot tolerate the long duration of the delay spread in the outdoor environment. • Using RAKE receiver cannot solve delay spread problem for all modulation scheme in 802.11. Other approach such as customized adaptive equalization may be used to relief the impact of the delay spread. Tricci So, Nortel Networks

9. System Performance Prediction Considerations – Interference • Adjacent and Co-channel Interferences • Interference is the reception of signals from source other than the intended source of which the signal can be the same or different frequency, the key differences between indoor and outdoor are the higher Tx power and antenna gain because of longer distances between the peer antennas. • A mesh auto-configuration system (i.e. no frequency planning) is more prone to interference as the radio interfaces of such system are designed to auto-scan possible neighbors to form the peer-to-peer adjacency and communication path. To minimize the interference, the mesh point has to rely on the limited number of non-overlapping channels to support the communication. • Traditionally co-channel interference is minimize by optimizing the frequency assignments of the APs, the transmit powers between the AP peers and the tilting of the antennae to limit the spread of the signals in the system. To apply these types of solution to the outdoor environment will require more sophistication and intelligence to be built into mesh AP due to the dynamic nature of the operation and may trade off some of the system performance. • Adjacent frequencies are supposed to be unable to interfere with each other, especially if directional antennae is used. Unfortunately, imperfect filtering allows adjacent channel signal to interfere with weak received signal. IEEE 802.11 requires system to continue to operate successfully even when the adjacent channel is 35 dB higher. For the outdoor mesh deployment, due to the nature of higher Tx power and strong Rx sensitivity, adjacent channels interference become even more harder to manage. Tricci So, Nortel Networks

10. System Performance Prediction Considerations – Link Budget • Outdoor Link Budget Computation • The link power budget is the computation of the whole Tx/Rx chain.  A radio Link consist of three basic elements in typical scenario:  • Effective transmitting power:  transmitter power [dBm] - (cable +connector) loss [dB] + antenna gain [dBi] • Propagation loss [dB]: propagation loss statistic for the given environment [dB] • Effective receiving sensitivity:  antenna gain[dBi]- cable loss [dB]- receiver sensitivity [dBm] • As transmitting and receiving properties are not always identical at both sides, a link budget calculation must be performed for BOTH directions. • These calculations are insufficient. In real life outdoor environment,  the atmospheric losses (air moisture, scattering, refraction etc.), type of antenna, reflections, can affect the link performance which are the additional contributors to the propagation loss. • To compensate for the propagation loss over the long distance in the outdoor environment, higher transmitter power and antenna gain within the bound of the regulatory restriction will also be added to the equations of • effective transmitting power, and • effective receiver sensitivity. Tricci So, Nortel Networks

11. Conclusions Most of the 802.11 deployment experiences have been indoor omni-directional links. Deploying outdoor 802.11 mesh network using sector or directional antenna enables new type of network deployment configuration over a long distance, however, it also introduces new challenges to the use of the 802.11 technology. Tricci So, Nortel Networks