Reliability of IEEE 802.11 Hi Rate DSSS WLANs in a High Density Bluetooth Environment

The issue of coexistence between IEEE 802.11 high speed Direct Sequence Spread Spectrum (DSSS) and Bluetooth radios with both radio types located within a mixed environment is studied. A network topology, propagation model, and user traffic loads are postulated. The reliability of IEEE 802.11 Hi Rate DSSS radios is then estimated under the stated conditions. 1.0 Introduction IEEE 802.11 and Bluetooth (BT) radios share common spectrum in the 2.45 GHz ISM band. In addition, both radio types are to a large degree targeted at the business user. With the advent of 11 Mbps data rates, IEEE 802.11 Direct Sequence Spread Spectrum (DSSS) radios can provide a mobile extension to wired networks in large enterprise installations. It can replace wired infrastructure entirely in Small Office / Home Office applications. At the same time, Bluetooth will become an important asset for the mobile worker and business traveler by servicing a number of applications which include downloading e-mail to a laptop via a cellular telephone, synchronizing palmtop devices, and accessing local printers. It is entirely likely that radios of both types will come in close proximity to each other within the enterprise setting. In order to determine the degree to which the radios will cause harmful interference to each other, a number of assumptions are necessary. It is difficult, if not impossible, to define a “typical” network topology. User scenarios and even indoor propagation models can be rather subjective. However, by using some reasonable assumptions, analysis of the interference caused by co-location of the two radio types can proceed. Assumptions include: a. a network topology and user density b. propagation model c. network traffic loads for IEEE 802.11 and BT A high density environment has been postulated. Large numbers of both types of devices are present within the topology analyzed. In addition, traffic loads are assumed for the BT piconets. This paper focuses exclusively on the reliability of the IEEE 802.11 high speed wireless network in the presence of interference from Bluetooth radios. The reverse situation, that being the effect of IEEE 802.11 DSSS radios upon Bluetooth piconets, is not analyzed. It must be emphasized that the results of this analysis should be considered preliminary. Further study is needed to verify key underlying assumptions, including the effective bandwidth over which a Hi Rate DSSS receiver is susceptible to Bluetooth interference. 2.0 Network Topologies In order to analyze the coexistence issue, a topology for the IEEE 802.11 Wireless LAN (WLAN) and the BT piconets must be postulated. It is assumed that there is one BT piconet co-located with each WLAN node. The WLAN network topology and the distributions of BT piconets are described more fully in the following paragraphs. 2.1 IEEE 802.11 WLAN Topology Harris Semiconductor Communications Products Bluetooth ‘99 2 In a typical WLAN installation, wireless stations (STAs) are associated with a fixed Access Point (AP) which provides a bridging function to the wired network. The combination of the AP and its associated STAs is referred to as a Basic Service Set (BSS). For the purpose of this analysis, the following assumptions were made: a. STAs may be located up to 20 meters from the AP b. The average density with in the BSS is one STA every 25 sq. meters c. The transmitter power for both STAs and the AP is +20 dBm Based on the above assumptions, there are 50 STAs associated with the AP. This topology is shown in Figure 2.1-1. The received signal strength is obviously a function of distance from the AP. It will be seen that the degree to which an IEEE 802.11 radio experiences interference is dependent upon its distance from the AP.