After another long and productive circuit-simulation session, Joe Engineer needed a break. "To reward myself, I'll download the complete works of Britney Spears MPEG collection," thought Joe. He figured that transferring the several-hundred-megabyte video file would tie up the office's 802.11b network for several minutes. To occupy himself, Joe began sending e-mails. He sent some large attached files over the Bluetooth link from his PDA to his 3G cell phone.

Did the interference between the Bluetooth and 802.11b networks frustrate the data transfers? Or did Joe have a positive wireless-networking experience? Don't underestimate the importance of these questions. The co-existence of IEEE 802.11b and 802.11g with Bluetooth wireless personal-area networks will be crucial to user satisfaction. These networks use the same 2.4-GHz spectrum. Often, they operate in the same locations. To determine how the networks can be used simultaneously, it is necessary to evaluate how they interact with one another. It then becomes possible to maintain acceptable performance on each network.

The IEEE 802.11b and 802.11g standards (referred to here as 802.11b/g) for WLANs will replace many wired-LAN computer networks. Specifically, the 802.11b standard provides for payload data rates of 1, 2, 5.5, and 11 Mbps. For 1- and 2-Mbps data transmission, respectively, it uses Differential Binary Phase Shift Keying (DBPSK) and Differential Quadrature Phase Shift Keying (DQPSK) direct-sequence spread-spectrum modulation. For data transmission at 5.5 and 11 Mbps, 802.11b specifies Complementary Code Keying (CCK) modulation.

In contrast, the 802.11g standard uses Orthogonal Frequency Division Multiplexing (OFDM) modulation to extend 802.11b with payload data rates up to 54 Mbps. In these WLANs, an access-point radio wirelessly connects terminal devices, such as personal computers, to each other and to the wired network. The maximum distance of terminal devices from the access point is 30 to 100 m, depending on the data rate.

According to the transmit spectrum mask of the IEEE 802.11b/g standards, a channel's occupied bandwidth must be less than 22 MHz. Three non-overlapping 25-MHz spaced channels can co-exist in the 80-MHz-wide ISM band. Although channel agility is an option for 802.11b and 802.11g access points, many implementations are expected to be fixed on a single channel.

Bluetooth wireless personal-area networks (WPANs) are intended to provide wireless datalinks between cell phones, wireless headsets, personal digital assistants, personal computers, and other devices in PANs. These devices communicate with one another within a range of approximately 10 m. Bluetooth signals are FSK modulated with a bit rate of 1 Mbps. The 2.4-GHz ISM band is divided into 79 Bluetooth channels, which are spaced 1 MHz apart. Bluetooth networks frequency hop through a pseudo-random selection of these channels at 1600 hops per second.

Typically, 802.11b and 802.11g WLAN access points will be stationary. Operating frequencies can then be planned to minimize interference between WLANs. This isn't the case for interaction between Bluetooth and 802.11b/g networks, however. Even if they're in areas where 802.11b/g networks are operating, devices like cell phones need to maintain their Bluetooth links to other devices. When Bluetooth networks hop onto a channel used by an 802.11b/g network, disruption of either or both networks is possible. Adaptive-hopping procedures are being considered for a new Bluetooth standard. They would help it avoid the frequencies being used by 802.11b/g networks.

Standards and designs must be created that will most effectively allow the two networks to co-exist. To begin this development, it's necessary to know the conditions that cause the interference between networks. The connected solutions of Agilent EEsof EDA's Advanced Design System (ADS) and Agilent's Vector Signal Analyzer software (virtual VSA) offer capabilities for the analysis of both Bluetooth and 802.11b/g networks.

Using ADS and the software-based VSA, several combinations of networks and interfering signals were simulated and analyzed. Here are the results for the following combinations of desired and interfering signals:

• Bluetooth performance with 802.11b interference
• Bluetooth performance with 802.11g interference
• 802.11g performance with Bluetooth interference

In all of these simulations, the results show bit-error rate (BER) when collisions occur between the desired and the interfering networks. The interfering signal is applied at 100% duty cycle at a constant frequency. In this worst-case scenario, every packet that is transferred on the network collides with interfering-signal packets. In actual applications, the probability that a collision will occur is less than 100%. To predict performance, one needs to know the interfering-signal power that will result in the specified BER when network collisions occur.

Over an 8-hr. day, an average 802.11b/g network may be expected to transmit only a small percentage of the time. The average BER that a network experiences during a day is therefore low, even though it is high during collisions. However, the average BER over a day may be a poor indicator of user satisfaction. The user may find network performance unacceptable if it's severely degraded during periodic intervals of simultaneous heavy activity.