Mobile networks hit noise hurdles

The emergence of wireless ad hoc networks means that multiple, diverse wireless devices will be operating in close proximity. In particular, the coexistence of IEEE 802.11b and IEEE 802.11g with Bluetooth personal-area networks will be an important user satisfaction issue. These networks use the same 2.4-GHz spectrum and will often operate in the same locations. Evaluating how the networks interact is needed to determine how they can be used simultaneously while maintaining acceptable performance on each.

The IEEE 802.11b and proposed 802.11g standards for WLANs will replace wired-LAN computer networks. In these WLANs, an access-point radio wirelessly connects terminal devices like personal computers to each other and to the wired network. The maximum distance of terminal devices from the access point is 30 to 100 meters, depending on the data rate.

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

Bluetooth personal-area networks are intended to provide wireless data links among 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 meters. Bluetooth signals are FSK-modulated with a rate of 1 Mbit/second. The 2.4-GHz ISM band is divided into 79 Bluetooth channels that are spaced 1 MHz apart. Bluetooth networks frequency hop through a pseudo-random selection of these channels at 1,600 hops/s.

Typically 802.11b/g WLAN access points will be stationary, and operating frequencies can be planned to minimize interference between WLANs. This is not the case for interaction between Bluetooth and 802.11b/g networks. Devices like cell phones need to maintain Bluetooth links to other devices in areas where 802.11b/g networks are operating. 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 to help it avoid frequencies that are being used by 802.11b/g networks.

Unacceptable degradations

To create standards and designs that most effectively allow the two networks to coexist, it is necessary to know under what conditions interference between networks produces unacceptable degradations in performance. The connected solutions of Agilent EEsof EDA's Advanced Design System (ADS) and Agilent's Vector Signal Analyzer (virtual VSA) software provide capabilities for analysis of Bluetooth and 802.11b/g networks.

Several combinations of networks and interfering signals were simulated and analyzed using ADS and the software-based VSA. Results for the following combinations of desired signals and interfering signals are presented here.

• Bluetooth performance with 802.11b interference

• Bluetooth performance with 802.11g interference

• 802.11g performance with Bluetooth interference

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

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

The bandwidth of a Bluetooth channel is less than 1 MHz, while 802.11b/g signals may be as wide as 22 MHz. To simplify calculation of a Bluetooth network's performance with 802.11b/g interference, a broadband noise source is sometimes used to represent the 802.11b/g interference. However, it is possible for simulations to include the details of the 802.11b/g modulation and full models of transmitter and receiver filters, amplifiers and mixers. These simulations show the extent to which broadband noise assumptions are valid and allow circuit designers to identify modulation and filtering-dependent effects.

The performance of a Bluetooth network was simulated with interference from an 802.11b source, an 802.11g source and broadband noise sources. The simulations determine the BER of the Bluetooth network as the power and frequency offset of the interfering signal are varied. The transmit filter and modulation characteristics that determine the interfering signal's power density as a function of frequency offset are included in these simulations.

To analyze the performance of the Bluetooth network, the threshold between acceptable and unacceptable performance is chosen to be a Bluetooth raw BER of 0.001. With Bluetooth packet lengths of 366 bits, this BER produces a raw packet error rate (PER) of 31 percent. Network performance with 31 percent raw PER is worse than insignificantly degraded and better than complete network failure. Bluetooth hops frequency through 79 channels over an 80-MHz frequency span. With the transmit filters used in this analysis, the 802.11b signal occupies 15 of the 79 channels. Collisions between an actual Bluetooth network and an 802.11b network will occur, at most, 19 percent of the time. With a BER of 0.001 during collisions, the total PER will be about 5.9 percent, assuming the error rate is much smaller when no collisions occur. This may not be acceptable for Bluetooth voice applications, but may be considered acceptable for data transmission.

Simulation results of a Bluetooth network with 802.11b, 802.11g and filtered broadband noise interference show the minimum interference power that degrades BER in a Bluetooth receiver to 0.001. When the center frequencies of the Bluetooth and 802.11b signals are the same, an interfering 802.11b signal power that is 8 dB lower than the Bluetooth signal power degrades the Bluetooth BER to 0.001.

The carrier-to-interference ratio in the Bluetooth receiver is calculated by dividing the interference power in the Bluetooth receiver noise bandwidth by the received Bluetooth signal power. In these simulations, the average carrier-to-interference ratio in the Bluetooth receive band required to produce 0.001 BER is 18.6 plus/minus 0.3 dB. Using broadband noise sources as interference instead of 802.11b/g modulated sources provides a reasonable estimate of performance. When making this approximation, the power of the noise source in the Bluetooth receiver bandwidth must be the same as would be produced by an 802.11b/g source. In many cases, designers may choose to include the full details of 802.11b/g modulations and filters in simulations, so that interference spectral densities are accurately modeled.

Orthogonal modulation

The 802.11a and 802.11g specs use orthogonal frequency-division modulation that divides 16.25 MHz of bandwidth into 52 subcarriers, each 312.5 kHz wide. An OFDM data packet consists of a preamble, header and data block. In the data block, 48 subcarriers transmit data. These carriers may be modulated with BPSK, QPSK, 16-QAM or 64-QAM depending on the data rate. Four subcarriers are pilot signals in the data block. The 802.11g receiver uses the pilot signal as reference for phase and amplitude to demodulate the data in the other subcarriers. The pilot signals allow the receiver to compensate for phase and amplitude distortion of the OFDM signal. Numbering the OFDM subcarriers of a channel from -26 to +26, pilot signals are on channels -21, -7, +7 and +21. When the 1-MHz-wide Bluetooth interference is applied to the 802.11g signal, only a few of the subcarriers are directly affected. Bluetooth interference that falls on the pilot subcarrier can produce errors in the phase and amplitude correction the receiver uses when demodulating the data subcarriers.

The increase in 802.11g network degradation at 2 MHz offset is due to the Bluetooth signal interfering with an 802.11g pilot subcarrier that's 2.19 MHz from the 802.11g center frequency. An increase in degradation also occurs at 6-MHz and 7-MHz offset, but is not as strong. The pilot subcarrier at 6.56-MHz offset is between the 6- and 7-MHz offset channels, so only the edges of the 1-MHz-wide Bluetooth signals fall on the pilot subcarrier.

The simulated EVM of each OFDM subcarrier was analyzed using the software-based VSA. For subcarriers No. -26 through 0, the EVM is between 2 and 5 percent rms. The subcarriers nearer to the Bluetooth interfering signal have larger EVM. Subcarrier No. 12 has an EVM greater than 100 percent. The power of the Bluetooth signal is 11 dB less than the total 802.11g power, but its power is 6 dB greater than the power of the individual OFDM subcarriers. Therefore, the EVMs of subcarriers with frequencies very close to that of the Bluetooth signal are expected to be very large.

Demodulation errors

Pilot subcarriers in an OFDM signal are used as phase and amplitude references for demodulating data subcarriers. Bluetooth interference on a pilot subcarrier can cause errors in the demodulation of data subcarriers.

Interference from 802.11b/g signals on Bluetooth networks can be approximated by determining the Bluetooth degradation that will be produced by broadband noise interference. By including models of 802.11b/g transmit filters in simulations of broadband noise interference with Bluetooth networks, performance degradation as a function of frequency offset can be determined.

Degradation of an 802.11g network by an interfering Bluetooth signal is more severe when the Bluetooth frequency is near an OFDM pilot subcarrier. The degradation of an 802.11g network by an interfering Bluetooth signal that falls directly on the pilot signal in the seventh OFDM subcarrier is the same as that produced by 10-dB-stronger Bluetooth signals falling on OFDM data subcarriers. Simulation can include the effects of 802.11g and Bluetooth modulation and system filters to determine 802.11g network performance as a function of frequency offset.

Simulations show that interference between 802.11b/g networks and Bluetooth networks can be significant when used simultaneously the way Joe Engineer does. His experience downloading files on his WLAN while there is large amount of activity on his PAN will be determined by the relative signal power of 802.11b/g and Bluetooth signals at each receiver. In some cases he may be frustrated by his simultaneous wireless-networking experience. In other cases, he will be unaware of the interference. ADS simulation of networks using all of these standards is a powerful tool for creating designs and standards that will ensure the highest possible level of user satisfaction.

See related chart