Performance Evaluation of Interference Between ...

Proceedings on the 15th CISL Winter Workshop · Kushu, Japan · February 2002

Performance Evaluation of Interference Between Bluetooth Networks using Bit Error Rate Soo-young Shin Control Information Systems Lab., School of Electrical Engr. and Computer Science, Seoul National University, Seoul, 151-742, Korea

Abstract: Bluetooth can form many piconets and scatternets within close proximity. This network topology can cause self channel interferences. As the interference increases, the performance will be degraded. The effect of interference is critical when a lot of Bluetooth networks coexist. In this paper, the performances of Bluetooth networks under interference is analyzed and simulated using the concept of bit error rate(BER). Keywords: Bluetooth, piconet, scatternet, interference, bit error rate(BER)

1

Introduction

Recently, a new universal radio interface, Bluetooth, has been developed enabling electronic devices to communicate wirelessly via short-range ad-hoc radio connections. The Bluetooth technology eliminates the need of wires, cables and the corresponding connectors between cordless or mobile phones, head-sets, personal digital assistants (PDAs), computers, notebooks, printers, projectors, etc. Bluetooth provides wireless interconnections among the electronic devices and peripherals used by individual. This network concept is called as personal area network, PAN [1, 2, 3, 4]. To provide global availability, the 2.4GHz industrial scientific and medical (ISM) unlicensed band are commonly used for low cost radios. The unrestricted access to the ISM spectrum exposes Bluetooth devices to a high level of interference. Bluetooth devices undergo interference problems that can be classified into two categories. The first category is the inference caused by non Bluetooth devices. New proposed solutions for wireless PANs such as IEEE 802.15 [5] and HomeRF [6] operate in the 2.4 GHz band while IEEE 802.11 [7] and HIPERLAN Type I [8] operate in this band for wireless LAN. The microwave ovens are also users of the band at 2.45GHz. This kinds of interference effect was widely researched[9, 10, 11, 12]. The second category is the interference from other

Bluetooth connections. It is called self interference. The Bluetooth networks adopts ad-hoc topology called by piconet and scatternet. This topology allows that many Bluetooth devices can coexist within close proximity. It is likely to have several persons in proximity, each having an open Bluetooth connection between a mobile phone and a headset or a mobile computer. Printer profile There are some former researches on this subject [13, 14]. However, there are some limitations on their models and assumptions. In [13], only packet error rate(PER) was considered using hit probability and co-channel carrier-to-interference ratio,(C/I)CO . The hit probability means the probability that two or more Bluetooth connections use the same frequency band. It was assumed that the transmitted and received power of all Bluetooth connections were equal. And all Bluetooth connections were synchronized. These are not real. In [14], packet error rate(PER) of asynchronous case was shown in detail. But there was no consideration about the signal power and (C/I)CO . It only considered the PER. In both [13] and [14], when the frame is hit by another frame using the same frequency, it is considered as corrupted. But if there are much differences between their signal powers, one frame may not be corrupted. In this paper, the bit error rate (BER) performance of Bluetooth networks under the self interference is an-

alyzed and simulated. Not only the hit probability but also the signal power and interference effect are considered. This paper is organized as follows. In Section 2, an Bluetooth technology is briefly described. In Section 3, the system model and assumptions are made. The BER of Bluetooth piconet system is analyzed in Section 4. In Section 5, simulation results are shown. Finally, this paper is concluded in Section 6.

2

Bluetooth Technology Overview

The user scenarios envisioned for first generation Bluetooth products are typically focused on travelling business people; portable devices that contain Bluetooth radio transceivers would enable to eliminate cables and connectors at home. These user scenarios share the following requirements:

the point-to-multipoint situation, a master sends every other slot and all slaves share the same time slot. Then, a master arbitrates between slaves by means of polling and is, thus, responsible for distributing the channel capacity among the slaves. Connections can be opened and closed dynamically. Connections are performed by a direct call from master to slave. Units may also respond to inquiry calls such that one unit can search for other Bluetooth units within the communication range.

3

System Model

It is assumed that n unsynchronized collocated piconets that are sufficiently close from one another such that a co-channel interference between two or more packets will affect each other. The interference system model is shown in Figure 1.

• Global operation • Peer connectivity : i.e. there is no wired infrastructure to provide call setup and networking functions ; connections are made based on an ad-hoc fashion • Simultaneous voice and data, for example, for multimedia applications • The radio transceiver must be small and operate at low power - i.e. the radio must fit into small, portable devices, such as mobile phones, headsets and personal digital assistants (PDA) To fulfill the requirements, Bluetooth operates at 2.4 GHz industrial scientific medical (ISM) band because this band is globally available and requires no license. The radio modem employs frequency hopping with 1600 hops/s to reduce the impact of jamming from other systems such as wireless LAN, micro oven and to comply with emission rule. The band is divided into 79 channels, each 1 MHz wide. The Bluetooth radio channel is assumed to be a burst-error channel without time dispersion [15]. Such a burst-error channel is rather good in between error bursts and very bad during bursts. Thus, a combination of fast automatic retransmission query (ARQ) scheme and simple coding is employed, rather than extensive coding, to increase the link robustness. Furthermore, the frequency hopping used at radio interface also improves the jamming resistance. Bluetooth communication protocol assumes that one unit is acting as master, the call-initiating, and all other participating units are slaves. A master together with slaves (up to seven slaves are supported) form an ad-hoc piconet. In the point-to-point case, a master and a slave sends and receives alternately to provide full duplex. In

Figure 1: Interference Model between Piconets Forward error correction and capture effect are neglected. Because of the strong adjacent channel rejection requirement required by the standard, adjacent channel interference is not considered. The traffic in each piconet is assumed periodic and the packet rate is packets per slot. Propagation of radio waves inside a building is a very complicated process, and it depends significantly on the indoor environment such as office,factory, and topography such as line-of-sight(LOS) and obstacles. The statistics of the indoor channel varies with time due to motion of people and equipment. Because Bluetooth radio communication aims indoor environment, the indoor radio propagation is considered in this paper. There were some researches for the indoor propagation model of Bluetooth transceiver [16, 17]. The link gain g = Pr /Pt , being the ratio between received and transmitted power, is given by the following short range propagation model. It consists of distance

dependent attenuation, shadowing, and multipath fading: g = g0 (d) · s (d) · r2 (d) . In this paper, only distance dependent attenuation, also called path loss, is considered. The communication channel is assumed to be additive white guassian noise(AWGN) channel.

4

Bit Error Rate Analysis between Bluetooth piconets

At first, n collocated synchronized piconets are assumed. It is also assumed that just overlap of bits will destroy all. The probability that two station in piconet A can successfully exchange a bit when one other synchronized piconet B is collocated, is equal to P0 = 78/79. It means that piconet B chooses another frequency as the one chosen by piconet A. With n collocated piconets, the piconet A has n-1 adversary piconets. The probability to successfully transmit is then Ps = P0n−1 . If the two piconets are not synchronized, which is the rule in Bluetooth, the probability to successfully transmit is not simple as that of synchronized case. Let L denote the packet length and TM denote the symbol duration. Then, the hop duration, Th , is equal to LTM . And, the delay of the kth user assumed to be uniformly distributed on [tkd , tkd + Th ) and in symbol transmitted by the kth user The hit pattern may be categorized into the following three cases. First, it is called a full hit by the kth transmitter has occurred for the nth symbol if the interference from the kth transmitter is present of the entire duration of the nth symbol [nTM , (n + 1)TM ). Second, it is said that a partial hit by the kth transmitter has occurred if the interference from the kth transmitter is present for only a fraction of the nth symbol duration. Finally, there is the case when the kth user’s signal is absent during the nth symbol duration in spite of the fact that a hop hit has occurred. The hit pattern are shown in Figure 2. The probability of a full hit denoted by pf , the probability of a partial hit denoted by pp , and the probability of a bit hit denoted by ph as a function of L are given as follows: n o (k) = Pr −Th + (n + 1)TM ≤ td < nTM

pf

=

L−1 2L

n³ ´ (k) pp = Pr −Th + nTM ≤ td < −Th + (n + 1) TM ³ ´o (k) ∪ nTM ≤ td < (n + 1) TM =

1 L

Figure 2: Patterns of Bit Hit

ps (L) ≡ pf (L) + pp (L) =

L+1 . 2L

The BER cannot be fully described by the hit probability and timing analysis. In collocated piconets, the interferences between piconets affect the BER. In other words, the BER is function of SIR(signal to interference ratio)    Ps  BER = f  P j  PI j

where Ps is the desired signal power,PIj is the interference power due to the jth interfering piconet. Therefore the gross BER is the sum of Equation (7) and Equation (8) like following Equation (9)    Ps  BERtotal = ps × f  P j  + BERr PI j

where BERr is the residual BER which is ignorable.

5

Simulation Results

In this Section, the simulation results are shown. Figure 3 shows the Bluetooh signal spectrum with frequency hopping. As shown in figure, the signal spectrum is spread out all over the 79 MHz bandwidth. This shows that frequency hopping is one of spread spectrum modulation technologies. At first, the BER under AWGN(additive white Guassian noise) is simulated. The results are shown in Figure 4. As shown in Figure 4, the BER under AWGN increases as the Es /N0 (Signal-to-noise ration, SNR) decreases. Assume that the transmitter power of interfering piconets have the same that of reference piconet. Then, the BER are shown like following Figure 5.

Figure 3: Signal FHSS signal spectrum of Bluetooth

Figure 5: BER performance when the T x powers of interfering piconets are all the same to that of reference one. [2] J. Haartsen, “Bluetooth-the unicersal radio interface for ad hoc, wireless connectivity,” Ericsson Review, vol. 1, pp. 110–117, 1998. [3] J. Haartsen, “The Bluetooth Radio Systems,” IEEE Personal Communications, vol. 1, pp. 28– 36, 2000.

Figure 4: BER performance under AWGN

6

Conclusion

In this paper, the interference effects are evaluated by not PER(packet error rate) but BER(bit error rate). As known in previous, the BER under AWGN increases as the SNR decreases. The BER is not only function of hit probability but also the function of SIR(signal-tointerference ratio). By evaluation of BER, performance of the Bluetooth networks can be estimated. This estimated performance can suggest coexistence criteria of Bluetooth piconets and scatternets. In this paper, only 2/3 FEC(forward error correction) payload is considered. The CRC(cyclic redundancy check), HEC(header error correction), and ARQ(automatic repeat request) should be contained to obtain more realistic result. These are future works.

References [1] SIG Bluetooth, “Specificaton of the Bluetooth system V1.1,” 2001.

[4] J. Haartsen, “Bluetooth-a new radio interface providing ubiquitous connectivity,” IEEE Vehicle Technology Conference, vol. 1, pp. 107–111, 2000. [5] Web document, “IEEE 802.15 WPAN Wokr Group,” http://www.ieee802.org/15/, 2002. [6] HomeRF, “Technical summary of the SWAP specification,” 1999. [7] IEEE Std.802-11, “IEEE Standard for Wireless LAN Medium Access COntrol(MAC) and Physical Layer(PHY) Specificaton,” 1997. [8] ETSI, “High Performance Radio Local Area Network(HIPERLAN) Type 1 Functional Specification,” 1998. [9] S. Zurbes J.C. Haartsen, “Bluetooth voice and data performance in 802.11 ds wlan environment,” Ericsson SIG Publication, pp. 1–13, 1999. [10] J. Ziren, “Reliability of ieee 802.11 hi rate dsss wlans in a high density bluetooth environment,” Bluetooth’99, pp. 1–12, 1999. [11] R. Nevo J. Lansford, A. Stephens, “Reliability of ieee 802.11 hi rate dsss wlans in a high density bluetooth environment,” IEEE Network, vol. 15, pp. 20 –27, 2001.

[12] I. Howitt, “Wlan and wpan coexistence in ul band,” IEEE Transactions on Vehiclar Technology, vol. 50, pp. 1114 –1124, 2001. [13] E.F.Meihofer S.Souissi, “Performance evaluation of a Bluetooth network in the presence of adjacent and co-channel interference,” IEEE Emerging Technologies Symposium: Broadband, Wireless Internet Access, p. 6, 2000. “Interference between Bluetooth [14] A.El-Hoiydi, networks-upper bound on the packet error rate,” IEEE Communications Letters, vol. 5, pp. 245 – 247, 2001. [15] S. Mattisson, “Low-power considerations in the design of bluetooth,” International Symposium on Low Power Electronics and Design, vol. 1, pp. 151– 154, 2000. [16] T.S. Rapparport, “Wireless communication : Principle and practice,” Prentice Hall, 1996. [17] B. Sklar, “Rayleigh fading channels in mobile digital communication systems,” IEEE Communication Magazine, vol. 1, pp. 136–146, 1997.