Packet Error Rate Analysis of IEEE 802.15.4 under

802.11g and Bluetooth Interferences

Alexandru Lavric, Valentin Popa, Ilie Finis, Adrian-Mihai Gaitan and Adrian-Ioan Petrariu

Stefan cel Mare University of Suceava, Department of Computers, Electronics and Automation, Suceava, Romania

lavric@eed.usv.ro

Abstract— This paper approaches the issues concerning the

coexistence of the Wireless Sensor Networks (WSN) IEEE

802.15.4, the IEEE 802.11g wireless networks and the ad-hoc

Bluetooth networks that function within the same 2.4 GHz

band. The quality of communication is assessed by analyzing

the PER (Packet Error Rate) parameter. The tests are based on

the worst case scenario presumption, which entails the

occurrence of the channel overlapping phenomenon. The paper

suggests a practical approach through the assessment of the

PER parameter at various separation distances between

networks and by varying the retransmissions of a packet for

the 802.15.4 network.

Keywords-802.14.5; WSN; Bluetooth; 802.11g; Packet Error

Rate.

I. INTRODUCTION

The presence of the wireless networks is becoming increasingly widespread and the radiofrequency environment is becoming more congested and overcrowded due to disturbances caused by electrical and electronic appliances, wireless communication systems and other networks that function within the same frequency band.

One consequence of such interferences is the decreased performance level caused by the collisions that require the retransmission of the packets. The consequences of this congestion vary from the loss of the packets, false commands and alarms, to loss of synchronization. This paper analyses and assesses the issue of the coexistence of the (WSN) IEEE 802.15.4 sensor networks, IEEE 802.11g (Wi-Fi) wireless networks and ad-hoc Bluetooth networks that function within the same 2.4 GHz band. According to [1], coexistence is defined as the ability of a system to operate within an environment where other systems operate and abide or not to the same set of rules.

The scientific literature that approaches the issue of coexistence can be divided into empirical studies [1]-[6], where the measured experimental data is collected by means of a practical approach, and analytical studies [7]-[9], that mathematically model and simulate the PHY (Physical) and MAC layers.

This paper presents an empirical study in which the results are obtained by means of a practical approach implementing different test scenarios. This paper is organized as follows. In Section II, we assess the channel overlapping problem of 802.11, 802.15.4 and Bluetooth communication protocols. In Section III, we describe the channel access techniques and the different modulations used. In Section IV, we present the proposed tests setup, and the results obtained. Finally, in Section V, we draw our conclusions.

II. CHANNEL OVERLAPPING

In this section the channel allocation of 802.11, 802.15.4 and Bluetooth communication protocols are analyzed. The 802.11b standard defines a total number of 14 channels, each with a 22 MHz band width as shown in Fig. 1. In the United States the 1-11 channels are regulated by the FCC, while channels 1-13 can be used in most European areas. The communication channels covered by the 801.11g standard have a band width of 20 MHz.

Figure 1. Channels 802.11 and 802.15.4 overlapping

The channels that do not overlap at one particular moment are 1, 6 and 11 for the US, and 1, 7 and 13 for the EU area respectively for 802.11b. In the case of 802.11g standard the channels that are not overlapped for the EU zone are 1, 5, 9 and 13. When designing a wireless network, channel allocation and avoiding channel overlapping are highly important. The IEEE 802.15.4 standard allocates 16 channels for the 2.4 GHz band, numbered from 11 to 26. The width of a channel is of 2 MHz, just as the frequency guard interval. As presented in Fig. 1, channels 802.15.4 numbered 15, 20, 25 and 26 for the US and channels 15, 16, 21 and 22 for the EU, respectively, are the only ones that do not overlap with the 802.11b channels. The Bluetooth protocol allocates 79 channels as shown in Fig. 1, or less, if there’s any interference and the AFH (Adaptive Frequency Hopping) is used.

III. CHANNEL ACCESS TECHNIQUES

A major problem that may occur in the analysis of coexistence is the presence of several devices operating on

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the same channel and using different modulations and channel access techniques. IEEE 802.11g uses the OFDM (Orthogonal Frequency Division Multiplexing) modulation for higher transfer rates (up to 54 Mbps) and DSSS (Direct-Sequence Spread Spectrum) at lower transfer rates. The channel access techniques range from DCF (Distributed Coordinated Function) which uses a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) algorithm to PCF (Point Coordination Function). In the 2.4 GHz band, the IEEE 802.15.4 uses the modulation O-QPSK (Orthogonal Quadrature Phase Shift Keying) which requires the encoding of 4 bits in a modulation symbol which is then conveyed with a 32 bits sequential code and thus reaches a maximum transfer rate of 250kbps [10]. The Bluetooth 2.0 EDR (Enhanced Data Rate) protocol uses a GFSK (Gaussian Frequency Shift Keying), /4 DQPSK (Differential Quadrature Phase Shift Keying), or 8DPSK (8-Differential Phase Shift keying) modulation schemes.

A first factor that causes the increased number of collisions is the presence of hidden nodes which, due to their limited communication range, can send packets simultaneously with other nodes. In order to settle the issue of the hidden node, the 802.11 standard provides another medium access control strategy. Since the channel must be clear both at the transmitter and at the receiver, the former will send the message only when the channel is clear. At that moment, it sends an RTS (Request To Send) frame and waits for a reply for a while, resending the request if too much time elapses. If clear, the receiver replies with a CTS (Clear To Send) frame. After receiving the CTS, the transmitter sends the packet and then awaits the receiver’s confirmation [11]. The Bluetooth communication technique addresses a frequency hopping mechanism where the channel access technique complies with the master-salve topology.

The IEEE 802.15.4 standard specifies the CSMA/CA mechanism as channel access technique. Delays and the possibility of retransmitting a packet are often directly related to the number of nodes in the network and the interferences affecting the environment. The CSMA/CA mechanism provides that every time a device wishes to send a message, it must first make a CCA (clear channel assessment) operation in order to make sure that the channel is clear and transmission can be initiated [12]. The CCA operation can be completed by using ED (energy detection), CS (carrier sense) or a combination of these two methods. If CS mode is selected the channel is declared occupied only if an 802.15.4 signal is detected. Another technique of clearing the channel is to measure the spectrum energy of the communication channel through the ED technique which doesn’t require the identification of the type of signal on the channel through demodulation. If the channel is busy, a second attempt will be made at a later time. A second channel access technique is the use of the GTS (guaranteed time slots). Thus, the PAN (Personal Area Network) network coordinator allocates a time slot to each device for transmissions. This access method is possible only if the network uses the beacon synchronization message. One disadvantage of this method is that the operating life of a battery of a device will be severely reduced as it frequently switches to active mode in order to receive the synchronization message. The packet collision probability is much higher if the network doesn’t use synchronization messages and, implicitly, GTS. The channel that is accessed through a CSMA/CA scheme can be accompanied by a channel access mechanism based on GTS (Guaranteed Time Slots). If the IEEE 802.15.4 adopts a

CSMA/CA channel access technique, when the carrier sensing mechanism is used in order to determine the state of the channel, instead of ED (Energy Detection), the Bluetooth nodes are hidden because they use different carrier frequencies. Since the Bluetooth communication uses frequency hopping through a Time Division Duplex access scheme (FH/TDD), the packets will be transmitted regularly, without checking the availability of the channel. Thus, the Bluetooth devices will be sending packets without considering the IEEE 802.15.4 interferences. Another factor that increases the probability of resending the packets is the fact that all the channel access techniques and collision avoidance techniques are designed to work within the same system but not between different systems.

IV. PROPUSED TEST SETUP

The purpose of this paper is to assess the performance of an IEEE 802.15.4 network that operates in the proximity of a 802.11g network and a Bluetooth network. The parameter under analysis is the PER (Packet Error Rate), expressed in percentages and calculated as the ratio between the total packets sent and received (by monitoring the ACK received in response) [13]. The packets are sent by using the CCA (clear channel assessment) mechanism, defined by the 802.15.4 standard. Should PER be 0%, no packets are lost, while a 100% PER signifies that no packet has been received. During the tests, the separation distance (between the 802.15.4 network and the 802.11 or the Bluetooth network) and the number of retransmissions of a packet is varied, while the effects on the PER parameter are assessed and analyzed. It is important to mention the fact that PER values we consider to be acceptable range between 0.3% and 3% for reaching an average performance level for the 802.15.4 network.

A. IEEE 802.15.4 and Bluetooth

Fig. 2 presents the suggested test scenario. The chosen distance between the Bluetooth nodes and the one between the IEEE 802.15.4 nodes is set to 0.5 m. The distance between the two networks (d) is 0.5m and 1m as shown in Fig. 2. The selected IEEE 802.15.4 module is Jennic JN5148 with a transmission power of 2dBm while the Bluetooth adapter 2.0 is a TRENDnet High Power USB TBW-102UB.

Figure 2. Test scenario for assessing the coexistence of IEEE 802.15.4

and Bluetooth

Fig. 3 shows the values of the PER parameter for each IEEE 802.15.4 channel, by taking into account the transmission of 5000 and 10000 packets with a payload of 100 bytes. In order to determine the value of the PER parameter three measurements are made and then the maximum value obtained is selected.

Figure 3. PER for IEEE 802.15.4 and Bluetooth coexistence at 0.5m

distance

The analysis of the values of PER obtained and presented above shows that the influence of the Bluetooth communication on the performance of the IEEE 802.15.4 network is not significant, as it hasn’t been severely affected. For a separation distance between the networks (d) of 0.5m, the value of the PER parameter is of maximum 1.5%. The same topology is used when the tests are again conducted for a separation distance (d) of 1m between the two networks. In this case, the PER value is of maximum 0.8% as shown in Fig. 4.

Figure 4. PER for IEEE 802.15.4 and Bluetooth coexistence at 1m

distance

According to the data collected for high performance in the presence of a Bluetooth interference, a separation distance (d) of more than 1m is recommended, as the PER value will not exceed 1%.

B. IEEE 802.15.4 and 802.11g

Fig. 5 presents the test scenario for assessing the coexistence between IEEE 802.15.4 and 802.11b networks.

Figure 5. Test scenario for assessing the coexistence between IEEE

802.15.4 and 802.11b

The wireless 802.11 access point is a Wavion WBS-2400 station, operating in the 2.4 GHz band. The station has an array of 6 omni-directional antennas of 7.5 dBi with a -105.5 dBm sensitivity and operating in 802.11g mode [14]. The WBS-2400 is connected to a FTP server as part of the test and a user is connected to the station and is running a FTP Client application. Thus, a large file is transferred and the channel usage exceeds 50%. The distance between the user and the station is established at 1m, the same as the one between the IEEE 802.15.4 nodes. The variation distance (d) between the two networks, 802.15.4 and 802.11, is set to 1, 2, 5 and 6m, respectively. All the measurements have been conducted in an environment where there are no other wireless networks or interferences, by using the RF environment scanning function integrated in the WBS-2400 station. The PER is analyzed for a number of 5000 packets generated. The tests have been conducted under the worst case scenario presumption which would entail the occurrence of the channel overlapping phenomenon. The 802.11 operating channel has been chosen at 1, which has the central frequency of 2412 MHz. This channel overlaps 802.15.4 channels numbered 11, 12, 13 and 14, respectively.

Fig. 6 presents the PER variation when the number of retransmissions of a packet (R) varies from 0 to 7 at a 1m separation distance (d). For R=0, the PER parameter reaches 68% and, as the number of retrials increases up to 7, the parameter reaches the minimum value of 15%.

Figure 6. PER for various Retry(R) values

The IEEE 802.15.4 standard states that the number of retransmission to be set to three. The maximum value measured for PER reaches 37% for R=3, thus triggering a series of problems and a low performance level for the 802.15.4 communication. The PER value is clearly high, for a 1m separation distance, and the 802.15.4 network performance is severely affected. For an acceptable PER value, the number of retransmissions must be considerably higher. Fig. 7 presents the measured values of the PER parameter for separation distances (d) of 2, 4 and 6m, where the number of retransmissions of a packet has been established at 3.

Figure 7. PER for 2, 4, and 6 m separation distances

The PER value reaches the maximum value 17% at a 2m separation distance, and 4% for a 4m distance respectively, but both values are not considered as acceptable (0.3-3%) and, therefore, maintain a low performance level of the 802.15.4 communication. In Fig. 6 and 7, only the channels from 11 to 14 were assessed, because the PER parameter measured for the 15 channel is below 1% due to the fact that there are no overlapping channels.

The offset between the center frequency of 802.11g channel number 1 (2412 MHz) and 802.15.4 frequency channel number 14 (2420 MHz) is 8 MHz. For an offset frequency of 8 MHz between the central operating frequencies at a distance of separation d = 2 m, the PER parameter value is 10%, which represents a low performance level of the 802.15.4 communication. If the selected 802.15.4 channel is number 15, the offset of the central frequencies of the operating channels is 13 MHz. Thus, according to the obtained results, if there is an offset of more than 8 MHz between the central channel operating frequencies, the PER parameter is within acceptable ranges because the overlapping channels phenomenon is clearly avoided. If the offset condition cannot be avoided and channel overlapping is inevitable, a physical separation of at least 6 m between the 802.11 and the 802.15.4 networks is required in order to reach a PER value below 1% and, implicitly, a high performance level.

V. CONCLUSIONS

The analysis of the coexistence of the 802.15.14 networks with the Bluetooth and the 802.11g networks should not be neglected, and certain solutions must be found without severely damaging the performance level. The present paper suggests a practical approach through the assessment of the PER parameter at various separation distances between networks and by varying the number of retransmissions of the packets. According to the results collected for high performance of 802.15.4 communication in the presence of a Bluetooth interference, a separation distance of more than 1m is recommended, as the PER value will not exceed 1%. The tests conducted in order to assess the coexistence problem between 802.15.4 and 802.11 networks have been developed for the worst case scenario, i.e. the occurrence of the channel overlapping phenomenon and a channel 802.11 usage of over 50%. The 802.11g channel is chosen as number 1. The PER parameter has been measured and analyzed for the 802.15.4 overlapping channels 11, 12, 13 and 14, respectively.

If the number of retransmissions of a packet vary from 0 to 7, and the separation distance between the 802.11 and 802.15.4 network is 1m, the value of the PER parameter will drop from 68% to 15%. The PER value reaches the maximum value 17% at a 2 m separation distance, and 4% for a 4m distance respectively, but both values are not considered as acceptable (0.3-3%) and, therefore, maintain a low performance level of the 802.15.4 communication. In order to reach an acceptable PER value of 0.1-3%, the offset between the centrals operating frequencies of 802.11 channels and the IEEE 802.15.4 channel should be higher than 8 MHz. If the offset condition cannot be met and

channel overlapping is inevitable because of the congested and overcrowded RF environment, a physical separation of at least 6 m between the 802.11 and the 802.15.4 networks is required in order to reach a PER value below 1% and, implicitly, a high performance level.

ACKNOWLEDGMENT

This paper was supported by the project "Improvement of the doctoral studies quality in engineering science for development of the knowledge based society-QDOC” contract no. POSDRU/107/1.5/S/78534, project co-funded by the European Social Fund through the Sectorial Operational Program Human Resources 2007-2013.

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