IEEE 802.15.4 Low Rate –Wireless Personal Area Network Coexistence Issues Ivan Howitt Jose A. Gutierrez Electrical & Computer Engineering Dept. RF/Communications Group University of North Carolina at Charlotte Innovation Center - Eaton Corporation Charlotte, NC 28223-001 4201 North 27th Street [email protected] Milwaukee, WI 53216 [email protected] Abstract – IEEE 802.15.4 is a proposed standard addressing the in order to provide WLAN support. Wireless devices based needs of low-rate wireless personal area networks or LR-WPAN with on these two standards are likely to be collocated and a focus on enabling wireless sensor networks. The standard is therefore their ability to coexist needs to be evaluated. Central characterized by maintaining a high level of simplicity, allowing for to the coexistence issue between wireless devices is the ability low cost and low power implementations. Its operational frequency to differentiate between operational conditions which will and band includes the 2.4GHz industrial, scientific and medical band will not result in the communication devices failing to meet providing nearly worldwide availability; additionally, this band is the requirements of an application. also used by other IEEE 802 wireless standards. Coexistence among Section II of this paper presents a technical overview of the diverse collocated devices in the 2.4 GHz band is an important issue proposed IEEE 802.15.4 standard. In Section III, an analytical in order to ensure that each wireless service maintains its desired model is presented to provide an approach for evaluating the performance requirements. This paper presents a brief technical coexistence between IEEE 802.15.4 and IEEE 802.11b. introduction of the IEEE 802.15.4 standard and analyzes the Conclusions are presented in Section IV. coexistence impact of an IEEE 802.15.4 network on the IEEE 802.11b devices. II. TECHNICAL OVERVIEW I. INTRODUCTION A summary of the high-level features of the IEEE 802.15.4 is shown in Table 1. With the success of wireless local area networks (WLANs), By favoring low-cost and low-power, IEEE 802.15.4 is the wireless networking community has been focused on enabling applications in the fields of industrial, agricultural, enhancing WLAN capabilities and developing new vehicular, residential and medical sensors and actuators. Until approaches to meet the needs of the growing pool of recently, these applications could not make use of current applications requiring wireless devices. In addition, there is a wireless technologies or would have to use proprietary movement towards standardized protocols and away from solutions (in most cases unidirectional) [2,3]. applications requiring inflexible wireless connectivity often The intent of IEEE 802.15.4 is to address applications based on proprietary technologies. Recently, the concept of a where existing WPAN solutions are too expensive and the standardized low rate wireless personnel area network (LR- performance of a technology such as BluetoothTM is not WPANs) has emerged [1-4]. Fuelled by the need to enable required. IEEE 802.15.4 LR-WPANs complement other inexpensive wireless sensor network applications, in WPAN technologies by providing very low power December 2000 Task Group 4, under the IEEE 802 Working consumption capabilities at very low cost, thus enabling Group 15, was formed to begin the development of a LR- applications that were previously impractical. Table 2 WPAN standard IEEE 802.15.4. The goal of Task Group 4 is illustrates a basic comparison between IEEE 802.15.4 and to provide a standard which has the characteristics of ultra-low other IEEE 802 wireless networking standards. complexity, low-cost and extremely low-power for wireless The IEEE 802.15.4 standard is being designed to be used in connectivity among inexpensive, fixed, portable and moving a wide variety of applications which require simple wireless devices [1]. communications over short-range distances with limited The IEEE 802.15.4 devices are proposed to operate in the 2.4 GHz industrial, scientific and medical (ISM) band. The same operational band used by other IEEE 802 wireless Table 1: IEEE 802.15.4 High Level Characteristics devices, such as IEEE 802.11b (WLAN) and IEEE 802.15.1 Low-Band 868 MHz 1 channel - 20 kb/s (BPSK) 915 MHz 10 channels - 40 kb/s (Bluetooth). IEEE 802.15.4 and IEEE 802.11b standards Frequency Band Two PHYs High-Band 2.4 GHz 16 channels - 250 kb/s support complimentary applications; e.g., IEEE 802.15.4 (O-QPSK) devices used to support a wireless sensor array within a home Channel Access CSMA-CA and slotted CSMA-CA or industrial complex could be collocated with IEEE 802.11b Range 10 to 20m Addressing Short 8 bit or 64-bit IEEE 0-7803-7700-1/03/$17.00 (C) 2003 IEEE 1481 power and relaxed throughput needs. IEEE 802.15.4 facilitates between the two PHYs is the frequency band. The 868/915 Wireless Sensor Networks (WSNs) with the goal of reducing MHz PHY (also called low-band) is specified for operation in the installation cost of sensors and actuators while enabling the 868 MHz band in Europe offering one channel with a raw sensor-rich environments. data rate of 20 kb/s and the 915 MHz ISM band in North America offering 10 channels with a raw data rate of 40 kb/s. Table 2: IEEE 802.15.4 High Level Characteristics The low-band uses binary phase shift key (BPSK) modulation. 802.11b WLAN 802.15.1 WPAN 802.15.4 LR-WPAN The 2.4 GHz PHY (also called high-band) specifies operation in the 2.4 GHz ISM band, with nearly worldwide Range ~100 m ~10 - 100 m 10 m availability. This band spans from 2.4 to 2.483 GHz and offers Raw Data Rate 11 Mbps 1 Mbps <= 0.25 Mbps 16 channels with channel spacing of 5 MHz, operating with a Power Consumption medium low ultra low raw data rate of 250 kb/s using offset quadrature phase shift key (O-QPSK) modulation. A. LR-WPAN Design The IEEE 802.15.4 standard specifies a receiver sensitivity A main design consideration for LR-WPANs is low power of -85 dBm for the 2.4 GHz band and -92 dBm for the consumption, thereby maximizing battery life. To achieve low 868/915 MHz band. Practical implementations are expected to average power consumption, IEEE 802.15.4 assumes that the improve this requirement. The standard specifies a transmit amount of data transmitted is short and that it is transmitted power capability of 1 mW, although it can vary within infrequently in order to keep a low duty-cycle. In addition, the governmental regulatory bounds. packet structure was designed to add minimal overhead over Both PHY layers use a common packet structure, enabling the transported payload. the definition of a common MAC interface. Each packet, or The standard allows the formation of two possible network PHY protocol data unit (PPDU), contains a preamble, a start topologies: the star topology or the peer-to-peer topology, of packet delimiter, a packet length, and a payload field, or Figure 1. In the star topology, the communication is performed PHY service data unit (PSDU). The 32-bit preamble is between network devices and a single central controller, called designed for acquisition of symbol and chip timing. The IEEE the PAN coordinator. A network device is either the initiation 802.15.4 payload length can vary from 2 to 127 bytes. This point or the termination point for network communications. structure is shown in Figure 2. The PAN coordinator is in charge of managing all the star PAN functionality. In the peer-to-peer topology, every PHY protocol data unit network device can communicate with any other within its (PPDU) Start of Length PHY layer payload range. This topology also contains a PAN coordinator, which Preamble packet Field PHY service data unit (PSDU) acts as the root of the network. Peer-to-peer topology allows delimiter 4 bytes 1 byte 1 byte 2-127 bytes more complex network formations to be implemented; e.g. ad hoc and self-configuring networks. The routing mechanisms Figure 2: IEEE 802.15.4 Packet Structure required for multi-hopping are part of the network layer and are therefore, not in the scope of IEEE 802.15.4. C. MAC sublayer An IEEE 802.15.4 LR-WPAN device is composed of a The IEEE 802 project divides the data link layer (DLL) in to physical (PHY) layer and a medium access control (MAC) two sublayers, the MAC and logical link control (LLC) sublayer that provides access to the physical channel for all sublayers. The LLC is standardized in IEEE 802.2 and is types of transfer and ensures the reliable transfer of frames. common among the IEEE 802 standards. The IEEE 802.15.4 medium access control (MAC) sublayer controls the access to the radio channel employing the CSMA- CA mechanism. If upper layers detect that the communications throughput has been degraded below a determined threshold, the MAC will be instructed to perform Star an energy detection scan through the available channels. Peer-to-peer Based on the detected energy, the upper layers will switch to Figure 1: Star and peer-to-peer topology (organized as a clustered network) the channel with the lowest energy. The IEEE 802.15.4 performs the energy scan by the use of a clear channel B. PHY Layer assessment procedure. This can be performed by following IEEE 802.15.4 was designed to support two PHY options either a simple in-band energy detection above a threshold, or based on direct sequence spread spectrum (DSSS); this an IEEE 802.15.4 carrier detection or a combination of both.