The Jennic JN5148 series IEEE 802.15.4 transceiver, including ZigBee PRO (Zig- Bee Compliant Platform) is an ultra low power, low cost wireless micro-controller that operates in 2.4GHz ISM band (λ = 0.125m) [1]. It uses 2MSps direct se- quencing spread spectrum (DSSS) with each symbol mapped to a 32 − chip PN sequence. The 32 − chip sequence represents each data symbol and therefore chip rate can be given as 32 times the symbol rate (62.5 kSps), and symbol duration is given as 1/symbol rate = 16 µs. The bandwidth of Zigbee is 2 MHz. A built- in ranging engine based on Time of Flight (ToF) calculates the time-of-flight of a radio signal between two wireless nodes using the two-way (round trip) ToF ranging. Its integrated power control system enables the system power consump- tion to be controlled carefully using different modes (i.e., active processing mode, sleep mode, deep sleep mode) to maximise battery life, hence network life. A 32-bit load and store RISC processor help to minimise the power consumption
for battery powered application and implementation of protocols with high per- formance and high level efficient programming. Table 3.1 lists the specification of IEEE 802.15.4 Compliant Device [19, 24].
Parameter Notation Value
Radio Frequency RF 2.4GHz
Spread Spectrum Ss DSSS
Data Rate Dr 250kbps
Transmit Power Pt 0dBm (1mW)
Modulation Scheme Ms OQPSK
Chip Spreading Sequence PN 32 chips
Symbol Sm PN = 32 chips
Symbol Rate Sr 62.5 kHz (ksymbols/s)
Symbol Duration Time Ts 1/Sr=16µs
Chip Rate Rc Sr× 32 = 2 Mchip/s
Chip Period Tc 1
Rc = 500 ns
Clock Rate (JN5148) fs 16MHz
Antenna Type Folded-monopole
Table 3.1: Specification of IEEE 802.15.4 Compliant Devices
3.4.1
Antenna Models
Anisotropy is a common and non-negligible phenomenon in wireless networks caused by different factors such as, antenna type, antenna gain, and environment dependant path loss [94,98]. In the age of compact devices, a light weight, small- sized and inexpensive antenna plays a important role to overcome the deployment concerns of external mounted antennas. There are many situations in which small size is important (particular in hand-held equipment). There are two different variants of Jennic’s wireless modules: modules with an integrated antenna and modules with an external antenna. Experimental results discussed in this work are based on the Jennic wireless modules those with an integrated antenna. The JN5148 modules with integrated antenna are based on a folded-monopole, omni- directional characteristic [1]. Integrated antenna are useful for many application (i.e. child locating solution, where a sensor node as wristband with integrated
antenna can be secured on the wrist of the child, tracking expensive items in a super market) where mounting an external antenna is not feasible due to the much required space or volume.
3.4.1.1 Integrated Folded Mono-pole Antenna
The Jennic experimental units include an antenna with a folded-monopole, omni- directional characteristic [1]. The main radiating lobe of this 2.4GHz ISM band antenna is projected at right-angles to the PCB ground plane. Fig. 3.1 shows the measurement planes for the folded mono-pole antenna [3], whereas Fig. 3.2 shows the radiation pattern of the three measurement planes [3]. In order to optimize the received power, it is important to match the polarization for both receiver and transmitter antennas particularly when there is a LOS or directional antenna in use. Considering the fact that antenna polarization is more dynamic for mobile nodes, a static and approximately aligned configuration is considered between nodes.
Figure 3.1: Integrated Folded Mono-pole antenna measurement planes [3]. Fig. (a). XY-Plane Fig. (b). XZ-Plane Fig. (c). YZ-Plane
3.4.2
Experimental Setup for Ranging
The experiments have been performed in an indoor and outdoor environment with both LOS and NLOS conditions. For each condition three different sets of experiments have been performed with the transceiver nodes mounted on a
−24 −24 −18 −18 −12 −12 −6 −6 0 dB 0 dB 90o 60o 30o 0o −30o −60o −90o −120o −150o 180o 150o 120o (a) XY Plane (a) −24 −24 −18 −18 −12 −12 −6 −6 0 dB 0 dB 90o 60o 30o 0o −30o −60o −90o −120o −150o 180o 150o 120o (b) XZ Plane (b) −24 −24 −18 −18 −12 −12 −6 −6 0 dB 0 dB 90o 60o 30o 0o −30o −60o −90o −120o −150o 180o 150o 120o (c) YZ Plane (c)
Figure 3.2: Measured antenna radiation pattern [3]. Fig. 3.2(a). XY-plane radiation pattern polar plot, Fig. 3.2(b). XZ-plane radiation pattern polar plot and Fig. 3.2(c). YZ-plane radiation pattern polar plot
tripod at one of 3 specified heights of 0.5m, 1.0m, 1.5m. A Coordinator node was fixed (connected to laptop) at one side of the field whereas an End device is fixed at variable distances (i.e. 1m, 2m, · · · 15m). The actual distance between the nodes was measured using Leica Disto A5 laser distance meter [103]. For each separation and height between sensor nodes, a total of 700 ranging samples were collected (a total of 10, 500 for all separations). A coordinator node was used to capture, process and save the data on a laptop.
3.4.2.1 Outdoor Experimental Setup
The left side of Figure 3.3 illustrates the outdoor test site setup for LOS path. The outdoor experiments were performed in Hyde Park situated just beside the University of Leeds. For all LOS ranging samples, a clear LOS is maintained on a plane open grassy field with no trees or obstacles between or near the transceivers. The NLOS outdoor experiments are executed in the same park at a location where the direct LOS was completely blocked for each height with trees and wooden benches. The obstructions e.g. trees, were of the order of 1m thick so no LOS existed. The dimensions of the field are 120m×200m.
Figure 3.3: Outdoor experimental setup with two nodes, tripods and data logger laptop for range measurements.
3.4.2.2 Indoor Experimental Setup
The indoor experimental setup is illustrated on the right of Figure 3.4. The indoor experiments were performed in a corridor at Edward Boyle library at the University of Leeds. The dimensions of the corridor are ∼ 3.5 m x 110m and floor to ceiling height is 2.5m. For LOS experiments the transceiver were kept in a straight line in the centre of the corridor. The NLOS experiments were executed in the extreme left of the corridor where the direct LOS was blocked by the side concrete walls, furniture and people.