FEATURE WIRELESS TECHNOLOGY
WIRELESS SENSOR CHALLENGES T
he wireless channel is unreliable in nature, and a number of phenomena
can prevent a transmitted packet from reaching a receiver. One such phenomenon is interference. If two independent transmitters transmit on the same channel such that their signals overlap, they may corrupt each other’s signal at a receiver’s radio. This requires the transmitter to retransmit, at the cost of time and energy. Interference can come from the same
network if the underlying medium access technology does not schedule contention-free communications. This is particularly problematic if the two transmitters can hear the receiver, but not hear each other - this is known as the ‘hidden terminal problem’, and it requires backoff and acknowledgement mechanisms to resolve collisions. Interference can also come from another network operating in the same radio space, or from a different radio technology using the same frequency band. The latter, known as ‘external’ interference, is especially present in unlicensed bands such as the 2.400- 2.485GHz instrumentation, scientific, and medical (ISM) band, crowded with WiFi, Bluetooth and 802.15.4. Figure 1 (top) was obtained by deploying
45 802.15.4 nodes in an office environment, and having them exchange 12 million packets, equally distributed over 16 802.15.4 channels. It plots the average packet delivery ratio of those packets as a function of the channel they are transmitted on - on channels overlapping WiFi channels, this delivery ratio is lower. A second phenomenon, multipath
fading, shown in Figure 2 (below), can prevent a transmitted packet from
Ross Yu, product marketing manager, Dust Networks Product Group, Linear Technology Corporation, details the key wireless sensor network challenges that industry faces today
reaching a receiver and is both more destructive and harder to quantify. Often described as ‘self interference’, this occurs when the recipient receives both the signal travelling over the line- of-sight path from the transmitter, as well as ‘echoes’ of the same signal that have bounced off objects in the environment (floors, ceilings, doors, people, etc). Since those copies travel different distances, they reach the receiver at different times, potentially interfering destructively. Fades of 20- 30dB are not uncommon.
MULTIPATH Figure 2 was obtained by having a transmitter transmit 1,000 packets to a receiver five metres away, and repeating this with the receiver positioned at each point in a 35cm by 20cm grid. The z-axis
represents the packet delivery ratio over that link. While the link is good at most positions, at some positions no packets are received successfully because of multipath fading. Multipath fading depends on the position and nature of every object in the environment, and is unpredictable in any practical set-up. One good property is that the ‘topography’ depicted in Figure 2 changes with the frequency. That is, if a packet is not received because of the multipath fading, retransmitting on a different frequency is likely to succeed. Because objects in the environment are not static, e.g. cars drive by and doors are opened and closed, the effect of
multipath changes over time. Figure 3 (above) shows the packet delivery ratio on a single wireless path between two industrial sensors over the course of 26 days, and for each of the 16 channels used by the system. There are weekly cycles where workdays and weekends are clearly visible. At any given time some channels are good (high delivery), others are bad, and still others vary greatly. Channel 17, while generally good, has at least one period of zero delivery. Each path in the network shows qualitatively similar behaviour, but with different channel performance, and there is never any one channel that is good everywhere in the network. Because of interference and multipath
fading, the key to building a reliable wireless system is to exploit channel and path diversity.
Linear Technology
www.linear.com T: 01628 477 066
20 FEBRUARY 2015 | AUTOMATION
Enter 208 /AUTOMATION
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56