Question 1. What is the typical application of the communication technology that you choose to study? Give some examples and pictures. What products use this technology?
IEEE 802.15.4 standard is intended to accommodate the residential, medical and industrial sectors. The standard was developed via a partnership between the Zigbee Alliance and the IEEE 802.15.4 committee.
The protocol is a low data rate Wireless Personal Area Network (WPAN) which provides low cost and low power consumption for devices that require battery life as long as several months and up to several years.
Another important aspect of the standard is that it supports the implementation of mesh networks. A mesh network is where each node in the network not only maintains its own data collection but can also serve as bridge for other nodes to relay information to a upstream/downstream endpoint.
Figure 1: Zigbee Protocol Network Example
Some products that use this technology are:
· SafePlug 1202
o gives power monitoring and other features such as on/off control and Appliance tracking
· Smart Gateway
o Connects “smart meters” to a collection server via a wireless communication in a smart grid application
· Digi: XBEE wireless radios
o Allows Drop-in-Networking for wireless and embedded systems solutions
Question 2. What is the frequency band allocated by FCC for this technology?
The Physical Layer or PHY provides the data transmission routine. The PHY maintains the actual RF transceiver and manages the selection of the RF channel and energy/signal strength routines.
The PHY can operate at the following unlicensed frequency bands:
· 868.0-868.6 MHz: Europe, allows one communication channel (2003, 2006)
· 902-928 MHz: North America, up to ten channels (2003), extended to thirty (2006)
· 2400-2483.5 MHz: worldwide use, up to sixteen channels (2003, 2006)
From the IEEE Std 802.15.4™-2006 document:
IEEE 802.15.4 regulatory requirements
“In the United States, the regulations are given under FCC CFR47 [B29]. FCC rules contained there provide allocations in the 260–470 MHz range (see Section 15.231 of FCC CFR47) and in the 902–928 MHz and 2400–2483.5 MHz ISM bands. In the ISM bands, Section 15.249 of FCC CFR47 provides for narrowband operation of up to approximately 1 mW effective radiated power (ERP) (if wideband, then 1 mW per 100 kHz below 1000 MHz, and 1 mW/MHz above 1000 MHz), and Section 15.247 of FCC CFR47 provides for wideband operation at up to 1 W transmitted power.”
Figure 2: IEEE 802.15.4 Protocol Stack
Figure 3: Schematic of the beacon frame and the PHY packet
Question 3. Who is the standard body? Where can one find the documents of the standard? Which part of the document is about physical layer?
As mentioned prior, the standard body is IEEE 802.15.4 in conjunction with the Zigbee Alliance. Inside the IEEE 802.15.4 document, the physical layer is addressed in section 5.4.1.
Question 4. Show/draw and explain system block diagrams that process information bits, perform error control encoding, and modulation.
Figure 4: Error Control Encoding
Question 5. What error control codes are used? (Convolutional codes, turbo codes, LDPC codes, Reed-Solomon codes, etc.)
Each 4-bit symbol is mapped to a 32-chip symbol that is RF modulated. Upon demodulation, a Hamming distance check is applied to find the closest symbol to the received chip sequence. This is designed for efficient implementation and not for optimal error correction.
Question 6. What constellations (BPSK, QPSK, 8PSK, 16-QAM, etc.) are used?
This standard is a revision of the 2003 edition. The revision specifies the following four PHY’s and is quoted as the following from the IEEE 802.15.4 document:
1. An 868/915 MHz direct sequence spread spectrum (DSSS) PHY employing binary phase-shift keying (BPSK) modulation 1
2. An 868/915 MHz DSSS PHY employing offset quadrature phase-shift keying (O-QPSK)modulation 1
3. An 868/915 MHz parallel sequence spread spectrum (PSSS) PHY employing BPSK and amplitude shift keying (ASK) modulation 1
4. A 2450 MHz DSSS PHY employing O-QPSK modulation 1
Figure 5: 868/915 MHz PHY using BPSK and DSSS
Figure 6: 868/915 MHz PHY using BPSK or ASK
Figure 7: 868/915 MHz PHY using O-QPSK (Note: this scheme is also used for the 2.4 GHZ band as well)
Question 7. Is IQ modulation used? What is in the I part of the base band complex signal and what is the Q part of the base band complex signal?
The first example of IQ modulation used in this standard is in the BPSK case. In binary phase shift keying, the carrier phase is shifted 180 degrees to signify a change from 0 to 1 or from 1 to 0. The BPSK lies only on one axis. The y-axis does not contain a projection. Thus the I and Q projections are the x and y axis projections.2
Figure 8: BPSK and IQ Modulation 2
Another example of the IQ modulation used in the above PHY’s is in the case of the O-QPSK modulation. In Offset QPSK, the I and Q bit streams are offset by one bit period. At any given time only one of the two bit streams can change values. This arrange gives increases in power efficiency.3
Figure 9: IQ "Offset" Modulation 3
Question 8. What kind of pulse shape is used? Can you show a picture of the pulse shape? Where can one find the pulse shape in a document of the standard?
Figure 10: 802.15.4 Pulse Shape for the 868/915 MHz PHY using O-QPSK
This information can be found on page 49 of the IEEE standard PDF.
Question 9. What multi-access and broadcast technologies are used? (TDM(A), CDM(A), FDM(A), OFDM(A), etc.) How do they work? (Multi-access, or uplink, means that multiple users send signals to the base station. Broadcast, or downlink, means that a base station sends different signals to different users.)
Uses both uplink and downlink where base station can send a signal to multiple devices at once in a broadcast mode setting. In addition the base station can receive multiple signals at once from endpoints/routers.
The 801.15.4 uses a multi-code modulation technique called parallel sequence spread spectrum (PSSS). This is commonly known as orthogonal code division multiplexing (OCDM).
Question 10. Does it use multi-antenna technology? How?
It does not use multi-antenna technology. The 801.15.4 protocol uses a single antenna per module.
Question 11. Is the standardization on-going? Which companies are the major players?
The Zigbee Alliance is a large committee that utilizes this protocol. They have comprised roughly 40% of the 2010 IEEE 802.15.4 chipset shipments and these numbers continue to grow.
Some major players are Texas Instruments, Philips, Emerson, Freescale, Kroger, and many other companies that continue to develop this technology.