Weightless System basic Overview
As we know internet of things have become popular for machine to machine communication. Due to requirement of low cost, longer battery life and use of existing TV white space spectrum has pioneered development of many wireless standards. The Whitespace radio spectrum is made available by the regulatory authority for unlicensed use. This falls under UHF band approx. between 470 MHz to 790 MHz frequency range based on country. Typically TV broadcasting uses 8MHz channel in Europe and 6MHz in US. Weightless standard has been developed to fulfill these requirements of the IoT system. This standard meets the basic machine requirements as outlined below.
• Need to support large number of terminals.
• Longer battery life, atleast about 10 years
• Low network and terminal costs, about $2 per chip or even less
• Efficient Small burst transmission, order of about 50 bytes
• Strong authentication as well as encryption for reliable guaranteed delivery
• Operation in TV white space which has many merits such as low levels of out of band emissions
Like other wireless systems, weithtless system consists of two main parts viz. network side systems and user terminals. It consists of base station, internet backbone systems on the network side and terminal having weightless radio module as well as protocol stack on the user side. Weightless system frame stucture is similar to wimax system frame structure having TDD topology.
Weightless frame consists of preamble followed by downlink part and uplink part. Downlink refers to transmissions from base station to one or more terminals. Uplink refers to transmissions from terminals to the base station. Usually weightless frame is of duration of about 2second. The frame is divided into slots which is the smallest possible length of resource allocation to the terminals in the weightless system.
The weightless system operates with single carrier modulation, broadband downlink, narrowband uplink, direct sequence spreading, long frame size, frequency hopping at the frame rate.
Following table-1 mentions the key specifications of the downlink in the weightless system designed to be used in IoT network.
|Specification||Weightless system support|
|Multiple Access scheme||TDMA, refer TDD and TDMA|
|Modulation scheme||Single carrier
16QAM, π/4 QPSK, π/2 BPSK
|Pulse shaping||RRC (beta = 0.4)|
|Chip rate|| 5 Mcpc for 8 MHz channels,
4 Mcpc for 6 MHz channels
|3dB Signal bandwidth||5MHz for 8MHz channels
4MHz for 6MHz channels
|Spreading factor||1 to 1023, provides processing gain of 30dB value|
|FEC coding scheme||None, rate 1/2(convolutional encoding) , rate 3/4|
|Interleaving||available when FEC encoding is available|
|Whitening||It is based on LFSR(Linear Feedback Shift Register), the seed value is derived from channel number and frame number|
|Frequency Range||UHF band|
|Channel Bandwidth|| 6MHz(America, South korea, Taiwan, Philippines, Japan)
8MHz(Europe, Africa, Asia, Oceania)
|Frequency accuracy|| The Base Station will maintain a frequency tolerance of +/- 1 ppm
When transmitting to the Base Station, a Terminal must achieve a frequency error of less than +/- 100Hz relative to the downlink reception portion of the same frame.
There are two uplink modes specified in weightless system viz. NB-FDMA(narrowband FDMA) and WB-FDMA (wideband FDMA). Both of these modes are single carries and uses burst format as specficied in the standard. Terminals are required to support either of these modes. If both are suppoted then it is advantageous. Following table-2 summarizes key specifications of both of these modes.
|Specification||Narrowband Uplink(NB-FDMA)||Wideband Uplink(WB-FDMA)|
|Multiple Access Scheme||Combined FDMA and TDMA||Combined FDMA and TDMA|
|Modulation scheme||Single carrier, 16QAM, π/4 QPSK, π/2 BPSK,
single carrier CPM(continuous phase modulation)
|Single carrier, 16QAM, π/4 QPSK, π/2 BPSK,
single carrier CPM(continuous phase modulation)
|Pulse shaping||RRC (Beta=0.4), no pulse shaping for CPM||RRC (Beta =0.4), no pulse shaping for CPM|
|Chip Rate||5/32 Mcps for 8 MHz channels
4/32 Mcps for 6 MHz channels
|5 Mcps for 8 MHz channels
4 Mcps for 6 MHz channels
|3dB Sub channel bandwidth||5/32 MHz for 8 MHz channels
4/32 for 6 MHz channels
| 5 MHz for 8 MHz channels
4 MHz for 6 MHz channels
|Comb finger||-||comb finger bandwidth(3dB) : 5/2048 MHz for 8MHz and 4/2048 for 6 MHz channels
comb finger separation: 5/64 MHz for 8MHz and 4/64 MHz for 6 MHz channels
|No. of available subchannels||22||16|
|Subchannel centre frequency||(2*s+1)*5/32 MHz (8 MHz channels)
(2*s+1)*4/32 MHz (6 MHz channels)
for s = -11,-10,..+9,+10
| (2*s+1)*5/2048 MHz for 8 MHz channals
(2*s+1)*4/2048 MHz for 6 MHz channels
for s = -8,-7,...,+6,+7
|Spreading factor||1 to 256 for PSK/QAM mode,
16 to 256 for CPM mode
|1 to 256 for PSK/QAM mode,
16 to 256 for CPM mode
|FEC coding scheme||None, rate 1/2(convolutional encoder), rate 3/4||None, rate 1/2(convolutional encoder), rate 3/4|
|Interleaving||incorporated in the chain when FEC is enabled||incorporated in the chain when FEC is enabled|
|Whitening||Based on LFSR(seed depends on frame number)||Based on LFSR(seed depends on frame number)|
|Frequency accuracy||+/-100Hz for terminal transmission relative to the received downlink frame||+/-100Hz for terminal transmission relative to the received downlink frame|
Fig-1 Weightless Network
As shown in the figure terminal(e.g. smart meter) will pass the reading information to the weightless PHY/radio module which encodes and transmits the information over weightless air interface. There are various modules in the weightless terminal transmitter. Typically it provides modulation, spreading and forward error correction functionalities to the data. Base station receiver receives the downlink frame and does the decoding and passes the frame to the core internet backbone network as per appropriate format. The informations transmitted by the terminals are routed via synchronization database to the clien information/management system. Fig-2 depicts protocol layers at each of this weightless system entities. This is referred as weightless protocol stack.
Weightless system channel types
As we know informations between various protocol layers in any system are exchanged using channels. The channels are categorized based on their functionalities such as control channels, logical channels, transport channels and physical channels.
There are three physical channels for baseband data processing in weightless system. They are downlink channel, uplink channel and uplink contended access channel.
The downlink physical channel is used to transmit data from
base station to one or multiple terminals in a cell.
The uplink physical channel is dedicated channel used to transmit data from terminal/terminals to the base station.
The uplink contended access physical channel is used for transmission of data from terminal to the base station. This channel is used for obtaining access of the network. This channel is used by all the terminals at the same time, hence the name.
Weightless system uses frequency hopping between the channels at frame rate. Hence in weightless system both the uplink and downlink portions of a weightless frame will use same channel.
Weightless protocol layers
Fig-2:Weightless protocol stack
The figure above mentions protocol layers of the weightless system at various entities such as
terminal, base station, data base and client system.
The core part of weightless system specifications is the MAC, PHY and radio layers. As per the requirement MAC layer forms the frame after inserting required header and CRC. This MAC frame is passed to the PHY layer which incorporates its header used for synchronization (time, frequency and channel) to the MAC payload. Weightless PHY layer also incorporates functionalities such as FEC, modulation, spreading, CP insertion and pulse shaping to the data need to be transmitted over the air. This PHY encoded data is passed to the radio layer for necessary radio frequency conversion as per RF channel allocated. Refer weightless physical layer for detailed description of weightless downlink PHY chain.
Refer our article on basics of OSI and TCPIP layers to understand functionalities of physical layer, MAC layer as well as upper layers.