While spread spectrum techniques such as adaptive frequency hopping work well, other interference avoidance methods are more efficient in congested 2,4 GHz environments like gyms, as James Fujimoto of Dynastream explains.
While several ISM bands are available, only one - the 2,4 GHz band - is accepted virtually globally. This makes it the perfect choice for manufacturers exporting products worldwide.
Many standards-based radios - for example IEEE.802.xx compliant products such as Wi-Fi, Bluetooth and ZigBee - as well as proprietary forms of wireless Ethernet and wireless USB, all use the 2,4 GHz band.
This means that chief among the technical challenges of deploying 2,4 GHz radios is ensuring that the radio link is able to handle the inevitable interference in what is becoming a very crowded band. In fact, regulations governing the ISM parts of the spectrum state that "a device must expect interference".
My company, Dynastream Innovations, and our partner, Nordic Semiconductor, are no exceptions. Nordic designs a 2,4 GHz silicon radio, the nRF24AP1, which incorporates the ANT wireless personal area network protocol. This combination is ideal for ultra-low power wireless links comprising sensors and transceivers that can run for years without any need for battery replacement.
Designers of low-power radios have come up with some innovative methods to avoid interference using a combination of modulation and channel management techniques. The most notable methods of channel management are time domain multiple access (TDMA), frequency hopping (FH) and direct sequence spread spectrum (DSSS, also known as DS-CDMA). These channel management methods are combined with various modulation techniques such as Gaussian frequency shift keying (GFSK) and frequency shift keying (FSK) to enable a variety of solutions with different strengths to best address the application requirements.
TDMA channel management works by subdividing the communication frequency into a number of timeslots allowing several users to share the same frequency. Each transceiver waits for its own timeslot before transmitting, thus avoiding interference. We will talk about TDMA in more detail below, as it is the basis for the nRF24AP1/ANT interference avoidance technique.
DSSS modulation and channel management transmissions multiply the data being transmitted by a 'noise' component. This noise signal is a pseudorandom sequence of values, at a frequency much higher than that of the original 2,4 GHz signal, thereby spreading the energy of the original signal across a much wider band. The noise is filtered out at the receiving end to recover the original data, by again multiplying the same pseudorandom sequence by the received signal.
For recovery to work correctly, transmit and receive sequences must be synchronised. This requires the receiver to synchronise its sequence with the transmitter's sequence via some sort of timing search process. DSSS works at a cost of transmitting excessive data packets, incurring both bandwidth usage and current consumption overheads.
Bluetooth and Nordic products, including the nRF24AP1, use the GFSK modulation technique. This technique results in a 'dampened' or gentler frequency swing between the high ('1') and low ('0') levels. The result is a narrower and 'cleaner' spectrum for the transmitted signal compared with FSK.
Bluetooth manages interference through the combination of GFSK modulation and FH. Bluetooth splits the 2,4 GHz ISM band into 79, 1 MHz channels (with a 1 MHz guard channel at the lower end of the band and a 2 MHz guard channel at the higher end). Bluetooth 1.2 uses a revised form of frequency hopping dubbed adaptive frequency hopping (AFH). This algorithm allows Bluetooth devices to mark channels as 'good', 'bad', or 'unknown'. Bad channels in the frequency-hopping pattern are then replaced with good channels via a look-up table.
With FH, a wireless technology transmits on a clear channel until it experiences interference (resulting in lost packets), whereby it relocates to a clear channel. Alternatively, the transmitter can periodically pseudorandomly retune to a different channel to minimise the chances of encountering interference that could occur when transmitting on a single channel for long periods.
Unfortunately, while Bluetooth includes this provision for interference management, there is little flexibility for the designer beyond what the architects of the standard have provided. In contrast, Nordic Semiconductor's nRF24AP1 allows the designer to benefit from interference management using GFSK modulation and TDMA channel management, but also permits the simultaneous use of frequency hopping for the ultimate flexibility in interference management.
The Bluetooth AFH system works well when it experiences interference from other radio sources, for example Wi-Fi (typically used for wireless LANs - or WLANs) because it simply hops to an alternative channel. But when there are dozens, or even hundreds of competing sources, AFH has its weaknesses. For example, there are only 79 channels, so only a relatively limited number of competing sources can be accommodated.
Another issue is that hopping between channels typically occurs when interference has been detected; this inevitably means unacknowledged packets will need to be retransmitted once the channel has been changed, consuming bandwidth, increasing power consumption and shortening battery life. This is the main reason why wireless links in the lab often exhibit much greater data transfer rates than those used in real world, practical situations.
In contrast to Bluetooth, the nRF24AP1 uses a TDMA scheme. The nRF24AP1 has been specifically designed as an ultra-low power consumption radio with a built-in protocol solution. It is targeted at applications where batteries have to last for years, or even for the entire life of the product. Embedded with the ANT protocol, the nRF24AP1 is ideal for applications such as heart rate monitors communicating with 'intelligent' sports watches, or large numbers of temperature sensors embedded in the ceiling of an office building or warehouse all communicating with a transceiver elsewhere in a given room or area. For these kinds of applications there are often dozens, or even hundreds of sensors attempting to transmit information on the same frequency in a physically confined space.
This is not a co-existence that engineers typically consider. They mainly think of one wireless technology comfortably existing alongside another wireless technology. Rarely does the application designer think about similar types of sensors close together doing the same thing and having to work correctly in a small enclosed area.
Let us consider an example. A large, commercial gym could have 30 or 40 rowing machines side-by-side. Many of the machine users may be wearing heart rate monitoring belts transmitting to their sports watches on the 2,4 GHz frequency band. If Bluetooth was chosen to power the wireless links, its greater radio on-air time required to manage interference and accommodate the larger message overhead would quickly consume a standard coin-cell battery, the preferred choice of battery used to power these devices.
The nRF24AP1 transceiver embedded with ANT, however, is ideally suited to this application. Heart rate monitoring (HRM) is a low duty-cycle task that requires a wireless solution with an ultra-low current 'sleep' mode that quickly comes to life periodically for a 'burst' of information before going back into sleep mode again. The nRF24AP1 is optimised for just this type of operation offering typical HRM battery lifetimes of approximately three years on a CR2032 coin cell battery with 1 hour per day usage.
The nRF24AP1's TDMA collision avoidance approach relies on each transceiver transmitting in a clear timeslot. If there are a number of discrete systems working side-by-side - such as the rowing machines in our gym example - by 'listening' for drifting transmission sources on its frequency, the wireless node can determine if there is approaching interference. It can then adapt its transmissions accordingly even if there are dozens of competing RF sources.
More information on the ANT protocol can be found at www.thisisant.com.
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