For wireless connectivity applications a fundamental decision that needs to be made,
is whether to go with a standards based RF interface like Wi-Fi, Bluetooth, or ZigBee,
or a proprietary RF design.
There are many reasons and tradeoffs to consider, such as cost, security, power
consumption, interoperability, design time, and certification requirements. Many
of these issues are interrelated so designers must first determine the design requirements,
to make the best decision..
Proprietary RF design is generally considered when an application requires high
security, low power, and a small footprint.
Security - proprietary designs may offer “security-through-obscurity.” That is,
an RF interface which isn’t generally known is harder to hack. There’s also the
tendency for proprietary interfaces to be point-to-point, or to operate in closed
systems that don’t connect to wider networks. Designers of proprietary interfaces
are free to develop their own advanced encryption algorithms or tweak established
ones, without concern for interoperability with security algorithms from other manufacturers.
Interoperability - proprietary designs can ensure a more robust connection (considering
interference from Wi-Fi networks, other electronic products/systems), or other low-power
wireless networks as designers have flexibility to optimize the wireless spectrum
using direct-sequence spread spectrum (DSSS) or frequency hopping spread spectrum
(FHSS). In addition, one can utilize coding schemes to increase throughput or lower
power consumption.
The same flexibility also applies to data packet structure. Without the packet overhead
required to ensure interoperability with established protocol standards, the packet
structure can be streamlined to meet needs of a specific application.
From a hardware design point of view, performance requirements may allow designers
of a proprietary RF interface to optimize for space, power, and performance.
While proprietary RF has many advantages, there are a number of factors/tradeoffs
to consider. The first is cost. Generally, to justify the non-recurring engineering
(NRE) cost of a custom RF IC design and software development requires a very high
volume application. Other key considerations, are design time, and time-to-market
goals.
Bluetooth was originally conceived as a point-to-point cable replacement technology
for HIDs and other devices. It soon became a wireless audio and device-to-device
connectivity solution. Bluetooth is well understood and designers can be confident
their devices will connect and be interoperable with other Bluetooth enabled devices,
regardless of the hardware source.
Bluetooth operates in the 2.4 GHz industrial, scientific, and medical (ISM) band
with throughput of 1 Mbit/s (Bluetooth Basic Rate). Its adaptive FHSS encoding scheme
allows it to continue to remain robust in the face of interferers. To get to higher
data rates, Bluetooth 2.0+Enhanced Data Rate (EDR) uses π/4-DQPSK (differential
quadrature phase shift keying) and 8DPSK modulation, yielding rates of 2 and 3 Mbits/s,
respectively.
While Bluetooth is tightly controlled, further changes came about with the introduction
of Bluetooth 4.0 Core Specification in 2010 which has to be considered. This introduced
Bluetooth low energy (BLE), formerly marketed as Bluetooth Smart, but is not backward
compatible with Bluetooth Classic.
The primary goal of BLE is low power consumption. This is accomplished by moving
from Bluetooth Classic’s connection-oriented approach (devices always connected),
to an unconnected approach where only connected when needed.
The latest version, Bluetooth 5, doubles the BLE data rate to 2 Mbits/s from 1 Mbit/s,
and increases the range of a 128 kbit/s connection by 4x to up to 50 m by using
stronger forward error correction (FEC). The higher data rate allows more packets
to be transmitted for a given time slot. Power consumption is reduced as the device
can stay in low-power or idle mode for extended periods.
The longer range gives designers more flexibility to trade-off data rate for distance
for any Bluetooth device such as beacons. Beacons are battery driven BLE devices
that broadcast their identifier to nearby mobile devices so those devices can perform
certain actions when close to the beacon.
What started as a simple cable replacement technology has now morphed into something
much more useful. As a result, designers are now more apt to look for a quick and
easy Bluetooth solutions rather than go through the cost and expense of designing
their own RF interface.
This inclination to opt for a Bluetooth interface is turning into a necessity as
time-to-market windows narrow and design budgets shrink.
Antenna matching and placement is one of the finer arts of RF design, so off-loading
it from the designer saves time and helps ensure optimal signal coupling. Off-the-shelf
modules provide a complete solution with established regulatory approvals. On-board
DC-DC converter, intelligent power control, efficient size and integrated antenna
options are all potential benefits.
When mounting a module in an enclosure, make sure there’s no metal near the antenna
or it will impact performance. As it is designed and tuned for free-air operation,
potting, epoxy, over-molding, or conformal coatings can affect performance, requiring
additional measurements after application to ensure the link budget is within specification.
Proprietary vs. Bluetooth sweet spot
Between a full custom proprietary radio design and standard Bluetooth, there is
another option: an off-the-shelf radio transceiver around which designers can develop
their own protocol and coding schemes, or adopt off-the-shelf versions such as Ant,
Thread, or ZigBee. These offer reduced costs and a wide range of software support,
and may be the “sweet spot” for designers looking for differentiation, latitude
for optimization, and enhanced security options, all while keeping costs low and
design schedules intact.
Proprietary vs. Bluetooth sweet spot
In summary there are many reasons to choose either a full proprietary RF design,
a standard Bluetooth radio, or flexible solutions enabling some level of proprietary
differentiation.