Favoured Smart City Networks

Michael Castle of Antenova Ltd takes a look at the wireless technology powering remote-controlled services in smart cities

LoRaWAN networks now rolling out in US and European cities are enabling the remote-controlled services that are making our cities smarter and more sustainable. Smart lighting alone could potentially cut cities’ energy bills by 33-62%, but smart cities offer greater potential than just smart lighting. Once a city-wide network is in place, it can also support other IoT services, such as smart parking, smart refuse collections, and air quality monitoring.

A network of water meters can measure the consumption of water and sensors can detect water leaks. Smart parking is an interesting application that can work more than one way. Either a network of sensors can detect vacant parking spaces and direct traffic to them – or maybe a city needs to eliminate illegal parking, in which case they can detect vehicles in a no-parking zone and sending a notification accordingly. Designing any kind of application for smart city networks means adding wireless communications to the air sensors, lights, or meters within the network, and possibly making the devices wireless for the first time.

This article considers the networks and the RF and antenna technology which allow connected devices to talk to each other as one connected, intelligent whole.

All smart city applications use networks of connected devices. They might have external antennas on the outside or embedded antennas hidden inside the device on the internal circuit board.

Figure 1 shows external antennas and an embedded antenna which is placed inside the device on the PCB.

The LP-WAN Networks

There are several options for the low powered, wide area networks known as LP-WAN. Originally designed for the industrial, scientific, and medical sectors (ISM), the networks offer long range communications with a low bit rate, which is good for small, intermittent data packets. The LP-WAN networks use low radio frequencies: 791-960MHz, 863-928MHz, 432-434MHz with the US using 915MHz and the EU using 868MHz and 433mHz.

These networks are not as fast as broadband, but they deliver long range communications with low power and low costs. They are unregulated, so unlicenced use is possible, but this also means that in some cases there is nothing to prevent interference on the networks and an unlicenced radio transmitter still needs to be certified. All of the LP-WAN networks offer the advantages of a simple network infrastructure and long battery life for connected devices.  LP-WAN is therefore a good choice for large scale IoT deployments or networks of sensors. It suits devices that only require low power, where the data packets are relatively small and not time critical.

Four technologies dominate LP-WAN: Sigfox, NB-IoT, LTE Cat-M1 and LoRaWAN.  Sigfox, NB-IoT and LoRaWAN are all capable of providing long range networking across a wide geographical area or a city, and LTE-M provides a similar connection for mobile devices.

The popular LP-WAN standards

Sigfox, which uses Ultra Narrow Band (UNB) has good coverage in France but is not used as much in the US. Sigfox uses a proxy server but offers some of the advantages of low-cost connections however, the network does not offer mobility or TCP/IP.

NB-IoT (Narrowband IoT) operates in the “wasted” bands in the carriers’ licensed mobile spectrum. This means that it operates in situations that are more challenging for radio signals, for example in buildings and even underground where cellular coverage may not usually function. NB-IoT is popular in Europe and is suitable for projects where large volumes of devices are connected in limited spectrum where the amounts of data are relatively small. With NB-IoT, each device uses a SIM card and the connections are secure and reliable.

LTE Cat M1 provides mobile connections, so it can be used to connect bikes and cars – it could be a good choice for rental bikes or scooters. LTE Cat M1 provides TCP/IP to the end device, i.e., a true internet connection. It has power saving modes, it can sleep and wake up again, and it can be programmed to wake up, to receive data reliably.

LoRaWAN dominates smart cities

LoRaWAN is usually the more favoured network for smart cities and is popular in the UK. Promoted by the LoRaWAN Alliance, it is an open standard based on chips and LoRaWAN gateways using IP from Semtech. It’s designed for networks of battery-operated devices and operates in the unlicensed spectrum around 868MHz. Using licenced-free spectrum reduces the cost of a connected solution. The combination of low cost and wide area coverage makes LoRaWAN attractive for M2M and IoT connected devices. The connection costs are lower than with the mobile networks, and the range is wide.

LoRaWAN is already available in more than 100 countries and many cities of the world.

It’s a popular choice for smart metering, probably due to its cost-effectiveness. It doesn’t require a mobile SIM, so it is also a good choice for smart parking, air sensors, tolls, and vending machines. It can support tele-health too, and situations where vulnerable people and patients are monitored in their homes.

All of these applications will usually work via a LoRaWAN gateway which connects to a central server and a control system, or direct to a cloud network service. The IoT service provider can either build a private network or use a 3rd party service.

Embedded antennas – how they work

Embedded antennas are placed on a PCB and usually use a ground plane to radiate effectively. This needs to be kept free of other components on the board, and the antenna itself may require a clearance underneath.

Figure 2 below shows an embedded antenna for ISM and the drawing shows the copper free keep-out area for the ground plane under the antenna. In this case the copper free keep-out area is simply defined as the same size as the antenna, with a 1mm additional clearance between the antenna to the main PCB ground.

Figure 3 shows an alternative - a flexible printed circuit (FPC) antenna which is attached to the PCB by a short I-PEX cable, which offers designers some alternative options for adding an antenna to their design. This antenna does not require a ground plane.

Wireless design questions

The roll-out of LoRaWAN is at different stages in different locations, so the first design question must be, “is a suitable network available”? The GSMA organisation website provides some resources on network rollouts and new spectrum as it becomes available.

How well will the network connections work across the area required? LP WAN and LoRaWAN gateways typically provide coverage for 1km or depending on the surroundings and the quality of the antennas can make a huge difference to the design of the project and the device’s battery capacity requirements.

Where are the devices located? The surrounding environment affects wireless communications, therefore, industrial buildings, comm. rooms and basements are not usually a suitable environment for radio equipment to operate. If the connected devices are sited in inaccessible areas such as basements and tunnels, as with meters, then LP-WAN with its low frequencies is the best option for making a working connection.

At frequencies below 1GHz antennas need to have 90-120mm of ground plane length to perform well. Additionally, the antenna itself needs to be larger, and this will require a larger footprint on your circuit board. If your device does not have the required ground plane size, there could be a compromise on antenna performance and range of coverage which would then reduce the battery life of your device.

Device design - consider the enclosure or casing for the device – most antennas do not work behind of near metal. Place the antenna correctly on or inside the device, following the antenna manufacturer’s instructions because the placement of the antenna and its proximity to other components will make a huge difference to its performance. 

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