Many operators are deploying LTE in low frequency bands (700 and 800MHz) to improve coverage in rural and in-building penetrations. Others are looking at LTE relays as an alternative approach. We examine the differences between a repeater, a relay and an LTE small cell and look at two specific operator use cases.
Technology recap – repeaters, small cells and relays
Cellular repeaters have been use for decades to boost weak signals, particularly indoors. They amplify the signal and rebroadcast it at the same frequency for both downlink and uplink frequencies. While they can work well if properly engineered, rogue repeaters have caused significant network disruption, demanding increased resources from distant macrocells and affecting the wider coverage pattern in the surrounding area.
Smarter repeater products, such as Nextivity Cel-Fi, can monitor more than one frequency band and automatically adapt to local conditions. These even allow remote control from the network which can limit or switch off devices by setting parameters in the broadcast control channel of the affected sector. Some repeater product come with two separate units, one positioned at the window to communicate with the outdoor macrocell and a second positioned within the building to broadcast the signal where reception is weakest. Larger buildings have been equipped with passive DAS systems that have multiple RF nodes and effectively act as larger scale repeaters.
LTE small cells provide a full set of basestation functionality, usually backhauled by dedicated IP connections which can be wired or wireless. Residential and Enterprise small cells often use existing Internet broadband services while urban outdoor small cells may be connected with dedicated fibre or microwave/millimetre wave backhaul radios.
An LTE relay combines a full LTE receiver and LTE small cell to use the cellular network for the backhaul link, operating in a different frequency than the small cell itself. This may be on a different channel within the same band or even use a completely different frequency band altogether.
Relay Capacity Gain
Vendors of LTE relays (and even repeaters) talk of the additional capacity they provide. The key factor is whether the signal quality from the donor macrocell is significantly better to the LTE relay than to the end user smartphone. A larger antenna, located outside and pointing in the right direction would typically have better reception. This increases the chances of using a higher modulation rate (say 64 or 256QAM) and thus transfer more data within the same spectrum – as much as 4 or even 16 times. While there is typically still a limit to the total data rate of a macrocell sector, it will free up radio resources to be expended serving other users in the wider area.
Unlike a regular small cell, an LTE relay does not require dedicated internet backhaul but does require a reasonable LTE signal to work with. I don’t believe it would make sense to use these for larger installation or to have groups of them in close proximity – they are more relevant for isolated coverage holes.
Sprint’s “Magic Box”
Sprint have launched an LTE Relay product aimed at residential and SOHO customers which they’ve branded as their “Magic Box”. It’s a compact LTE relay which can receive at 1900 or 2600MHz and retransmit on 2600MHz. The unit was developed by Airspan, which has a lot of LTE small cell experience and had previously embedded a WiMAX-based 2600MHz backhaul in some of their outdoor products.
As we reported recently, Sprint has very large amounts of 2600MHz LTE TDD spectrum and is making as much use of it as it can. This strategy may make a lot less commercial sense for operators elsewhere with fewer and more precious spectrum resources.
Small Cells deployed in the US all incorporate GPS receivers which are used to determine location, which is used to work out what spectrum is available for use in that area and may also be used in handling emergency 911 calls. This means that they must be located near to windows (or in some cases had long GPS antenna cables). Apart from that, the small cell could be located anywhere.
The Magic Box would work best if closer to the window and ideally positioned wherever the best macrocell signal is to be found. We understand that trials have completed successfully and a new version of the product is starting to be rolled out to customers.
Sprint also have other options including Nextviity’s Cel-Fi, CommScope’s S1000 small cell. Their decision not to adopt VoLTE means that voice calls remain handled entirely by 3G CDMA and so these LTE only products won’t help. Customers would need to use their 3G femtocell, provided by Airvana (now Commscope) or a CDMA compatible repeater for that.
The Magic Box also has an RJ45 Ethernet socket and I understand can be used as a standalone small cell, so for those out of coverage but with adequate internet broadband may also provide a useful solution.
Airspan have a range of SON capabilities, including those from their partner AirHop, to configure and optimise system operation.
Vodafone is known to have some LTE coverage issues and has previously highlighted difficulties in securing suitable rooftop cellsites in London. They have developed an LTE relay unit called the CrowdCell. It can be used in a fixed location or embedded into a car. This presentation summarises the product features. The indoor variant supports 2x 250mW LTE radios at 2600MHz TDD using either 800 or 1800MHz for backhaul.
An embedded SIM Card in the LTE relay is used for security. To the macrocell, the LTE relay just looks like any other end user device. To a smartphone, it just looks like a macrocell. These are open for any Vodafone subscriber to use.
A CrowdCell Controller provides centralised SON (Self Organising Network) functionality to configure and optimise the use of CrowdCells alongside the macrocell network.
Vodafone have conducted live trials in Turkey and Spain but appear to be considering use throughout any of the subsidiaries worldwide.
Parallel Wireless CWS
We reported earlier this year about three Parallel Wireless rural deployments in Wales, of which two separate rural villages were connected using LTE backhaul. Their CWS basestation product incorporates an LTE receiver which uniquely can be shared for both 3G and 4G backhaul.
Their architecture routes traffic back to a central HetNet gateway which separates and distributes the 3G and 4G components as well as providing SON configuration and optimisation capabilities.
The use of inband LTE for backhaul appears to be seeing growing interest, mostly for those operators with spectrum resources to spare. The low cost of an LTE receiver and the very high performance it provides could make it attractive compared to some of the more specialist microwave backhaul alternatives. However it does consume precious cellular frequencies that may best be used for end users.
I can see that this approach would be attractive to serve areas of low traffic, especially rural and for those few operators awash with high band spectrum. I doubt it would be so relevant for areas with high capacity demand although some might argue about the value of "relay capacity gain".
Small cell products which incorporate the option to operate as an LTE relay for relatively little additional cost may seem attractive and provide greater flexibility. You can deploy them almost immediately anywhere and return later to install dedicated out of band wireless or wired backhaul.