Reading some commentators' blogs and opinion pieces, you'd think small cells are consigned to the graveyard. All you need to stay connected are numerous Wi-Fi access points supplemented by a few outdoor macrocells. Does this fit with reality or are we witnessing some "over-enthusiasm" from Wi-Fi proponents?
I asked Nick Johnson, CTO of ip.access, to spell out the key technical differences between Wi-Fi and Small Cells. These help clarify some of the limitations of both systems and demonstrate there is room for both.
Some of the key differences between Wi-Fi and licenced cellular
Fixed packet scheduling limits bandwidth and voice capacity
Wi-Fi transmits using TDD (Time Division Duplexing), alternating between send and receive on the same frequency. One packet is sent in each direction every millisecond, addressed to a single user at a time. Applications such as voice telephony use tiny packets, but still take a whole timeslot consuming a disproportionate amount of capacity. This inherently limits the number of concurrent voice calls that a single access point can handle.
LTE (predominantly FDD today, sending and receiving packets on separate paired frequencies) has a packet scheduler that shares out the available timeslots depending on the traffic. This makes more efficient use of the available capacity and allows a macrocell basestation to carry many hundreds of voice calls and/or data sessions simultaneously. Small cells typically handle a smaller number, but that's a design choice to save cost rather than an inherent constraint.
Interference Co-ordination
Wi-Fi access points are standalone devices which are very simple and straightforward to install. They try to avoid interfering with their neighbours by being polite – not speaking until everyone nearby is quiet, and not hogging the frequency endlessly when they have a lot of data to send. That works well in many situations, but becomes a problem with high traffic loads or when too many hotspots are co-located. There is anecdotal evidence that the 2.4GHz band is permanently congested in many public settings, resulting in devices spending more time waiting for a quiet period than actively using it.
This is far less of an issue in the 5GHz band which has much more spectrum and shorter range (so lower surrounding interference). LTE circumvents the problem by co-ordinating transmission timeslots between cells, and even selectively steering the data traffic onto each sub-carrier for optimum performance.
Mobility vs Nomadicity
Wi-Fi is designed to be used when stationary and doesn't incorporate many features that support active handover as you move around between access points. Cellular systems natively support that.
Long Range
Wi-Fi was aimed at relatively short range, mostly indoor applications using low RF power. Some jurisdictions, such as the US, permit up to 1 Watt RF (4W EIRP) to be used in these unlicensed bands – and even more on fixed point-to-point links (see this table). Most other countries, such as Europe, have much lower limits of 100mW EIRP. Simply transmitting unidirectionally at higher power may not give you the range increase you were expecting. The higher power results in many signals travelling in different directions, being reflected from and via other buildings, resulting in multi-path signal reception.
This needs a more complex receiver design that can cope with a longer duration receive window. It all contributes towards a need for higher processing capacity, more silicon area, higher power consumption and increased costs. This conflicts with the Wi-Fi industry's focus on low cost, mass market devices.
Cellular systems are inherently capable of serving long range, fast moving users (eg in cars, trains etc.). Urban small cells may not have all the range of their macro cousins, saving cost and power, but they have the ability to capture all this multipath energy and deliver a higher sensitivity, higher performance receiver as a result. And smartphones have this capability built in as standard.
Roaming, Security and Encryption
Another strong benefit of cellular networks has been their simplicity and security, even when abroad. Strong authentication algorithms, encryption and interworking agreements mean that you can switch on your smartphone almost anywhere and instantly be securely connected. Wi-Fi standards have caught up, with Hotspot 2.0 allowing the same seamless sign-on and secure encryption. The WBA has facilitated roaming between both Wi-Fi providers and mobile network operators.
Nick observes that Hotspot 2.0 deployments are on the increase, expects them to increase further, and become coupled with licensed radio small cells as the networks address load balancing between WiFi and cellular. Connection managers in handsets need to be augmented by end-to-end quality monitoring solutions that move load intelligently between radios, rather than the simplistic always Wi-Fi first predominantly in use today.
Listen before Talk
Unlicensed spectrum use by many uncoordinated transmitters requires some rules to keep things in order. Some regulators are insisting that polite behaviour is achieved through "Listen before Talk". LTE doesn't have that today – transmission timeslots are scheduled by the basestation – though it does respond to interference after the fact by steering around busy sub-carriers.
Fewer frequency variants
With licensed spectrum at such a premium, LTE standards have been stretched to address over 50 different frequency bands and modes. This makes smartphones extremely complex and use only a selected subset. Multiple variants of popular smartphones are required to meet different choices worldwide. LTE roaming may not work everywhere.
By contrast, Wi-Fi only comes in two bands (2.4GHz and 5GHz) and although there are a few minor variations, the latest Wi-Fi hotspots are backward compatible with every certified Wi-Fi device ever sold.
What of the future?
Wi-Fi and LTE standard will both continue to evolve on a convergent path and become even more similar and equivalent in capability over coming years. Many cellular operators look upon Wi-Fi as a commercial opportunity to offload traffic at low cost or for free. Few seem to have a fully integrated traffic management plan that incorporates it alongside 3G/LTE cellsite and small cell planning.
With Carrier Wi-Fi investment growing and Small Cells incorporating Wi-Fi by default, never mind the potential upcoming LAA, we can expect radio planning departments to have to get to grips with the Wi-Fi footprint that complements their cellular coverage.
I don't see Wi-Fi replacing cellular for voice usage, especially in public/semi-public areas for the reasons discussed, but it can go a long way to help serve the burgeoning demand for always connected data service.
As to whether this will be reflected in industry opinions, we'll be watching the clock to see if the pendulum swings back over time...
Our thanks to Nick Johnson, CTO ip.access, for providing many of the technical insights shared above.
