The threat of more governments blocking Huawei and ZTE from bidding for 5G infrastructure contracts (see separate item) has thrown a harsh spotlight on one of the myths of 5G. There have been high hopes that the 5G network would turn out to be more open than its predecessors, making it easier for operators to mix equipment from multiple vendors in the same network zones. In fact, not only does it look as awkward as ever to deploy a multivendor RAN, but backwards compatibility – a key claim of successive 3GPP standards generations – is less than perfect too.
This has been highlighted as an issue because operators which have 4G RANs supplied by Huawei or ZTE are saying it will not be a simple matter to migrate them to 5G equipment from another supplier. In some cases, an expensive rip-and-replace might be necessary.
With Huawei under pressure, its rivals discover openness:
Of course, with the prospect of the Chinese vendors being out of the picture in some countries, Nokia and Ericsson are sounding more open than usual. Both are promising ways for their 5G or multi-radio equipment to be deployed, while enabling MNOs to hang onto their Huawei LTE systems.
Of course, these efforts could backfire. If very few nations follow Australia in barring Huawei and ZTE (the USA already being a Chinese-free zone, at least for tier one operators’ networks), the Nordic suppliers may have fixed one of the broader problems which would otherwise help them to keep their own customers faithful. The X2 interface, which supports handover between 4G base stations – and will also support handover between 4G and 5G – is not fully interoperable. Like so many other interfaces, notoriously the CPRI ‘standard’ for linking basebands to remote radio heads, X2 has been implemented differently by different vendors.
“Each vendor tends to implement it slightly differently to get superior performance on their own systems,” acknowledged Nokia’s CTO and head of Bell Labs, Marcus Weldon.
That threatens to maintain the current situation, where a multi-supplier RAN usually means no more than different vendors providing equipment for different geographical areas, with minimal interaction required. As operators start to deploy 5G New Radio, they will want plenty of interworking with their LTE networks.
In the first phase, Non-Standalone, they will retain the LTE core, and in most cases, will only introduce 5G base stations for particular locations or applications with high demands. So difficulty in enabling those new base stations to communicate with the existing LTE majority would clearly be a show-stopper. Even when operators move to the Standalone mode, and the 5G core, there will need to be close interworking with LTE for many years to come.
The problems with the X2 interface:
So the Huawei issue has only highlighted an issue that is important to the MNOs on a broader level as they dream of multivendor 4G/5G networks, or at least a free choice of 5G supplier.
In fact, one of the 11 options for the functional split in a disaggregated RAN, defined by the 3GPP in Release 15, can support multivendor interworking by avoiding the use of X2. Option 3a, which defines how dual connectivity works in a 4G/5G, NSA deployment, calls for a direct S1-U interface from the 4G core to the 5G NR site, and the traffic flow for each 4G or 5G NR is split at the core. This reduces the processing demand on the 4G site and works well in multivendor scenarios.
However, many operators are more interested in deploying one of the other two dual connectivity options for NSA, particularly 3x, which reduces the need for expensive backhaul and LTE capacity upgrades because the 5G NR is connected simultaneously to both the EPC and the LTE cell (via the traffic plane S1-U and the X2 interfaces, respectively).
Nokia and Ericsson look to DSS to allow coexistence with Huawei 4G:
So vendors are looking for other solutions which may help MNOs to get the best of both worlds. Nokia has proposed an overlay approach to 4G/5G coexistence (see Wireless Watch April 10 2019) and Weldon has now provided more detail on a solution that was rather sketchily described before. It is based on dynamic spectrum sharing (DSS), a new standardized feature which is just appearing in some radios, and allows the existing LTE to communicate with the new base stations without X2. With DSS, spectrum can be reallocated to different radios every millisecond.
The DSS technology allows an MNO to move spectrum between different radio technologies as required, and was designed mainly to give greater flexibility and efficiency in spectrum usage, in high bandwidth situations. However, Weldon says it could also help with interoperability, as an MNO could buy 5G equipment, with DSS support, from a new vendor and initially run the system in 4G mode, using some existing 4G spectrum.
This overlay would support interoperability between the incumbent 4G and the new 5G systems, with roaming between the two 4G radios happening at packet core level as usual.
Ericsson is also looking to offer DSS-based workarounds. “We don’t think X2 is the way forward because it would slow things down and be expensive,” Thomas Noren, Ericsson’s head of 5G commercialization, told LightReading. “DSS is a much better way forward. It means you can be totally flexible between 4G and 5G and allow any mix of the two to run in a band.”