LTE & Wi-Fi: Options for Uniting Them for a Better User Experience

2

Most national governments consider the radio spectrum a valuable national resource and heavily regulate its commercial use. Governments typically auction off licenses for the right to transmit over a portion of the spectrum, which can be very expensive.

The traditional business model for cellular carriers is based on access to this licensed spectrum. They license slices of spectrum from the local regulator and sell their customers access to it. After decades of parallel evolution on the two sides of the Atlantic through multiple generations of technologies, the business has coalesced worldwide around a single 4th generation (4G) radio technology standard called Long Term Evolution, commonly referred to as LTE.

However, if a wireless device promises to “play nice,” most regulators will allow it to transmit on a slice of spectrum set aside for that purpose: the license-exempt, license-free or simply unlicensed bands. Playing nice means adhering to certain rules that will be verified when the device is certified. The rules are derived from basic human civility: I will not shout too loudly—there is a limitation on transmit power, with the Effective Isotropic Radiated Power (EIRP) limited to anything between 4W (36 dBm) to 25 mW (14 dBm). I will share the resource, not monopolize it—these are rules about the duty cycle, the maximum duration of transmit bursts, minimum duration of silence after transmission, and an obligation to “listen before talk (LBT).” I will yield to users who have been deemed by society to be serving a higher need than I have—as we would pull over to give way to a fire engine or ambulance, unlicensed spectrum users must move away from a frequency if they detect equipment like airport weather radar operating on it.

While the exact situation varies by country, generally speaking there are three unlicensed

bands of greatest interest today when it comes to wireless broadband: The 2.4 GHz band (λ ≈ 12 cm) has 83 MHz between 2.400 and 2.483 GHz. This band is almost uniformly available worldwide and is heavily used by consumer devices. The 5 GHz band (λ ≈ 5½ cm) has 775 MHz between 5.150 and 5.925 GHz. This band is gaining popularity in consumer devices—mostly in premium and high-end devices for now—but its allocation is fragmented and less uniform across countries. The 60 GHz band (λ ≈ ½ cm) has 9,000 MHz between 57 and 66 GHz. This band is relatively new and promising, though the laws of physics put some limitations on the ways it can be used.

The dominant wireless broadband technology in these three bands is Wi-Fi, which is based on the IEEE 802.11 wireless LAN standard. Wireless LAN technology is now heavily used for private networks in homes as well as in the workplace. Then there is public Wi-Fi, which is common in cafés, restaurants, airports, hotels, shopping malls, and increasingly on trains and planes. Sometimes it is complimentary, and sometimes we have to pay for it. In fact, in several small, densely populated developed nations such as Singapore, Wi-Fi can be found almost anywhere.

While cellular carriers have been good at providing coverage—especially outdoors—they face both coverage and capacity challenges as the demand for broadband internet access grows. There can be a coverage problem at the network’s edge, in locations where installing radio infrastructure, such as towers, cannot be justified financially. There is a coverage problem indoors, because the materials a building is made of—especially stone, concrete, steel and metallized sun-control film—can block radio signals to and from the carrier’s cell tower outdoors. There can be a capacity problem in “hotspots” where many people congregate. This isn’t a problem if the carrier has enough licensed spectrum to address this demand. However, spectrum licenses are expensive, and budgeting spectrum for the capacity demands of hotspots would leave most of that spectrum unused

3

In recent years, cellular carriers have been looking at the unlicensed spectrum for ways to address all of these challenges.

01 —

02 —

03 —

over most of the carrier’s coverage footprint. The other way to handle the demand is to put radio infrastructure equipment closer together—for example cell towers—thereby having smaller cells where the demand is higher. The downside of this approach is it increases interference and causes more handovers—which do not help beyond a point. Such hotspots may be: Outdoors, such as stadiums and entertainment venues, as well as locations like Times Square, Piccadilly Circus and Shibuya Crossing. Indoors, such as shopping malls, airports and railway stations—public indoor spaces inside buildings that get significant numbers of walk-in users. Clearly, as explained above, a coverage problem is also present in such locations, so the carrier gets hit with a double whammy which is difficult to address for two reasons: First, the cellular carrier, the walk-in user and the building owner are distinct and separate entities. Since the building blocks radio signals to and from its network outside, the obvious answer is to install radio infrastructure—a distributed antenna system (DAS) fed by one of the carrier’s base stations—within the building. Unfortunately, while the carrier “owns” the user and the spectrum license, it has no rights to the space inside the building.

Second, the owner of the building has no spectrum license, but he controls the space inside the building. He would presumably like to monetize it by cutting some kind of deal with the carrier, but since there are typically multiple carriers in the area—anywhere from two to six on an average—it may be against the building owner’s interest to play favorites between them by allowing only one of them to install a DAS.

Unlicensed spectrum infrastructure can substitute licensed spectrum coverage in network-edge locations. Due to the regulatory limitations on transmit power, however, all such coverage using unlicensed spectrum can only be of the hotspot or “hot-zone” variety: that is, only terminals that are in the vicinity of the infrastructure can be served.For indoor terminals in poor signal-to-noise (SNR) locations, it is possible to use indoor unlicensed spectrum infrastructure to compensate for higher path loss between outdoor cell towers and indoor locations. This offers an option to keep only low-bitrate signaling on licensed spectrum while using unlicensed spectrum to deliver most of the payload traffic.In situations of capacity crunch, it is possible to augment the available air-link capacity by diverting overflow traffic from licensed spectrum to be delivered over unlicensed spectrum. This allows for the combination of:

serving more terminals in locations that have many users in one place; andproviding a thicker data pipe to terminals, which potentially provides substantially higher bit rates for all applications that use “best effort” Quality of Service.

In the specific instance of public indoor spaces, solutions based on unlicensed spectrum are also attractive for building owners, as they can install unlicensed-band radio infrastructure in their buildings. They can then “rent out” the use of this infrastructure equitably to all carriers in the area, and the carriers can use it to serve their respective subscribers who walk into those buildings. The end user gets high-quality connectivity, the carrier gets the goodwill from satisfied customers, and the building owner makes some money. It’s a win-win situation all around.

Building owners may actually be ahead of the game. Indeed, an increasing percent of them are investing in public Wi-Fi, which is the dominant form of radio access in the unlicensed spectrum. Quite a lot of the public Wi-Fi is running without the involvement of any carrier, however, and in the rest of the public Wi-Fi deployment the involvement of the carrier is very loose. As a consequence, the end-user experience is not “seamless.”

In fact, it can be argued that there is a threat to the carriers in this case. When the end user connects through the unlicensed spectrum infrastructure belonging to the building owner, the carrier adds value by being a broker between the two—it’s a trusted intermediary known to both parties. However, this role can be fulfilled equally well by a Wi-Fi aggregator, such Boingo or iPass, that exist for that very purpose.

10

So how should a carrier play this game? To

understand the challenge, we have to familiarize ourselves with the structure of a cellular broadband network. Figure 1 shows what a LTE network looks like when only the internet-access service is considered.

The User Equipment (UE) is the end-user’s smartphone, which may be a carrier-locked model purchased from the carrier or an “unlocked” model bought in the open market. The Subscriber Identity Module (SIM) is the tiny smart card that goes into the phone: it is in fact a computer in its own right, and holds the subscription credentials that authenticate the phone to the network when required.

The Evolved Node B (ENB) is the LTE radio base station. The Mobility Management Entity (MME) and the Serving Gateway (SGW) are functions of the LTE core network that, in case of roaming, must be in the “visited network” to which the end-user is connected.

Figure 1: The LTE internet service flowThe red line is the path taken by the data packets, the green lines are for authentication, the blue lines are for signal control, and the yellow line is for credit control and authorization.

ENBUE

MMEHSS

OCSAAA

PGWSGW Internet

Licensed

SIM

The PDN Gateway (PGW) is the point where the LTE network connects to the Internet. It is the function that allocates dynamic IP addresses to each UE as they connect, and it is the last router that IP packets addressed to the UE have to pass through. It is a function of the packet core that may be located in the visited network or the home network—the choice is up to the carrier.

The Home Subscriber Server (HSS), Online Charging System (OCS) and AAA Server are systems that must be in the subscriber’s home network whether or not they are roaming. The HSS/AAA holds the subscription information for the end user and their authentication credentials, while the OCS maintains his credit balance to enable prepaid pay-as-you-go service.

Figure 2

The key to this solution is to leverage the user’s SIM for authentication using a technique known as EAP-SIM/EAP-AKA. Additionally, technologies such as Hotspot 2.0 can be used by the Wi-Fi network to advertise its willingness to accept visitors from the user’s carrier, which can help automate the process. After the user’s Wi-Fi session is over, the Wi-Fi network will send the accounting records to the carrier, billing settlement will take place offline, and the user will be charged for the usage in their next phone bill.

For now, this technique has one advantage over methods discussed below: almost any smartphone on sale today can access Wi-Fi using unlicensed spectrum. However, there are some shortcomings: Service continuity—commonly called handover—with the cellular mobile broadband service offered by the carrier is not possible. Simply put, when the user comes into the Wi-Fi network from outside, any data sessions, such as TCP/IP connections, will need to be restarted. It will be up to the application whether that is acceptable.

The red line indicates the path taken by the data packets flowing between the UE and the Internet. The green lines indicate paths used primarily for authentication.The blue lines indicate paths used by more general control signaling that is used to set up, modify and tear down the connectivity. The yellow line indicate the path taken by ‘online charging’ credit control interactions that make prepaid services possible.

With Figure 1 as the reference point, consider Figure 2 which shows the entry-level way carriers can tap into the unlicensed spectrum.

Prepaid service is not possible. This may not be a big problem in economies such as the US where postpaid and contracts are standard practice. However, in some of the largest cellular markets in the world, including China and India, prepaid service is the dominant model.

UE

HSSOCS

AAA

InternetWi-Fi

Unlicensed

SIM

Figure 3

In fact, it is difficult to distinguish this model from the business models pursued by Wi-Fi aggregators. Something is required to make it more attractive, which is captured in Figure 3.

In this new model, called the trusted WLAN, the carrier extends its core network to the site where the Wi-Fi is deployed, and provides a connection between the Wi-Fi network and the PGW. In one fell swoop, the carrier solves the problems of connecting to the user:

To be sure, there are a few blemishes:

12

Since the user traffic is not carried by the carrier at all—the red line does not touch any of the carrier’s infrastructure—the carrier will probably be able to claim only a small share of revenue.

The user traffic is transported over the carrier’s infrastructure (the PGW), so the carrier can claim a larger share of the revenue.Since the PGW knows about credit control, all of a sudden the carrier can now handle users who have prepaid plans. The PGW is, in fact, the same as the one used for cellular access, so it is possible to maintain service continuity as the UE moves between LTE and Wi-Fi radio accesses: the PGW can ensure the IP address allocated to the UE will be the same.

To take advantage of the network’s new-found ability to maintain service continuity, the UE has to learn about some new signaling procedures. Specifically, it has to replace the ubiquitous Dynamic Host Configuration Protocol (DHCP) protocol with something else. Luckily, this is a software change, and the UE can execute the new tricks with a software update.Whenever the UE moves between Wi-Fi and LTE radios, there is signaling traffic to and from the PGW. This is undesirable: not only does it create potentially avoidable load on the PGW when the user is roaming, but it can slow down the handovers.

The decision to move to Wi-Fi and back to LTE is left to the end-user (or at least the policy they configured in the phone,) so sometimes the UE will be on Wi-Fi when the carrier would rather have it on LTE, or vice versa. Also when the UE chooses Wi-Fi, the entire traffic to and from the UE is subject to the interference-prone uncertainties of the unlicensed spectrum. It may be preferable to set aside a portion of the traffic to be transported over the licensed spectrum which the carrier has more control over.

This model is as good as it gets in locations where the carrier is depending on unlicensed spectrum to substitute licensed spectrum coverage. However, if the UE is located where both LTE and Wi-Fi are available, it may be less than satisfactory from the carrier’s point of view.

UE

HSSOCSAAA

PGW InternetWi-Fi

Unlicensed

SIM

There is a fork on the road ahead when addressing these two challenges. The left fork addresses challenge 1, as captured in Figure 4.

Figure 4

Figure 5

In this approach—called Network Based IP Flow Mobility (NBIFOM)—the UE maintains simultaneous LTE and Wi-Fi connections, and offloads only a part of its traffic to Wi-Fi. The data traffic is partitioned based on each packet’s:

Remote IP Address (i.e. of the server in the Internet)Protocol (i.e. TCP, UDP, SCTP, ICMP, ESP, GRE, …)Local Port Number (i.e. at the UE end)Remote Port Number (i.e. at the server in the Internet)Security Parameter Index (for ESP packets)Flow Label (for IPv6 packets)“Type of Service” or DSCP “Traffic Class” field

In this solution, called RAN Controlled LTE WLAN Interworking (RCLWI), the decision to pick LTE or W-Fi is handled by the ENB. The UE maintains an LTE connection whenever it can see the LTE network—irrespective of whether it is using offload-to-Wi-Fi or not. The UE keeps the ENB informed of the measurements of the Wi-Fi networks around itself. Based on this information, the ENB commands the UE to start or stop offloading to Wi-Fi, and specifically to which Wi-Fi network.

This approach can pinpoint all communications from the terminal to a particular server on the internet. In the world of 3GPP/LTE protocols, the data traffic to and from the internet can be split into a maximum of 11 distinct partitions, called bearers.

Unfortunately, RCLWI doesn’t address challenge 2 and NBIFOM does not address challenge 1. Fortunately, there is a third approach captured in Figure 6, named LTE WLAN Aggregation (LWA), that addresses both challenges.

HSSOSS

AAA

PGW InternetWi-Fi

Unlicensed

UELicensed

ENBMeasurements →

← Orders

SIM

ENB

UE

MME HSSOCS

AAA

PGWSGW Internet

Wi-Fi

Unlicensed

Licensed

SIM

Figure 6

Figure 7

This solution incorporates both UE-reported Wi-Fi measurements from RCLWI and the use of different radios for different bearers—or IP flows, sometimes called packet pipes—from NBIFOM. Unlike NBIFOM, the Wi-Fi leg no longer requires a separate AAA operation—instead, the Wi-Fi infrastructure can “borrow” the authentication from the LTE side.

Recognizing that the choice between the LTE radio and Wi-Fi radio is ultimately significant only between the UE and the base station, this solution splits or merges the traffic at the base station (ENB) and leaves the packet core (PGW) entirely out of the picture. LWA is completely transparent to the core network, which remains entirely unaware of whether each data packet is carried over Wi-Fi or LTE. Not only does this reduce the signaling overhead between the access and core networks, it simplifies charging enormously: no per-user charging is required for traffic that is offloaded to Wi-Fi.

12

Switched Bearer mode, where the entire bearer is diverted over LWA. It may be diverted back to LTE but only by signaling between the UE and ENB.Split Bearer mode, where the decision of whether the packet goes over Wi-Fi or LTE is taken on a packet-by-packet basis. No signaling exchange is necessary.

Typically, only non-GBR (guaranteed bitrate), best-effort bearers will be diverted. Diversion of GBR bearers is permitted, but it is understood that the bit rate is not guaranteed in Wi-Fi.

But how is the data packet actually transported? Figure 7 explains the protocol stack applicable for LWA user traffic, which is the Payload IP layer. The stack to the left is used to transport data in regular Wi-Fi, while the one to the right is used in regular LTE. The stack in the middle is used when LTE data is diverted over Wi-Fi in LWA, and it inherits qualities from both sides.

The control plane continues to use LTE on the licensed spectrum, which is a path the carrier has more control over. For the bearers carrying user traffic, there are two possible modes of operation:

ENB

UE

MMEHSS

OCSAAA

PGWSGW Internet

Wi-Fi

Unlicensed

Licensed

SIM

Wi-Fi MAC

Wi-Fi PHY

2.4, 5 or 60 GHz

Payload IP

LTE MAC

LTE PHY

Licensed Band

LTE PDCP

LTE RLC

Payload IP

Wi-Fi MAC

Wi-Fi PHY

2.4, 5 or 60 GHz

LWAAP

Payload IP

LTE PDCP

Wi-Fi U-Plane Stack LWA U-Plane Stack LTE U-Plane Stack

Figure 8

3

Consider what this means in the context of the building depicted in Figure 8. The Wi-Fi Installation belongs to the building owner. The two carriers in the area are represented by the colors red and blue. The red and blue lines in the picture denote the Xw interfaces for the respective carriers.

In the Wi-Fi Medium Access Control (MAC), the packet header—technically the SNAP header—contains a 2-byte field called EtherType. When IP packets are transported over Wi-Fi, this field is hexadecimal ‘0800’ for IPv4 packets and hexadecimal ‘86DD’ for IPv6 packets. For LWA, this field is set to hexadecimal ‘9E65’, which tells the Wi-Fi infrastructure that what follows in the packet is not a naked IP packet but is encapsulated using LTE PDCP. LWA introduces a 1-byte ‘LWAAP header’ that identifies the bearer to which the packet belongs.

In Release 13 LWA, only downlink traffic can be diverted. Diversion of uplink will be allowed from the Release 14 version of the standards.

LWA is derived from a 3GPP architecture called dual connectivity (DC) that will eventually enable network infrastructure that uses multiple, and possibly very different radio technologies simultaneously to transfer data fast and efficiently to and from the UE. This architecture is very flexible. It works with simple IP connectivity between the infrastructure-side radios without the need for strict synchronization between them. This feature—along with the standardization of the interface (named Xw) between the ENB and Wi-Fi infrastructure—allows LWA to address a variety of configurations.

The Wi-Fi installation in the building can serve multiple carriers—potentially all the carriers in the area.Each outdoor ENB can serve multiple LWA-capable Wi-Fi installations. This makes it possible for the Wi-Fi installation in any number of buildings in the outdoor base station’s coverage footprint to support the ENB’s LTE coverage. It does this by augmenting its capacity and by compensating for any coverage degradation that may have occurred due to radio-penetration losses.

This is a win-win-win situation for everyone: building owners, carriers and end users.

The possibilities extend beyond LWA, to License Assisted Access (LAA), which is shown in Figure 9. Here, we dispense with Wi-Fi altogether, and carry traffic on the unlicensed spectrum using a form of the LTE air interface.

Small Cell

ENB

Lightweight AP

DAS Remote

Outdoor (Tower)

ENB

Outside Basement Ceiling

DAS Headend

Carrier A @(

UE

Floor

PacketCore

PacketCore

To another building

A B

uild

ing

Carrier B

Wi-Fi Installation

DAS Installation

LTE

LTE

Wi-Fi

Wi-Fi

Wireless LANController

Figure 9

Every LTE smartphone already has a Wi-Fi radio, making LWA possible right now with just a software update. LAA would require new smartphones with LTE-over-unlicensed-band radios.Carrier aggregation integrates the packet schedulers of the component radios, which requires the radios to be tightly synchronized. In practice, the radios must be integrated into the same base-station hardware in the same location. Separating the licensed and unlicensed radios would require a very demanding “ideal backhaul” between them and limit the deployment flexibility of LAA considerably.

Regulatory requirements such as “listen before talk (LBT)” can reduce throughput of the LTE air interface (which was not designed for such restrictions) to below what is offered by Wi-Fi (which was designed for such restrictions).

This idea is derived from the Carrier Aggregation (CA) architecture developed for LTE Advanced. The 3GPP has already welcomed the 5 GHz unlicensed band into the LTE fold, designating it TDD Band 46 (5150-5925 MHz). While LAA can bring additional efficiencies over LWA, in practice LWA appears to be the optimal solution for the following reasons: Still, every one of the options above is

available to the carriers and equipment manufacturers today. There are even side roads, including options based on running IPSEC (IKEv2/ESP) over Wi-Fi on the UE that allow carriers to partner with Wi-Fi networks that wish to join the game but are unwilling to upgrade their Wi-Fi infrastructure to support new 3GPP-specific interface protocols.

All the options that involve Wi-Fi in the unlicensed spectrum may eventually be supported by handsets through software updates, making them available to carriers on equal terms. In the meantime, however, Aricent believes LWA is a technology that has great potential and capabilities that could make it the eventual winner.

16

ENBUE

MMEHSS

OCSAAA

PGWSGW Internet

Unlicensed

Licensed

SIM

17

Aricent is a global design and engineering company innovating for the digital

era. With more than 12,000 design and engineering talent and over 25 years of

experience, we help the world’s leading companies solve their most important

business and technology innovation challenges - from Customer to Chip.

© 2016 Aricent. All rights reserved.

All Aricent brand and product names are service marks, trademarks, or registered marks of Aricent in the United States and other countries.

About Aricent

Contact

Avijit Ghosh, AVP and Global R&D LeadEmail: avijit.ghosh@aricent.com