Automatic Neighbor Relation (ANR) in LTE

Manually adding neighbor cells in network is indeed a very hectic process and prone to errors as well. While  networks are becoming more and more complex, it is required to find an automatic and a more optimized way of adding neighbor cells.

ANR comes under the umbrella of Self Organizing Networks ( SON) features. ANR relies on UE to detect unknown cells and report them to eNB. There are two major types:

i) UE based ANR
ii) ANR with OAM Support

UE based ANR

• No OAM support is required.
• UE detects PCI of unknown cell when it needs to do measurement (as configured by network)
• In case of inter-frequency or inter-RAT measurements, eNB needs to configure measurement gaps/or DRX so UE can detect PCI to different frequencies as well.
• UE reports the unknown PCI to eNB via RRC-Reconfiguration message.
• eNB request UE to report Eutran Cell Global ID (ECGI).
• UE reports ECGI by reading BCCH channel.
• eNB retrieves the IP address from MME to further setup the x2 interface. 

ANR with OAM Support

• OAM support is required
• Every new eNB registers to OAM and download the table with information of PCI/ECGI/IP related to neighbors
•  Neighbors also update their own table with new eNB information
• Now like "UE based ANR", UE will detect unknown PCI and report it to the eNB
• eNB doesn't request for ECGI and does not need support from MME
• eNB setups x2 interface with the help of mapping table created in second step above

Default Bearer, Dedicated Bearer... What exactly is bearer ?

While trying to get a better understanding of VoLTE and IMS , I came across bearers stuff which I thought might be useful to share with you guys. Before we talk about what default and dedicated bearer is, let's understand what bearer itself is :
 
Bearer is just a virtual concept. It defines how the UE data is treated when it travels across the network. Network might treat some data in a special way and treat others normally. Some flow of data might be provided guaranteed bit rate while other may face low transfer.  In short, bearer is a set of network parameter that defines data specific treatment   e.g. Person A will always get at least 256 Kbps download speed on his LTE phone while for person B there is no guaranteed bit rate and might face extremely bad download speed at times

Now let me talk briefly about Dedicated and Default bearer one by one.

Default Bearer in LTE

When LTE UE attaches to the network for the first time, it will be assigned default bearer which remains as long as UE is attached. Default bearer  is best effort service. Each default bearer comes with an IP address. UE can have additional default bearers as well. Each default bearer will have a separate IP address. QCI 5 to 9 (Non- GBR) can be assigned to default bearer. 

Dedicated Bearer

To put it simple, dedicated bearers provides dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc). Dedicated bearer acts as an additional bearer on top of default bearer. It does not require separate IP address due to the fact that only additional default bearer needs an IP address and therefore dedicated bearer is always linked to one of the default bearer established previously. Dedicated bearer can be GBR or non-GBR (whereas default bearer can only be non-GBR). For services like VoLTE we need to provide better user experience and this is where Dedicated bearer would come handy. Dedicated bearer uses Traffic flow templates (TFT) to give special treatment to specific services

Example

Usually LTE networks with VoLTE implementations has two default and one dedicated bearer

Default bearer 1: Used for signaling messages (sip signaling) related to IMS network. It uses qci 5
Dedicated bearer: Used for VoLTE VoIP traffic. It uses qci 1 and is linked to default bearer 1
Default bearer 2: Used for all other smartphone traffic (video, chat, email, browser etc)

Why dont we use dedicated bearer for ims signaling as well ?
IMS network is separate network from normal internet. And it comes with its own APN. Therefore we need separate default bearer for IMS network. This also helps in separating IMS traffic from normal internet traffic as well

How it is decided to send voip traffic (for example) on dedicated bearer?
Both UE and eNB has TFT which has rules for certain services. For example, incase of VoLTE voip traffic, the rule are defined on the basis of protocol number , destination network ip network etc

How is dedicated bearer linked to default bearer?
The value of "Linked EPS bearer identity" defined in setup info of dedicated bearer is used to link dedicated bearer to default bearer

Please also visit Quality of Service (QoS) in LTE to get more better understanding on bearers and QoS

TTI Bundling

With all the hype created around IMS and LTE, operators have started questioning network vendors if they are supporting RAN specific features for VoLTE. TTI bundling is one of the features among many others that can help VoIP (VoLTE) calls in LTE.

TTI Bundling is LTE feature to improve coverage at cell edge or in poor radio conditions. UE has limited power in uplink (only 23dBm for LTE) which can result in many re transmissions at cell edge (poor radio). Re transmission means delay and control plan overhead which may not be acceptable for certain services like VoIP. To understand TTI bundling one need to have the basic idea of Hybrid Automatic Repeat Request (HARQ) and Transmission Time interval (TTI).

HARQ

HARQ is a process where data at mac layer is protected against noisy wireless channels through error correction mechanism. There are couple of different versions of HARQ but in LTE we have a type known as 'Incremental Redundancy Hybrid ARQ'. When receiver detects erroneous data, it doesn't discard it. On the other hand, sender will send the same data again but this time, with different set of coded bits. The reciever will combine the previously recieved erroneous data with newly attempted data by the sender. This way the chances of successfully decoding the bits improve every time. This will repeat as long as the receiver is not able to decode the data. The advantage of this method is  that with each re-transmission, the coding rate is lowered. Whereas in other types of HARQ, it might use the same coding rate in every re-transmission

TTI

TTI is LTE smallest unit of time in which eNB is capable of scheduling any user for uplink or downlink transmission. If a user is receiving downlink data, then  during each 1ms, eNB will assign resources and inform user where to look for its downlink data through PDCCH channel. Check the following figure to understand the concept of TTI

Now coming to TTI Bundling ...

HARQ is a process where receiver combines the new transmission every time with previous erroneous data. There is one drawback however, that it can result in delay and too much control overhead in case of poor radio conditions if the sender has to attempt many transmissions. For services like VoIP this means bad end user experience. Well, there is another way- Instead of re-transmitting the erroneous data with new set of coded bits, why not send few versions (redundancy versions) of the same set of bits in consecutive TTI and eNB sends back Ack when it successfully decodes the bits. I hope  the figure below will make it clear. This way we are avoiding delay and reducing control plane overhead at mac layer 

Voice solutions in LTE

The original idea behind LTE is that it would provide only wireless internet services. However, major revenue for cellular operators comes from voice calls and SMS and therefore Voice in LTE has become a hot topic. Recently, I got an opportunity  to work with various voice solutions-the experience which I believe would be useful to share here.

LTE does not have a 'circuit switch core' which means that we cannot have voice calls as it is in 2G and 3G technologies. In the initial LTE deployment cases however, operators are using their legacy networks along with their 4G network for voice services.

So far we have heard of the following available voice solutions which I will discuss briefly.

• Circuit Switched Fall Back (CSFB)
• Simultaneous Voice and LTE (SV-LTE)
• Voice over LTE (VoLTE)
• Voice over LTE via Generic Access (VoLGA)
• Over the top (OTT)

Circuit Switched Fall Back 

An operator who deployed LTE network, already owning a 3G or 2G network can take benefit from the feature called "Circuit switched fall back'. The main idea is that 4G smartphones are going to have a radio capabilities for 3G/2G networks as well. Such handsets can connect at a time either to LTE or 2G/3G . The shortcoming is that someone on voice call will not be able to use LTE network for browsing or chatting etc.

CSFB for operator means very little investment since only few modifications are required in the network. Additional interface between MME and MSC is required (SGs). CSFB solution has also been standardized by 3GPP and has gained large industrial support.

Simultaneous Voice and LTE

SV-LTE is handset specific in which handset is capabile of using two radios (LTE and WCDMA/GSM/CDMA) at one time. So a user can use packet services from LTE while voice call can be made on other networks simultaneously unlike CSFB. The shortfall here is high battery utilization due to dual radio operation. 

For CDMA and  LTE pair, the SV-LTE is the standard solution and being widely adopted. There are already SV-LTE smartphones available in the market. I came across a few available for LGU+ in Korea and Verizon in USA. Both operate LTE networks as an overlay to their old CDMA networks. 

SV-LTE is the cheapest option for operators as no new modification is required to the network. Nevertheless, as mentioned earlier it is at the cost of high battery utilization

Voice over LTE (IMS)

I believe this is going to be the most popular and widely adopted future voice solution for LTE. Instead of using legacy networks, VoLTE utilizes IP Multimedia Subsystem (IMS) and provides voice services  using the application layer on LTE.
IMS is a group of core network entities responsible for providing rich multimedia services over IP network. VoIP call, SMS, MMS, LIVE TV are a few such services. IMS has been in the communication industry for long but with the emergence of 4G networks, it is gaining popularity again.

Voice over LTE via Generic Access

I think, operators will accept VoLGA as a last option for voice capability. This solution uses CS core only from legacy networks and also require new network elements. Therefore LTE handsets do not need 3G/2G radio capabilities since radio part won't be used from legacy networks. Good thing about this solution however is that unlike CSFB, LTE handset will be able to use voice and data simultaneously.

Over the top VoIP application

OTT is actually not LTE specific but a generic solution that we already have been using on 3G/WiFi networks. OTT application is completely transparent to network and also out of operators' control. I am talking about generic VoIP clients like Viber, skype, Tango etc. They do not give the real taste of voice flexibility as in other 3GPP networks and also lack the QoS for voice. Nonetheless,  these will be widely used by the consumers as an alternative, because of the fact that it gives them full flexibility to choose their own service.

Battery consumption issues and Connected Mode DRX (Discontinuous Reception) in LTE Network

There are various 4G Smartphones emerging in the market now. Apple recently introduced its 4G LTE capable ipad "The new ipad", promising much higher download speed (100 Mbps) to the consumers. We are observing similar devices from other vendors like Samsung, HTC and Nokia. Few examples are HTC Raider, Nokia Lumia 900, Samsung Galaxy S2 & S3 and many more.

All of these 4G capable devices are going to face one common issue of high battery consumption. From technological perspective, large Bandwidth, MIMO, OFDMA are factors which result in high battery drainage. High download speeds would encourage consumers to use data hungry applications on their LTE smartphones, thus adding more to the battery consumption issue.

We know that 2G and 3G terminal uses discontinuous reception in idle mode, whereas in LTE we have similar DRX however but both in idle and connected mode. By idle mode, we understand that the smartphone is not utilizing radio resources. Whereas in connected mode, smartphone utilizes radio resources and battery consumption is very high due to 'over the air' communication between mobile terminal and network antenna (base station). In LTE when there is no data to receive or transmit, smartphone would switch off its transceiver for a very short interval. It will start similar "wake up and sleep " cycle to check whether there is some data that it has to either receive or send. This DRX feature in connected is likely to save a lot of battery usage for consumers.

Currently,most of the Qualcomm based LTE handsets are not fully implementing the connected mode DRX feature. Therefore, LTE smartphone fans have to be patient until Qualcomm and other radio chipset vendors come up with fully connected mode DRX capabilities.

Signaling load in LTE as compared to 3G networks

"Always-On" connectivity of applications in Smartphone is causing many problems in 3G networks world wide. Many smartphone applications (Facebook, email, Skype to name a few) are always running in the background and constantly sending updates and keep-alive packets to servers in the internet.

Every time a smartphone application sends a keep-alive packet, radio resources from the network are taken. The situation can be very problematic when many smartphones in the network are running similar applications in the background. Such behavior was observed by 3G operators when the data storm was seen as a result of high influx of iPhone in operators networks. To make the situation worse, smartphone's vendors introduced another feature i.e. "Fast dormancy" enabling UE to send fake error message to network and go in to the idle state in order to save battery power.This also results in large amount of signaling. Now the question arises how LTE is going to handle this problem.

LTE Networks promise a lot of improvements over 3G Networks,yet facing many challenges.
The first improvement is that we have only two RRC states (RRC-Idle & RRC-Connected) as compared to four RRC states in WCDMA networks. This can largely improve RRC efficiency. LTE Flat architecture will also help in decreasing radio networks signaling load. To improve the battery consumption in LTE terminals, discontinuous reception in RRC connected mode has been introduced, which will switch off smartphone transceiver even in RRC connected state. All these improvements in LTE somehow are very encouraging for the operatiers. In real scenario however,when dealing with millions of users in the networks, this still can be very challenging. There's also a possibility that LTE core network may not be capable of handling the signaling storm in coming years.