VoLTE Bearers

While going through Anritsu whitepaper on VoLTE, I found this picture that explains the concepts of bearers in a VoLTE call well. From the whitepaper:

All networks and mobile devices are required to utilize a common access point name (APN) for VoLTE, namely, “IMS”. Unlike many legacy networks, LTE networks employ the “always-on” conception of packet connectivity: Devices have PDN connectivity virtually from the moment they perform their initial attach to the core network. During the initial attach procedure, some devices choose to name the access point through which they prefer to connect. However, mobile devices are not permitted to name the VoLTE APN during initial attach, i.e., to utilize the IMS as their main PDN, but rather to establish a connection with the IMS AP separately. Thus, VoLTE devices must support multiple simultaneous default EPS bearers.

Note that because the VoLTE APN is universal, mobile devices will always connect through the visited PLMN’s IMS PDN-GW. This architecture also implies the non-optionality of the P-CSCF:

As stated, VoLTE sessions employ two or three DRBs. This, in turn, implies the use of one default EPS bearer plus one or two dedicated EPS bearers. The default EPS bearer is always used for SIP signaling and exactly one dedicated EPS bearer is used for voice packets (regardless of the number of active voice media streams.) XCAP signaling may be transported on its own dedicated EPS bearer – for a total of three active EPS bearers – or it may be multiplexed with the SIP signaling on the default EPS bearer, in which case only two EPS bearers are utilized.

My understanding is that initially when the UE is switched on, a default bearer with QCI 9 (see old posts on QoS/QCI here) is established that would be used for all the signalling. Later on, another default bearer with QCI 5 is established with the IMS CN. When a VoLTE call is being setup, a dedicated bearer with QCI 1 is setup for the voice call. As the article says, another dedicated bearer may be needed for XCAP signalling. If a Video call on top of VoLTE is being used than an additional dedicated bearer with QCI 2 will be setup. Note that the voice pat will still be carried by dedicated bearer with QCI 1.

Do you disagree or have more insight, please feel free to add the comment at the end of the post.

The whitepaper is embedded below and is available to download from slideshare.

Related posts:

• Voice-Over-LTE (VoLTE) Signalling
• Voice over LTE – a step towards future telephony (Ericsson Whitepaper)
• SPS and TTI Bundling Example
• On LTE Roaming ...
• LTE QCI and End-to-end bearer QoS in EPC

3GPP Rel.11 Technology Introduction whitepaper

Another excellent summary whitepaper by R&S, embedded below:

LTE- Advanced (3GPP Rel.11) Technology Introduction from Zahid Ghadialy

Decision Tree of Transmission Modes (TM) for LTE

4G Americas have recently published whitepaper titled "MIMO and Smart Antennas for Mobile Broadband Systems" (available here). The above picture and the following is from that whitepaper:

Figure 3 above shows the taxonomy of antenna configurations supported in Release-10 of the LTE standard (as described in 3GPP Technical Specification TS 36.211, 36.300). The LTE standard supports 1, 2, 4 or 8 base station transmit antennas and 2, 4 or 8 receive antennas in the User Equipment (UE), designated as: 1x2, 1x4, 1x8, 2x2, 2x4, 2x8, 4x2, 4x4, 4x8, and 8x2, 8x4, and 8x8 MIMO, where the first digit is the number of antennas per sector in the transmitter and the second number is the number of antennas in the receiver. The cases where the base station transmits from a single antenna or a single dedicated beam are shown in the left of the figure. The most commonly used MIMO Transmission Mode (TM4) is in the lower right corner, Closed Loop Spatial Multiplexing (CLSM), when multiple streams can be transmitted in a channel with rank 2 or more.

Beyond the single antenna or beamforming array cases diagrammed above, the LTE standard supports Multiple Input Multiple Output (MIMO) antenna configurations as shown on the right of Figure 3. This includes Single User (SU-MIMO) protocols using either open loop or closed loop modes as well as transmit diversity and Multi-User MIMO (MU-MIMO). In the closed loop MIMO mode, the terminals provide channel feedback to the eNodeB with Channel Quality Information (CQI), Rank Indications (RI) and Precoder Matrix Indications (PMI). These mechanisms enable channel state information at the transmitter which improves the peak data rates, and is the most commonly used scheme in current deployments. However, this scheme provides the best performance only when the channel information is accurate and when there is a rich multi-path environment. Thus, closed loop MIMO is most appropriate in low mobility environments such as with fixed terminals or at pedestrian speeds.

In the case of high vehicular speeds, Open Loop MIMO may be used, but because the channel state information is not timely, the PMI is not considered reliable and is typically not used. In TDD networks, the channel is reciprocal and thus the DL channel can be more accurately known based on the uplink transmissions from the terminal (the forward link’s multipath channel signature is the same as the reverse link’s – both paths use the same frequency block). Thus, MIMO improves TDD networks under wider channel conditions than in FDD networks.

One may visualize spatial multiplexing MIMO operation as subtracting the strongest received stream from the total received signal so that the next strongest signal can be decoded and then the next strongest, somewhat like a multi-user detection scheme. However, to solve these simultaneous equations for multiple unknowns, the MIMO algorithms must have relatively large Signal to Interference plus Noise ratios (SINR), say 15 dB or better. With many users active in a base station’s coverage area, and multiple base stations contributing interference to adjacent cells, the SINR is often in the realm of a few dB. This is particularly true for frequency reuse 1 systems, where only users very close to the cell site experience SINRs high enough to benefit from spatial multiplexing SU-MIMO. Consequently, SU-MIMO works to serve the single user (or few users) very well, and is primarily used to increase the peak data rates rather than the median data rate in a network operating at full capacity.

Angle of Arrival (AoA) beamforming schemes form beams which work well when the base station is clearly above the clutter and when the angular spread of the arrival is small, corresponding to users that are well localized in the field of view of the sector; in rural areas, for example. To form a beam, one uses co-polarized antenna elements spaced rather closely together, typically lamda/2, while the spatial diversity required of MIMO requires either cross-polarized antenna columns or columns that are relatively far apart. Path diversity will couple more when the antennas columns are farther apart, often about 10 wavelengths (1.5m or 5’ at 2 GHz). That is why most 2G and 3G tower sites have two receive antennas located at far ends of the sector’s platform, as seen in the photo to the right. The signals to be transmitted are multiplied by complex-valued precoding weights from standardized codebooks to form the antenna patterns with their beam-like main lobes and their nulls that can be directed toward sources of interference. The beamforming can be created, for example, by the UE PMI feedback pointing out the preferred precoder (fixed beam) to use when operating in the closed loop MIMO mode TM4.

For more details, see the whitepaper available here.

Related posts:

• LTE-A: Downlink Transmission Mode 9 (TM-9)
• R&S Whitepaper: LTE Transmission Modes and Beamforming

Is it too early to talk '5G'

While LTE/LTE-A (or 4G) is being rolled out, there is already a talk about 5G. Last week in the LTE World Summit in Amsterdam, there was a whole track on what should 5G be without much technical details. Couple of months back Samsung had announced that they have reached 5G breakthrough. In my talk back in May, I had suggested that 5G would be an evolution on the Radio Access but the core will evolve just little. Anyway, its too early to speculate what the access technology for 5G would be.

Ericsson has published a '5G' whitepaper where they talk about the vision and why and what of 5G rather than going into any technical details. It is embedded below:

5G: What is it? from Ericsson

NEC on 'Radio Access Network' (RAN) Sharing

Its been a while we looked at anything to do with Network Sharing. The last post with an embed from Dr. Kim Larsen presentation, has already crossed 11K+ views on slideshare. Over the last few years there has been a raft of announcements about various operators sharing their networks locally with the rivals to reduce their CAPEX as well as their OPEX. Even though I understand the reasons behind the network sharing I believe that the end consumers end up losing as they may not have a means of differentiating between the different operators on a macro cell.

Certain operators on the other hand offer differentiators like residential femtocells that can enhance indoor coverage or a tie up with WiFi hotspot providers which may provide them wi-fi access on the move. The following whitepaper from NEC is an interesting read to understanding how RAN sharing in the LTE would work.

“NEC’s Approach towards Active Radio Access Network Sharing” from Zahid Ghadialy

eMBMS Release-11 enhancements

Continuing on the eMBMS theme. In the presentation in the last post, there was introduction to the eMBMS protocols and codecs and mention about the DASH protocol. This article from the IEEE Communications magazine provides insight into the working of eMBMS and what potential it holds.

Evolved Multimedia Broadcast/Multicast Service (eMBMS) in LTE-Advanced: Overview and Rel-11 Enhancements from Zahid Ghadialy

Myths and Challenges in Future Wireless Access

Interesting article from the recent IEEE Comsoc magazine. Table 1 on page 5 is an interesting comparison of how different players reach the magical '1000x' capacity increase. Even though Huawei shows 100x, which may be more realistic, the industry is sticking with the 1000x figure. 

Qualcomm is touting a similar 1000x figure as I showed in a post earlier here.

Direct Communication in 3GPP Release-12

Here is a presentation from LG on this topic:

Direct Communication in 3GPP from Zahid Ghadialy

And a magazine article from IEEE on this topic:

Design Aspects of Network Assisted Device-to-Device Communications from Zahid Ghadialy

If interested in doing more research on this topic, also see this link.

Related posts:

• Public Safety Communications using LTE
• 3GPP Release-12 and beyond

VoLTE, Battery Issues and Solutions

Sometime back we had news about how VoLTE is battery killer and how it would suck our 4G phones dry. Well, I agree. I am no fan of VoLTE and think that CSFB solution can suffice in mid-term. Having said that, there is a solution which would be soon available to sort this battery issue during VoLTE call. I had a post on this topic earlier titled SPS and TTI Bundling. I am not sure about exactly how much saving would occur if either of the features are implemented.

ST Ericsson has recently released a whitepaper on this topic that is embedded below. If you have more idea on this, please add it in comments.

LTE, Telephony and Battery Life from Zahid Ghadialy

IMS Whitepapers from Spirent

Couple of old but interesting whitepapers from Spirent available, in case you are interesting in IMS. Available to download from here (registration required)

Related blog posts:

• IMS Release 10 Tutorial
• ETSI INT IMS/EPC Interoperability Standardisation: Motivation, Roadmap & First Results
• UICC and ISIM (IMS SIM)

Quick Introduction to LTE-Advanced

An article written by me for the Mobile Europe magazine where I try and explain LTE-A without going in technical details. This also includes the state of market on who is doing what.

LTE, LTE-A and 4G from eXplanoTech

Developing and Integrating a High Performance HetNet

I have seen on Twitter some people think that HetNets (Heterogeneous Networks) is just a new name for the Hierarchical Cell Structures (HCS). The main difference between then is that while HCS requires all layers to have different frequencies, HetNets can use the same or the different frequency. In case the same frequency is used, there needs to be a way to manage interference between the different layers. In fact the term 'layers' is hardly used with HetNets as there is nothing strictly hierarchical with different types of cells that co-exist in a HetNet. Typically a HetNet comprises of Macro cells, Micro/Pico cells, other Small Cells (including Femtocells) and WiFi as well (if used to offload traffic).

This recent whitepaper from 4G Americas is an excellent source to understand more about HetNets

Available to download from Slideshare here.

LTE-Advanced: An Operator Perspective

3GPP SA1 Release 11 Standardisation Trends

An Interesting article from the NTT Docomo technical journal:

3GPP SA1 Release 11 Standardisation Trends from Zahid Ghadialy

Related posts:

• Non-Voice Emergency Services (NOVES)
• Network Improvements for Machine Type Communications (NIMTC)
• Evolution of Machine Type Communications (MTC)
• 3GPP Release-12 and beyond

HSPA+ Evolution: Standards and Data Rates

From the 4G Americas white paper.

IMS Release 10 Tutorial

IMS Release 10 Tutorial

View more documents from Zahid Ghadialy

Available to download from here.

Voice over HSPA (VoHSPA) and CS over HSPA (CSoHS)

4G Americas has recently released a whitepaper entitled, "Delivering voice over HSPA". This paper describes the technological features that are being developed to make Voice over HSPA (VoHSPA) a reality. It describes the two potential options for VoHSPA. The first option leverages IP Multimedia Subsystem (IMS) technology developed in conjunction with Long Term Evolution (LTE), and is referred to as IMS Voice over HSPA or simply IMS Voice. The other option delivers voice by modifying existing circuit-switch based techniques so that those communications can be transmitted over an HSPA infrastructure, and is referred to as CS Voice over HSPA (CSoHS). Both the options are shown in the picture above. Note that there is no discussion about Over the top (OTT) type voice services like Skype, etc. 

The chief among benefits anticipated from VoHSPA are increases in the spectral efficiency of mobile networks. With these new techniques, voice calls can be delivered more efficiently from a spectral standpoint over Packet Switched (PS) rather than Circuit Switched (CS) networks freeing up radio resources for additional data traffic.

The 4G Americas report defines work completed by the GSMA for a minimum mandatory set of features defined in existing 3GPP Release 8 specifications (IR 58: IMS Profile for VoHSPA) that should be implemented in order to insure an interoperable, high quality, IMS-based telephony service over an HSPA radio access layer. In the white paper, 4G Americas recommends additional features, above the minimum mandatory features in IR 58, for VoHSPA either under an IMS or a CS approach, in order to minimize packet losses and variations in packet arrival times that can impair the quality of voice communications.

The whitepaper is available to download from here.

R&S Whitepaper: LTE Transmission Modes and Beamforming

Interesting technical whitepaper from R&S:

LTE Transmission Modes and Beamforming

View more documents from Zahid Ghadialy

Available to download from here or here.

HSPA vs LTE

Interesting report to remind the differences between HSPA and LTE available here.

Quick update on LTE Release 11 Work and Study Items

LTE Release 11 Work and Study Items

View more documents from Zahid Ghadialy

Nice work by Nomor. Available to download from here.