Enhancing voice services using VoLTE

VoLTE has been a very popular topic on this blog. My overview of the LTE Voice Summit missed out narrowly from the Top 10 posts of 2014 but there were other posts related to VoLTE that made it.

In this magazine article, NTT Docomo not only talks about its own architecture and transition from 3G to 4G for voice and video, it provides some detailed insights from its own experience.

There is also discussion into technical details of the feature and examples of signalling for VoLTE registration and originating/terminating calls (control, session and user plane establishment), SMS, SRVCC, Video over LTE (ViLTE) and voice to video call switching.

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

VoLTE for Enhancing Voice Services from Zahid Ghadialy

Related links:

• DOCOMO VoLTE and Next Steps
• VoLTE Roaming with RAVEL (Roaming Architecture for Voice over IMS with Local Breakout)
• 2015 will finally be the year of Voice over LTE (VoLTE)

Detailed whitepaper on Carrier Aggregation by 4G Americas

4G Americas has published a detailed whitepaper on Carrier Aggregation (CA). Its a very good detailed document for anyone wishing to study CA.

Two very important features that have come as part of CA enhancements were the multiple timing advance values that came as a part of Release-11 and TDD-FDD joint operation that came part of Release-12

While its good to see that up to 3 carriers CA is now possible as part of Rel-12 and as I mentioned in my last post, we need this to achieve the 'Real' 4G. We have to also remember at the same time that these CA makes the chipsets very complex and may affect the sensitivity of the RF receivers.

Anyway, here is the 4G Americas whitepaper.

LTE Carrier Aggregation Technology Development and Deployment Worldwide from Zahid Ghadialy

You can read more about the 4G Americas whitepaper in their press release here.

Envelope Tracking for improving PA efficiency of mobile devices

I am sure many people would have heard of ET (Envelope Tracking) by now. Its a technology that can help reduce the power consumption by our mobile devices. Less power consumption means longer battery life, especially with all these new features coming in the LTE-A devices.

As the slide says, there are already 12 phones launched with this technology, the most high profile being iPhone 6/6 Plus. Here is a brilliant presentation from Nujira on this topic:

Envelope Tracking : Fuel injection for the RF Front End from Zahid Ghadialy

For people who are interested in testing this feature may want to check this Rohde&Schwarz presentation here.

Elevation Beamforming / Full-Dimension MIMO

Four major Release-13 projects have been approved now that Release-12 is coming to a conclusion. One of them is Full dimension MIMO. From the 3GPP website:

Leveraging the work on 3D channel modeling completed in Release 12, 3GPP RAN will now study the necessary changes to enable elevation beamforming and high-order MIMO systems. Beamforming and MIMO have been identified as key technologies to address the future capacity demand. But so far 3GPP specified support for these features mostly considers one-dimensional antenna arrays that exploit the azimuth dimension. So, to further improve LTE spectral efficiency it is quite natural to now study two-dimensional antenna arrays that can also exploit the vertical dimension.

Also, while the standard currently supports MIMO systems with up to 8 antenna ports, the new study will look into high-order MIMO systems with up to 64 antenna ports at the eNB, to become more relevant with the use of higher frequencies in the future.

Details of the Study Item can be found in RP-141644.

There was also an interesting post by Eiko Seidel in the 5G standards group:

The idea is to introduce carrier and UE specific tilt/beam forming with variable beam widths. Improved link budget and reduced intra- and inter-cell interference might translate into higher data rates or increased coverage at cell edge. This might go hand in hand with an extensive use of spatial multiplexing that might require enhancements to today’s MU-MIMO schemes. Furthermore in active antenna array systems (AAS) the power amplifiers become part of the antenna further improving the link budget due to the missing feeder loss. Besides a potentially simplified installation the use of many low power elements might also reduce the overall power consumption. 

At higher frequencies the antenna elements can miniaturized and their number can be increased. In LTE this might be limited to 16, 32 or 64 elements while for 5G with higher frequency bands this might allow for “massive MIMO”. 

WG: Primary RAN1 (RP-141644) 
started 06/2014 (RAN#64), completion date 06/2015 (RAN#68)
work item might follow the study with target 12/2015 (RAN#70) 

Supporting companies
Samsung/NSN, all major vendors and operators 

Based on RAN1 Rel.12 Study Item on 3D channel model (TR36.873) 

Objectives 
Phase 1: antenna configurations and evaluation scenarios Rel.12 performance evaluation with 3D channel model 

Phase 2: study and simulate FD-MIMO enhancement identify and evaluate techniques, analyze specification impact performance evaluation for 16, 32, 64 antenna elements enhancements for SU-/MU-MIMO (incl. higher dimension MU-MIMO) (keep the maximum number of layer per UE unchanged to 8)

An old presentation from Samsung is embedded below that will provide more insight into this technology:

Full Dimension MIMO and Beamforming Technologies for B4G from Zahid Ghadialy

Related post:

• 3D-Beamforming and 3D-MIMO

New LTE-A UE Category 9 and 10 in Rel-11

Its been a while since we saw any new UE categories coming but then I noticed some new categories came earlier this year for Release-11. The latest 3TPP TS 36.306 have these new Category 9 and Category 10 as follows.

For those who are aware of the categories of the UE's being used in practice may be aware that the most common ones have been 'Category 3' with 100Mbps max in DL and 50Mbps max in UL. The new 'Cat. 4' devices are becoming more common as more manufacturers start bringing these devices to the market. They support 150Mbps max in DL and 50Mbps max in UL. Neither of them supports Carrier Aggregation.

Having said that, a lot of Cat. 4 devices that we may use in testing actually supports carrier aggregation. The next most popular devices soon to be hitting the market is Cat. 6 UE's with 300Mbps max in DL and 50Mbps max in UL. Category 6 UE's support 2 x 20MHz CA in downlink hence you can say that they can combine 2 x Cat. 4 UE's in DL but they do not support CA in uplink hence the UL part remains the same as Cat. 4 device.

Cat. 9 and 10 are interesting case as Car. 8 was already defined earlier to meet IMT-A requirement as shown below.

To meet IMT-A requirements of peak data rates of 1Gbps in UL and DL, LTE-A had to define category 8 with 5 band CA and 8x8 MIMO to be able to provide 3Gbps max in DL and 1.5Gbps max in UL. No one sees this device becoming a reality in the short term.

The new categories will have to be defined from Cat. 9 onwards.

Cat. 9 allows 3 x Cat. 4 device CA in the downlink to have the maximum possible downlink data rates of 450Mbps but there is no CA in the uplink. As a result, the UL is still 50Mbps max. Cat. 10 allows carrier aggregation in the uplink for upto 2 bands which would result in 100Mbps max in UL.

The LG space website gives a better representation of the same information above which is shown below:

A UE category 9 transmits Rel 11 category 9 + Rel 10 category 6 + Rel 8 category 4

With Release-12 due to be finalised later in the year, we may see new UE categories being defined further.

Multi-Frequency Band Indicator (MFBI)

I am sure we all know that LTE bands have been growing, every few months. All the 32 bands for FDD have now been defined. The 33rd band is where TDD bands start. What if we now want to have more FDD bands? Well, we will have to wait to fix that problem.

Picture Source: LG Space

Anyway, as can be seen in the above picture, some of the frequency bands overlap with each other. Now you may have a UE thats camped onto one frequency that is overlapping in different bands. Wouldn't it be useful to let the UE know that you are camped in more than one band and you can change it to another frequency which may be a different band but you were already on it in the first place (it may sound confusing).

Here is a much simpler table from the specs that show that when a UE is camped on band 5, it may also be camped on bands 18, 19 and 26. Remember the complete bands may not be overlapping but may only be partially overlapping.

An example could be Sprint that used Band 38 TDD (BW 50MHz) for its legacy devices but is now able to use Band 41 (BW 194MHz) as well. The legacy devices may not work on Band 41 but the new devices can use much wider band 41. So the transmission would still say Band 38 but the new devices can be informed of Band 41 using the System Information Block Type 1. AT&T has a similar problem with Band 12 and 17.

Even though this was implemented in Release-8, it came as a part of Late Non-critical extensions. Its a release independent feature but not all UE's and Network have implemented it. The UE indicates the support for MFBI using the FGI (Feature Group Indicator) bits. 

Observed Time Difference Of Arrival (OTDOA) Positioning in LTE

Its been a while I wrote anything on Positioning. The network architecture for the positioning entities can be seen from my old blog post here. 

Qualcomm has recently released a whitepaper on the OTDOA (Observed Time Difference Of Arrival) positioning. Its quite a detailed paper with lots of technical insights.

There is also signalling and example of how reference signals are used for OTDOA calculation. Have a look at the whitepaper for detail, embedded below.

Observed Time Difference Of Arrival (OTDOA) Positioning in 3GPP LTE from Zahid Ghadialy

LA-LTE and LAA

Recently came across a presentation by Ericsson where they used the term LA-LTE. I asked a few colleagues if they knew or could guess what it means and they all drew blank. I have been blogging about Unlicensed LTE (a.k.a. LTE-U) on the Small Cells blog here. This is a re-branding of LTE-U

LA-LTE stands for 'Licensed Access' LTE. In fact the term that has now been adopted in a recent 3GPP workshop (details below) is Licensed Assisted Access (LAA).

Couple of months back I blogged in detail about LTE-U here. Since then, 3GPP held a workshop where some of the things I mentioned got officially discussed. In case you want to know more, details here. I have to mention that the operator community is quite split on whether this is a better approach or aggregating Wi-Fi with cellular a better approach.

The Wi-Fi community on the other hand is unhappy with this approach. If cellular operators start using their spectrum than it means less spectrum for them to use. I wrote a post on the usage of Dynamic Spectrum Access (DSA) Techniques that would be used in such cases to make sure that Wi-Fi and cellular usage does not happen at the same time, leading to interference.

Here is a presentation from the LTE-U workshop on Use cases and scenarios, not very detailed though.

Finally, the summary presentation of the workshop. As it says on the final slide "The current SI proposal focuses on carrier aggregation operations and uses the acronym LAA (Licensed Assisted Access)", you would be seeing more of LAA.

3GPP workshop on LTE in unlicensed spectrum, June 13, 2014: Chairman Summary from Zahid Ghadialy

Case Study: Migrating from WiMAX to TD-LTE

I was glad to hear this case study by Mike Stacey where they have a WiMAX network already deployed and are in process of moving to TD-LTE. Along with the technical issues there are also business and customer issues that need to be taken into account while doing this technology swap. Surprisingly 3.5GHz is also not a very popular band because there are very few deployments in this spectrum. On the other hand, most of the companies worldwide that have been able to get their hands on this spectrum, generally got a big chunk (60-100MHz) so they would be able to do CA easily (bar the technical issues of Intra-band interference).

Anyway, the presentation is embedded below. Hope you find it useful. If you know of similar experiences, please feel free to add them in the comments.

Overcoming Interference Challenges when migrating from WiMAX to LTE from Zahid Ghadialy

Diamater: Market Status, Roaming, NFV and Case Studies

Some more interesting presentations from the Signalling Focus Day of LTE World Summit. Good overview of market by Greg Collins of Exact ventures is embedded below.

Diameter Signaling Market Update from Zahid Ghadialy

A good presentation by Tieto where they presented some good case studies for Diameter Interworking. Presentation embedded below:

Diameter routing & interworking in NFV from Zahid Ghadialy

The final presentation by Diametriq is very interesting because they presented interesting way of mining the control plane. Thee case study presented was of a 'silent roamer' who is not going to spend money while roaming because he is not sure how much money is spent. This can be exploited by the operator to offer flat packages, 1 day pass, etc. to get some revenue from these roamers. Their presentation included some animations that cannot be shown while being embedded. Please download the PPT from Slideshare to view them.

Unlocking New Value in the Diameter Signaling Path from Zahid Ghadialy

International LTE Data and VoLTE Roaming - NTT Docomo

Quick recap of the Bearer Architecture: Remember the interface between S-GW and P-GW is known as S5/S8. S5 in case the S-GW and P-GW are part of the same network (non-roaming case) and S8 in case where P-GW belongs to another network than S-GW (roaming case). The S5/S8 interfaces are generally exactly the same. There is a possibility of different types of S5/S8 interfaces like GTP based and PMIP based but lets not discuss that here.

NTT Docomo published an excellent article in their magazine recently showing the different approaches to International Data roaming.

The different scenarios above are based on the guidelines provided in GSMA PRD IR.88. Each operator has to adopt one of the scenarios above, NTT Docomo has selected scenario 4. The Home PLMN (HPLMN) and the Visited PLMN (VPLMN) connect via IP eXchange (IPX).

As can be seen above, the MME in VPLMN communicates with HSS in HPLMN using Diameter Edge Agent (DEA).

Finally, it is well known that NTT Docomo is not launching VoLTE untill 2015. The above is their proposal on how they handle VoLTE while in Japan and when roaming.

The paper is an interesting read, embedded below:

NTT Docomo: Implementing LTE International Data Roaming from Zahid Ghadialy

Another article worth a read is the VoLTE roaming with RAVEL here.

VoLTE Roaming with RAVEL (Roaming Architecture for Voice over IMS with Local Breakout)

Voice over LTE or VoLTE has many problems to solve. One of the issues that did not have a clear solution initially was Roaming. iBasis has a whitepaper on this topic here, from which the above picture is taken. The following is what is said above:

The routing of international calls has always been a problem for mobile operators. All too often the answer—particularly in the case of ‘tromboning’ calls all the way back to the home network—has been inelegant and costly. LTE data sessions can be broken out locally, negating the need for convoluted routing solutions. But in a VoIMS environment all of the intelligence that decides how to route the call resides in the home network, meaning that the call still has to be routed back.

The industry’s solution to this issue is Roaming Architecture for Voice over LTE with Local Breakout (RAVEL). Currently in the midst of standardisation at 3GPP, RAVEL is intended to enable the home network to decide, where appropriate, for the VoIMS call to be broken out locally. 

Three quarters of respondents to the survey said they support an industry-wide move to RAVEL for VoLTE roaming. This is emphatic in its enthusiasm but 25 per cent remains a significant share of respondents still to be convinced. Just over half of respondents said they plan to support VoIMS for LTE roaming using the RAVEL architecture, while 12.3 per cent said they would support it, but not using RAVEL.

Until RAVEL is available, 27.4 per cent of respondents said they plan to use home-routing for all VoLTE traffic, while just under one fifth said they would use a non-standard VoLTE roaming solution.

Well, the solution was standardised in 3GPP Release-11. NTT Docomo has an excellent whitepaper (embedded below) explaining the issue and the proposed solution.

In 3GPP Release 11, the VoLTE roaming and interconnection architecture was standardized in cooperation with the GSMA Association. The new architecture is able to implement voice call charging in the same way as circuit-switched voice roaming and interconnection models by routing both C-Plane messages and voice data on the same path. This was not possible with the earlier VoLTE roaming and interconnection architecture.

Anyway, here is the complete whitepaper

NTT Docomo: VoLTE Roaming and Interconnection Standard Technology from Zahid Ghadialy

Different flavours of SRVCC (Single Radio Voice Call Continuity)

Single Radio Voice Call Continuity (SRVCC) has been quietly evolving with the different 3GPP releases. Here is a quick summary of these different flavors

In its simplest form, SRVCC comes into picture when an IMS based VoLTE call is handed over to the existing 2G/3G network as a normal CS call. SRVCC is particularly important when LTE is rolled out in small islands and the operator decided to provide VoLTE based call when in LTE. An alternative (used widely in practice) is to use CS Fallback (CSFB) as the voice option until LTE is rolled out in a wider area. The main problem with CSFB is that the data rates would drop to the 2G/3G rates when the UE falls back to the 2G/3G network during the voice call.

The book "LTE-Advanced: A Practical Systems Approach to Understanding 3GPP LTE Releases 10 and 11 Radio Access Technologies" by Sassan Ahmadi has some detailed information on SRVCC, the following is an edited version from the book:

SRVCC is built on the IMS centralized services (ICS) framework for delivering voice and messaging services to the users regardless of the type of network to which they are attached, and for maintaining service continuity for moving terminals.

To support GSM and UMTS, some modifications in the MSC server are required. When the E-UTRAN selects a target cell for SRVCC handover, it needs to indicate to the MME that this handover procedure requires SRVCC. Upon receiving the handover request, the MME triggers the SRVCC procedure with the MSC server. The MSC then initiates the session transfer procedure to IMS and coordinates it with the circuit-switched handover procedure to the target cell.

Handling of any non-voice packet-switched bearer is by the packet-switched bearer splitting function in the MME. The handover of non-voice packet-switched bearers, if performed, is according to a regular inter-RAT packet-switched handover procedure.

When SRVCC is enacted, the downlink flow of voice packets is switched toward the target circuit-switched network. The call is moved from the packet-switched to the circuit-switched domain, and the UE switches from VoIP to circuit-switched voice.

3GPP Rel-10 architecture has been recommended by GSMA for SRVCC because it reduces both voice interruption time during handover and the dropped call rate compared to earlier configurations. The network controls and moves the UE from E-UTRAN to UTRAN/GERAN as the user moves out of the LTE network coverage area. The SRVCC handover mechanism is entirely network-controlled and calls remain under the control of the IMS core network, which maintains access to subscribed services implemented in the IMS service engine throughout the handover process. 3GPP Rel-10 configuration includes all components needed to manage the time-critical signaling between the user’s device and the network, and between network elements within the serving network, including visited networks during roaming. As a result, signaling follows the shortest possible path and is as robust as possible, minimizing voice interruption time caused by switching from the packet-switched core network to the circuit-switched core network, whether the UE is in its home network or roaming. With the industry aligned around the 3GPP standard and GSMA recommendations, SRVCC-enabled user devices and networks will be interoperable, ensuring that solutions work in many scenarios of interest.

Along with the introduction of the LTE radio access network, 3GPP also standardized SRVCC in Rel-8 specifications to provide seamless service continuity when a UE performs a handover from the E-UTRAN to UTRAN/GERAN. With SRVCC, calls are anchored in the IMS network while the UE is capable of transmitting/ receiving on only one of those access networks at a given time, where a call anchored in the IMS core can continue in UMTS/GSM networks and outside of the LTE coverage area. Since its introduction in Rel-8, the SRVCC has evolved with each new release, a brief summary of SRVCC capability and enhancements are noted below

3GPP Rel-8: Introduces SRVCC for voice calls that are anchored in the IMS core network from E-UTRAN to CDMA2000 and from E-UTRAN/UTRAN (HSPA) to UTRAN/GERAN circuit-switched. To support this functionality, 3GPP introduced new protocol interface and procedures between MME and MSC for SRVCC from E-UTRAN to UTRAN/GERAN, between SGSN and MSC for SRVCC from UTRAN (HSPA) to UTRAN/GERAN, and between the MME and a 3GPP2-defined interworking function for SRVCC from E-UTRAN to CDMA 2000.

• 3GPP Rel-9: Introduces the SRVCC support for emergency calls that are anchored in the IMS core network. IMS emergency calls, placed via LTE access, need to continue when SRVCC handover occurs from the LTE network to GSM/UMTS/CDMA2000 networks. This evolution resolves a key regulatory exception. This enhancement supports IMS emergency call continuity from E-UTRAN to CDMA2000 and from E-UTRAN/UTRAN (HSPA) to UTRAN/ GERAN circuit-switched network. Functional and interface evolution of EPS entities were needed to support IMS emergency calls with SRVCC.

• 3GPP Rel-10: Introduces procedures of enhanced SRVCC including support of mid-call feature during SRVCC handover (eSRVCC); support of SRVCC packet-switched to circuit-switched transfer of a call in alerting phase (aSRVCC); MSC server-assisted mid-call feature enables packet-switched/ circuit-switched access transfer for the UEs not using IMS centralized service capabilities, while preserving the provision of mid-call services (inactive sessions or sessions using the conference service). The SRVCC in alerting phase feature adds the ability to perform access transfer of media of an instant message session in packet-switched to circuit-switched direction in alerting phase for access transfers.

• 3GPP Rel-11: Introduces two new capabilities: single radio video call continuity for 3G-circuit-switched network (vSRVCC); and SRVCC from UTRAN/GERAN to E-UTRAN/HSPA (rSRVCC). The vSRVCC feature provides support of video call handover from E-UTRAN to UTRAN-circuitswitched network for service continuity when the video call is anchored in IMS and the UE is capable of transmitting/receiving on only one of those access networks at a given time. Service continuity from UTRAN/GERAN circuitswitched access to E-UTRAN/HSPA was not specified in 3GPP Rel-8/9/10. To overcome this drawback, 3GPP Rel-11 provided support of voice call continuity from UTRAN/GERAN to E-UTRAN/HSPA. To enable video call transfer from E-UTRAN to UTRAN-circuit-switched network, IMS/EPC is evolved to pass relevant information to the EPC side and S5/S11/Sv/Gx/Gxx interfaces are enhanced for video bearer-related information transfer. To support SRVCC from GERAN to E-UTRAN/HSPA, GERAN specifications are evolved to enable a mobile station and base station sub-system to support seamless service continuity when a mobile station hands over from GERAN circuit-switched access to EUTRAN/ HSPA for a voice call. To support SRVCC from UTRAN to EUTRAN/ HSPA, UTRAN specifications are evolved to enable the RNC to perform rSRVCC handover and to provide relative UE capability information to the RNC.

NTT Docomo has a presentation on SRVCC and eSRVCC which is embedded below:

Improvement of VoLTE Domain HO with Operators Vision from Zahid Ghadialy

Advancements in Congestion control technology for M2M

NTT Docomo recently published a new article (embedded below) on congestion control approaches for M2M. In their own words:

Since 3GPP Release 10 (Rel. 10) in 2010, there has been active study of technical specifications to develop M2M communications further, and NTT DOCOMO has been contributing proactively to creating these technical specifications. In this article, we describe two of the most significant functions standardized between 3GPP Rel. 10 and Rel. 11: the M2M Core network communications infrastructure, which enables M2M service operators to introduce solutions more easily, and congestion handling technologies, which improve reliability on networks accommodating a large number of terminals.

Complete article as follows:

Core Network Infrastructure and Congestion Control Technology for M2M Communications from Zahid Ghadialy

Other related posts:

• Rise of the "Thing"
• Access Class Barring in LTE using System Information Block Type 2
• Extended Access Barring (EAB) in Release 11 to avoid MTC overload
• RAN mechanisms to avoid CN overload due to MTC
• Service Specific Access Control (SSAC) in 3GPP Release 9

eIMTA: Enhanced Interference Mitigation & Traffic Adaptation

eIMTA is one of the features being discussed in 3GPP Rel-12. The pictures above and below provide the details.

As can be seen, at the moment all the eNodeB's associated with a network has to transmit the same UL/DL pattern throughout out the system. With eIMTA, each eNodeB can decide the UL/DL pattern itself depending on the load.

The main challenge would be interference management while using this scheme.

See also, this slideshare presentation for details:

White Paper: Dynamic TDD for LTE-a (eIMTA) and 5G from Eiko Seidel

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

Multi-RAT mobile backhaul for Het-Nets

Recently got another opportunity to hear from Andy Sutton, Principal Network Architect, Network Strategy, EE. His earlier presentation from our Cambridge Wireless event is here. There were many interesting bits in this presentation and some of the ones I found interesting is as follows:

Interesting to see in the above that the LTE traffic in the backhaul is separated by the QCI (QoS Class Identifiers - see here) as opposed to the 2G/3G traffic.

This is EE's implementation. As you may notice 2G and 4G use SRAN (Single RAN) while 3G is separate. As I mentioned a few times, I think 3G networks will probably be switched off before the 2G networks, mainly because there are a lot more 2G M2M devices that requires little data to be sent and not consume lots of energy (which is an issue in 3G), so this architecture may be suited well.

Finally, a practical network implementation which looks different from the text book picture and the often touted 'flat' architecture. Andy did mention that they see a ping latency of 30-50ms in the LTE network as opposed to around 100ms in the UMTS networks.

Mark Gilmour was able to prove this point practically.

Here is the complete presentation:

Timing Accuracy and Phase Performance Requirements in LTE/LTE-A/4G

Nice quick summary videos from Chronos.



If you are interested in learning more on this topic or discussions, I would recommend joining the Phase Ready Linkedin group.

New Carrier Type (NCT) in Release-12 and Band 29

One of the changes being worked on and is already available in Release-11 is the Band 29. Band 29 is a special FDD band which only has a downlink component and no uplink component. The intention is that this band is available an an SCell (Secondary cell) in CA (Carrier Aggregation). 

What this means is that if this is only available as an SCell, any UE that is pre-Rel-11 should not try to use this band. It should not read the system information, reference information, etc. In fact the System Information serves little or no purpose as in CA, the PCell will provide the necessary information for this SCell when adding it using the RRC Reconfiguration message. This gives rise to what 3GPP terms as New Carrier Type for LTE as defined here. An IEEE paper published not long back is embedded below that also describes this feature in detail. 

The main thing to note from the IEEE paper is what they have shown as the unnecessary information being removed to make the carrier lean.

China Mobile, in their Rel-12 workshop presentation, have suggested 3 different types/possibilities for the NCT for what they call as LTE-Hi (Hi = Hotspot and Indoor).

Ericsson, in their Rel-12 whitepaper mention the following with regards to NCT:

Network energy efficiency is to a large extent an implementation issue. However, specific features of the LTE technical specifications may improve energy efficiency. This is especially true for higher-power macro sites, where a substantial part of the energy consumption of the cell site is directly or indirectly caused by the power amplifier.

The energy consumption of the power amplifiers currently available is far from proportional to the power-amplifier output power. On the contrary, the power amplifier consumes a non-negligible amount of energy even at low output power, for example when only limited control signaling is being transmitted within an “empty” cell.

Minimizing the transmission activity of such “always-on” signals is essential, as it allows base stations to turn off transmission circuitry when there is no data to transmit. Eliminating unnecessary transmissions also reduces interference, leading to improved data rates at low to medium load in both homogeneous as well as heterogeneous deployments.

A new carrier type is considered for Release 12 to address these issues. Part of the design has already taken place within 3GPP, with transmission of cell-specific reference signals being removed in four out of five sub frames. Network energy consumption can be further improved by enhancements to idle-mode support.

The IEEE paper I mentioned above is as follows:

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