3GPP Release-12 and beyond

3GPP Recently held a workshop on "Release 12 and Onward" to identify common requirements for future 3GPP radio access technologies. The goal of the workshop is to investigate what are the main changes that could be brought forward to evolve RAN toward Release 12 and onward. It is recommended that presentations in the workshop include views on:
- Requirements
- Potential technologies
- Technology roadmap for Releases 12, 13 and after

The discussions from the workshop should be used to define the work plan for Release 12 and onward in TSG-RAN.

The list of presentations and links, etc. are below and I have also embedded the Summary and Draft report, both of which can be downloaded from 3GPP website or slideshare. Here is a list of different topics and the presentations that covered them:

AdHoc Networks
AdHoc Networks - RWS-120035


Antennas, Beamforming, Transmitters, Receivers
3D-beamforming - RWS-120002
Vertical sectorization/3D beamforming via AAS - RWS-120005
Advanced receivers and joint Tx/Rx optimisation - RWS-120005
Network assistance for IC receivers - RWS-120005
Support of Active Antenna Systems - RWS-120006
Advanced transmitter beamforming - RWS-120010
Advanced receiver cancellation - RWS-120010
Vertical and 3D beamforming - RWS-120011
MIMO Enhancements - RWS-120014
New antenna configurations and 3D MIMO - RWS-120014
UE AAS (Active Antenna System) [Detailed] - RWS-120015 / RWS-120049
Cloud of Antennas (CoA) Concept - RWS-120016
Support of Massive MIMO Technology - RWS-120016
Full Dimension MIMO (FD-MIMO) System [Detailed] - RWS-120021 / RWS-120046
Cloud-RAN: Benefits and Drawbacks - RWS-120021 / RWS-120046
Further Enhanced Receivers - RWS-120022
Multiple antenna evolution - RWS-120025
3D beamforming - RWS-120026
Vision of 3D MIMO - RWS-120029
Massive MIMO & 3D MIMO - RWS-120034
Potential MIMO Enhancements - RWS-120035
Advanced Antenna Technology - RWS-120035
DL MIMO Enhancement - RWS-120037
Performance Requirement for 8Rx at eNB - RWS-120037
UE Receiver Enhancements - RWS-120039
DL MU-MIMO Enhancement - RWS-120039
Enhancement of MIMO, CoMP - RWS-120040
Advanced MIMO - RWS-120040
MIMO and COMP - RWS-120041
Role of Advanced Receivers - RWS-120041
Advanced Interference Handling - RWS-120041
Interference Suppression Subframes (ISS) and IRC Receiver [Detailed] - RWS-120051


Applications (Apps)
Efficiency for diverse small data applications - RWS-120011
Device Service/Application Awareness - RWS-120018
I-Net:”I”-centric mobile network design philosophy - RWS-120024
Application Aware Comm - RWS-120036 / RWS-120050


Backhaul and Relay
Relay backhaul enhancement - RWS-120011
LTE Backhaul - RWS-120013
Relay - RWS-120025
CoMP, backhaul and X2 interface - RWS-120027 / RWS-120048
Mobile Relay And Relay Backhaul Enhancement - RWS-120029


Baseband
Baseband resource pooling and virtualization - RWS-120011


Capacity and Coverage
Higher system capacity - RWS-120010
Capacity for Mobile Broadband: Requirements and Candidate technologies - RWS-120012
Increase N/W capacity by 1000 times - RWS-120020
Coverage Enhancement - RWS-120037
Capacity Enhancement - RWS-120038 / RWS-120047
Cell-edge Throughput Improvement - RWS-120038 / RWS-120047


Carrier Aggregation, Flexible Bandwidths and Multiflow
LTE multiflow / Inter-site CA - RWS-120002
LTE/HSDPA Carrier Aggregation - RWS-120002
Multiflow Enhancements - RWS-120002
Multi-Stream Aggregation - RWS-120006
Provide mechanisms for Flexible Bandwidth Exploitation - RWS-120008
Carrier aggregation enhancement - RWS-120019
Inter-eNB Carrier Aggregation - RWS-120021 / RWS-120046
Evolution of Carrier Aggregation - RWS-120036 / RWS-120050
CA of Alternative Spectra - RWS-120042


Cells, Carriers, C/U Planes
C/U plane split & Phantom cell - RWS-120010
Phantom cell by single/separate nodes - RWS-120010
Phantom cell: Other topics - RWS-120010
New Carrier Type for Primary Component Carrier - RWS-120011
Flexible/Reconfigurable Cells - RWS-120023
New carrier-type (NCT) enhancements - RWS-120026
Amorphous cells - RWS-120034
New Carrier Types - RWS-120035
Non-Orthogonal Access - RWS-120039
Dynamic Area Construction for UE - RWS-120040


Cognitive Radio
Cognitive radio - RWS-120034
Cognitive Networking - RWS-120036 / RWS-120050


Coordinated MultiPoint (CoMP)
CoMP Enhancements - RWS-120014
CoMP/ICIC enhancement - RWS-120019
CoMP Enhancements - RWS-120023
CoMP enhancements - RWS-120026
CoMP Technologies - RWS-120027 / RWS-120048
Enhanced CoMP - RWS-120029
Potential CoMP Enhancements - RWS-120035
CoMP - RWS-120037
CoMP Enhancement for Indoor Environment - RWS-120040
Overhauling DL CoMP - RWS-120042


Device, Handsets, UE's
Additional UE Enhancements - RWS-120018
Coordination : Multi-mode UE - RWS-120024


D2D / Device-to-Device
Device-to-Device - RWS-120003
LTE Device to Device - Proximity Based Services - RWS-120004
LTE device to device - RWS-120007
LTE direct communication - RWS-120007
Device-to-Device Communications - RWS-120014
D2D Discovery/Communication - RWS-120016
3GPP Proximity Services (ProSe) / D2D - RWS-120022
Device-to-Device communications - RWS-120026
Device-to-Device communication - RWS-120036 / RWS-120050


Data Rates and Throughputs
Higher data rate and user-experienced throughput - RWS-120010
Fairness of user throughput - RWS-120010


Deployments
LTE in Local Area Deployments & Enhancements - RWS-120004
Energy Efficient Local Area Deployments - RWS-120004
Scaling for Mass Deployment - RWS-120008
Flexible and cost-efficient NW deployments - RWS-120010
Considerations on dense NW deployment - RWS-120019


Energy Consumption, Efficiency and Savings
Energy efficiency - RWS-120005
Reduce energy consumption - RWS-120008
Energy Saving - RWS-120014
UE Power Saving - RWS-120036 / RWS-120050
NB Power Saving - RWS-120036 / RWS-120050
Energy Saving Enhancements with CoMP - RWS-120040
Energy Saving with Centralized eNB - RWS-120040


Herogeneous Networks (HetNets)
Optimisation of Het Nets performance - RWS-120005
Improved Support for Heterogeneous Networks - RWS-120006
Network hyper-densification: LTE HetNet2.0 - RWS-120007
Multi-layer HetNet Deployments - RWS-120016
HetNet for HSPA - RWS-120017
HetNet Enhancements - RWS-120023
HetNet Mobility - RWS-120029
Small cells & HetNet - RWS-120031
HetNet - RWS-120037
HetNet Enhancements for HeNB - RWS-120040


HSDPA / HSUPA / HSPA+ Enhancements
HSPA UL Enhancements - RWS-120003
Uplink Enhancements - RWS-120006
UMTS evolution: enhancing CS voice on DCH - RWS-120007
High Speed Packet Access - RWS-120012
HSPA RRM enhancement - RWS-120024
HSPA+ further evolution - RWS-120034


Interworking (HSPA, LTE)
Coordination : HSPA/LTE e-interworking - RWS-120024
Inter-RAT Coordination/CA - RWS-120037


Local-Area Access (Small Cells)
Local-Area Access - RWS-120003
LTE in Local Area Deployments & Enhancements - RWS-120004
LTE Local Area Enhancements - RWS-120004
LTE Local Area Enhancement Areas - RWS-120004
enhanced Local Area (eLA) - RWS-120010
Local Area Enhancements - RWS-120022
Improved Local Area Mobility - RWS-120022


LTE
LTE for Nomadic and Fixed Use - RWS-120018
E-PDCCH enhancement - RWS-120019
Efficiency : Paging Optimization - RWS-120024


LTE Hotspot and Indoor Enhancements (LTE-Hi)
Hotspot and Indoor Enhancements (LTE-Hi) - RWS-120006
Hotspot/indoor Scenario (LTE-Hi) - RWS-120025
Indoor & Hotspot Enhancements (LTE-Hi) [Detailed] - RWS-120029
Possible Study Items for Indoor Environment - RWS-120040


M2M / Machine Type Communications (MTC)
Machine Type Communications - RWS-120003
Improved Support for MTC - RWS-120006
Machine-to-Machine: The Internet of Things - RWS-120014
Machine Type Communications: a new ecosystem - RWS-120014
Wireless MTC and RAN optimizations for MTC - RWS-120016
Low-Cost MTC UE - RWS-120017
MTC + eDDA (enhanced Diverse data application) - RWS-120019
Further Enhancements to Support MTC - RWS-120023
MTC - RWS-120025
MTC enhancements - RWS-120026
M2M - RWS-120029
MTC and migration of traffic from 2G - RWS-120031
Machine Type Communications enhancements - RWS-120034
Machine Type Communications - RWS-120035
Extension triggered by growing M2M traffic - RWS-120038 / RWS-120047
LTE-based M2M - RWS-120041


MBMS / eMBMS
eMBMS Enhancements - RWS-120007
eMBMS - RWS-120013
UHD Multimedia Broadcast/Multicast Service - RWS-120036 / RWS-120050


Mesh Networks
Mesh Networks - RWS-120018


Network Density
Network density: Scenarios - RWS-120010


Network Architecture and Operation
Easier network operation, tolerance to failure - RWS-120005
System Architecture - RWS-120032
Evolution of LTE Networks - RWS-120034


Positioning
Positioning Enhancements - RWS-120006


Public Safety
Public Safety - RWS-120030
Operation of Public Safety System via LTE - RWS-120031
Public safety’s future in LTE [Detailed] - RWS-120033


Self Organising Networks (SON) and Minimisation of Drive Testing (MDT)
SON Evolution - RWS-120002
Enhanced MDT - RWS-120011
Network Self-Optimisation - RWS-120014
SON and MDT - RWS-120017
HetNet SON - RWS-120029
MDT & Energy Saving - RWS-120029
Autonomous Interference Coordination - RWS-120029
Large scale multi-layer centralized cooperative radio - RWS-120034
MDT Enhancement - RWS-120036 / RWS-120050
SON Enhancements - RWS-120036 / RWS-120050
MDT and eDDA - RWS-120041


Small Cells (HNB/HeNB)
UMTS evolution: small cells - RWS-120007
Wide & Local area enhancements - RWS-120010
Small Cells - RWS-120014
Small Cell Enhancement in Rel-12 - RWS-120021 / RWS-120046
HeNB Enhancement - RWS-120036 / RWS-120050
Efficient Usage of Macro and Small Cells - RWS-120038 / RWS-120047
Low-cost Low Power Nodes (LC-LPN) - RWS-120038 / RWS-120047
Small-Cell Improvements: System Aspects - RWS-120041


Spectrum
Enhanced spectrum efficiency - RWS-120005
Spectrum efficiency: eLA topics - RWS-120010
Scenarios for spectrum extension - RWS-120010
Spectrum and spectrum usage - RWS-120012
Wider Spectrum Utilization - RWS-120016
Spectral efficiency for LTE - RWS-120017
New Spectrum for Mobile Broadband Access - RWS-120021 / RWS-120046
Enabling Technologies for New Spectrum - RWS-120021 / RWS-120046
Radio Propagation - RWS-120021 / RWS-120046
Opportunistic Use of Unlicensed Spectrum for D2D Local Traffic - RWS-120023
Flexible Spectrum Utilization - RWS-120024
Spectrum Related: New Bands And CA Band Combinations - RWS-120029
Spectrum - RWS-120032
Hybrid access scheme - RWS-120034
Spectrum - RWS-120035
Spectrum and Transmission Efficiency - RWS-120039
Spectrum-Agile LTE - RWS-120041


TDD / TD-LTE
TD-LTE - RWS-120014
TDD-specific aspects - RWS-120014
TDD adaptive reconfiguration - RWS-120034
Efficient Usage of Dual Duplex Modes - RWS-120038 / RWS-120047
LTE TDD Small-Cell versus WiFi - RWS-120041


Testing
Testing and Certification - RWS-120022


Traffic and Signalling Overhead
Efficient support of diverse traffic characteristics - RWS-120005
Efficient support for variety of traffic types - RWS-120010
Enhancements for variety of traffic types - RWS-120010
Very high traffic (and signalling) scenarios - RWS-120017
Control Plane Overhead Reduction - RWS-120021 / RWS-120046
Further Enhancements to Support Diverse Data Applications - RWS-120023
Efficiency : Small data services in high mobility - RWS-120024


User Experience
Improve User experience - RWS-120009
User Challenges - RWS-120032


Video streaming, call
RAN Enhancements for Video Streaming QoE - RWS-120023
RAN Enhancements for Internet Video Call - RWS-120023


WiFi / WLAN
Cooperation between LTE/HSPA and WiFi - RWS-120005
Unlicensed spectrum: LTE & WLAN - RWS-120007
LTE integration with other RATs - RWS-120014
WiFi integration: For Beyond Rel-12 - RWS-120017
LTE-WLAN Interworking - RWS-120023
Coordination With WiFi - RWS-120029
Smarter opportunistic usage of Wi-Fi - RWS-120031
LTE TDD Small-Cell versus WiFi - RWS-120041


Others
Other identified techniques for LTE - RWS-120005
Efficient Transactions - RWS-120035
Link Enhancement Considerations - RWS-120035
Intra-RAT cooperation / Inter-RAT cooperation - RWS-120036 / RWS-120050

Here is the summary from the workshop:

Summary of 3GPP TSG-RAN Workshop on Release 12 and Onward

View more PowerPoint from Zahid Ghadialy

Complete list of Presentations

RWS-120002	Release 12 and beyond for C^4 (Cost, Coverage, Coordination with small cells and Capacity)	NSN
RWS-120003	Views on Rel-12	Ericsson & ST-Ericsson
RWS-120004	LTE evolving towards Local Area in Release 12 and beyond	Nokia Corporation
RWS-120005	Views on Release 12	Orange
RWS-120006	Views on Rel-12 and onwards for LTE and UMTS	Huawei Technologies, HiSilicon
RWS-120007	3GPP RAN Rel-12 & Beyond	Qualcomm
RWS-120008	New Solutions for New Mobile Broadband Scenarios	Telefonica
RWS-120009	Telecom Italia requirements on 3GPP evolution	Telecom Italia
RWS-120010	Requirements, Candidate Solutions & Technology Roadmap for LTE Rel-12 Onward	NTT DOCOMO, INC.
RWS-120011	Where to improve Rel-12 and beyond: Promising technologies	NEC
RWS-120012	Deutsche Telekom Requirements and Candidate Technologies	Deutsche Telekom
RWS-120013	Release 12 Prioritization Concepts	Dish Networks
RWS-120014	Towards LTE RAN Evolution	Alcatel-Lucent
RWS-120015	UE AAS (Active Antenna System)	Magnolia Broadband
RWS-120016	Requirements and Technical Considerations for RAN Rel.12 & Onwards	Fujitsu Limited
RWS-120017	Operator requirements on future RAN functionality	TeliaSonera
RWS-120018	AT&T View of Release 12 in the North America Marketplace	AT&T
RWS-120019	Major drivers, requirements and technology proposals for LTE Rel-12 Onward	Panasonic
RWS-120020	Efficient spectrum resource usage for next-generation N/W	SK Telecom
RWS-120021	Technologies for Rel-12 and onwards	Samsung Electronics
RWS-120022	LTE Rel-12 and Beyond	Renesas Mobile Europe
RWS-120023	LTE Rel-12 and Beyond: Requirements and Technology Components	Intel
RWS-120024	Considerations on further enhancement and evolution of UMTS/LTE network in R12 and onwards	China Unicom
RWS-120025	Views on LTE R12 and Beyond	CATT
RWS-120026	A proposal for potential technologies for Release 12 and onwards	ETRI
RWS-120027	A view on requirements on Rel-12 and onwards from an operator’s viewpoint	Softbank Mobile
RWS-120028	India market Requirements for Rel. 12 and beyond	CEWiT
RWS-120029	Views on LTE Rel-12 & Beyond	CMCC
RWS-120030	LTE addressing the needs of the Public Safety Community	IPWireless
RWS-120031	Vodafone view on 3GPP RAN Release 12 and beyond	Vodafone
RWS-120032	An Operator’s View of Release 12 and Beyond	Sprint
RWS-120033	Public Safety Requirements for Long Term Evolution REL-12	U.S. Department of Commerce
RWS-120034	Views on 3GPP Rel-12 and Beyond	ZTE
RWS-120035	Considerations for LTE Rel-12 and beyond	Motorola Mobility
RWS-120036	LG’s view on evolution of LTE in Release 12 and beyond	LG Electronics
RWS-120037	Views on REL-12 and Onwards	China Telecom
RWS-120038	KDDI’s Views on LTE Release 12 onwards	KDDI
RWS-120039	Evolving RAN Towards Rel-12 and Beyond	SHARP
RWS-120040	Views on enhancement of system capacity and energy efficiency toward Release12 and onward	Hitachi
RWS-120041	Beyond LTE-A: MediaTek’s view on R12	MediaTek
RWS-120042	Potential Technologies and Road Map for LTE Release 12 and Beyond	ITRI, HTC
RWS-120043	New concept to maximize the benefit of interference rejection at the UE receiver: interference suppression subframes (ISS)	Broadcom
RWS-120046	Technologies for Rel-12 and onwards	Samsung Electronics
RWS-120047	KDDI’s Views on LTE Release 12 onwards	KDDI
RWS-120048	A view on Rel-12 and onwards from an operator’s viewpoint	Softbank Mobile
RWS-120049	UE AAS (Active Antenna System)	Magnolia Broadband
RWS-120050	LG’s view on evolution of LTE in Release 12 and beyond	LG Electronics
RWS-120051	New concept to maximize the benefit of interference rejection at the UE receiver: interference suppression subframes (ISS)	Broadcom

More technically minded people want to explore the 3GPP website for the workshop links here: http://3gpp.org/ftp/workshop/2012-06-11_12_RAN_REL12/

Draft report that gives more insight into the presentations as follows:

Draft Report of 3GPP RAN Workshop on Release 12 and onwards

View more documents from Zahid Ghadialy

LTE 'Antenna Ports' and their Physical mapping

People who work with LTE Physical layer and maybe higher layers would be aware of this term called 'Antenna Ports'. I have always wondered how these antenna ports are mapped to physical antennas.

The following is from R&S whitepaper:

The 3GPP TS 36.211 LTE standard defines antenna ports for the downlink. An antenna port is generally used as a generic term for signal transmission under identical channel conditions. For each LTE operating mode in the downlink direction for which an independent channel is assumed (e.g. SISO vs. MIMO), a separate logical antenna port is defined. LTE symbols that are transmitted via identical antenna ports are subject to the same channel conditions. In order to determine the characteristic channel for an antenna port, a UE must carry out a separate channel estimation for each antenna port. Separate reference signals (pilot signals) that are suitable for estimating the respective channel are defined in the LTE standard for each antenna port. 

Here is my table that I have adapted from the whitepaper and expanded. 

The way in which these logical antenna ports are assigned to the physical transmit antennas of a base station is up to the base station, and can vary between base stations of the same type (because of different operating conditions) and also between base stations from different manufacturers. The base station does not explicitly notify the UE of the mapping that has been carried out, rather the UE must take this into account automatically during demodulation (FIG 2).

If there is another way to show this physical mappings, please feel free to let me know.

The R&S Whitepaper is available here if interested.

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.

MU-MIMO (and DIDO)

Late last month a guy called Steve Perlman announced of a new technology called DIDO (Distributed-Input-Distributed-Output) that could revolutionise the way wireless transmission works and can help fix the channel capacity problem as described by Shannon's formula. A whitepaper describing this technology is available here.

I havent gone through the paper in any detail nor do I understand this DIDO very well but what many experienced engineers have pointed out is that this is MU-MIMO in disguise. Without going into any controversies, lets look at MU-MIMO as its destined to play an important part in LTE-Advanced (the real '4G').

Also, I have been asked time and again about this Shannon's channel capacity formula. This formula is better known by its name Shannon-Hartley theorem. It states:

C <= B log2 (1 + S/N)

where:

C = channel capacity (bits per second)

B = bandwidth (hertz)

S/N = Signal to Noise ratio (SNR)

In a good channel, SNR will be high. Take for example a case when SNR is 20db then log2 (1 + 100) = 6.6. In an extremely noisy channel SNR will be low which would in turn reduce the channel capacity.

In should be pointed out that the Shannon's formula holds true for all wireless technologies except for when multiuser transmission like MU-MIMO (or DIDO) is used.

Anyway, I gave a simple explanation on MU-MIMO before. Another simple explanation of what an MU-MIMO is as explained in this video below:

The picture below (from NTT) gives a good summary of the different kinds of MIMO technology and their advantages and disadvantages. More details could be read from here.

Click to enlarge

As we can see, MU-MIMO is great but it is complex in implementation.

Click to enlarge

Multiuser MIMO technology makes it possible to raise wireless transmission speed by increasing the number of antennas at the base station, without consuming more frequency bandwidth or increasing modulation multiple-values. It is therefore a promising technology for incorporating broadband wireless transmission that will be seamlessly connected with wired transmission in the micro waveband (currently used for mobile phones and wireless LAN, and well suited to mobile communications use), where frequency resources are in danger of depletion. Since it also allows multiple users to be connected simultaneously, it is seen as a solution to the problem specific to wireless communications, namely, slow or unavailable connections when the number of terminals in the same area increases (see Figure 9 above).

There is a good whitepaper in NTT Docomo technical journal that talks about Precoding and Scheduling techniques for increasing the capacity of MIMO channels. Its available here. There is also a simple explanation of MIMO including MU-MIMO on RadioElectronics here. If you want to do a bit more indepth study of MU-MIMO then there is a very good research paper in the EURASIP Journal that is available here (Click on Full text PDF on right for FREE download).

Finally, there is a 3GPP study item on MIMO Enhancements for LTE-Advanced which is a Release-11 item that will hopefully be completed by next year. That report should give a lot more detail about how practical would it be to implement it as part of LTE-Advanced. The following is the justification of doing this study:

The Rel-8 MIMO and subsequent MIMO enhancements in Rel-10 were designed mostly with homogenous macro deployment in mind. Recently, the need to enhance performance also for non-uniform network deployments (e.g. heterogeneous deployment) has grown. It would therefore be beneficial to study and optimize the MIMO performance for non-uniform deployments where the channel conditions especially for low-power node deployments might typically differ from what is normally encountered in scenarios considered so far.

Downlink MIMO in LTE-Advanced has been enhanced in Release 10 to support 8-layer SU-MIMO transmission and dynamic SU-MU MIMO switching. For the 8-tx antenna case, the CSI feedback to support downlink MIMO has been enhanced with a new dual-codebook structure aimed at improving CSI accuracy at the eNB without increasing the feedback overhead excessively. Precoded reference symbols are provided for data demodulation, allowing arbitrary precoders to be used by the eNB for transmission. In many deployment scenarios, less than 8 tx antennas will be employed. It is important to focus on the eNB antenna configurations of highest priority for network operators.

The enhancement of MIMO performance through improved CSI feedback for high priority scenarios not directly targeted by the feedback enhancements in Release 10, especially the case of 4 tx antennas in a cross-polarised configuration, in both homogeneous and heterogeneous scenarios should be studied.

MU-MIMO operation is considered by many network operators as important to further enhance system capacity. It is therefore worth studying further potential enhancement for MU-MIMO, which includes UE CSI feedback enhancement and control signaling enhancement. Furthermore, open-loop MIMO enhancements were briefly mentioned but not thoroughly investigated in Rel-10.

In addition, the experience from real-life deployments in the field has increased significantly since Rel-8. It would be beneficial to discuss the experience from commercial MIMO deployments, and identify if there are any potential short-comings and possible ways to address those. For example, it can be discussed if robust rank adaptation works properly in practice with current UE procedures that allow a single subframe of data to determine the rank. In addition the impact of calibration error on the performance could be discussed.

This work will allow 3GPP to keep MIMO up to date with latest deployments and experience.

Quick Recap of MIMO in LTE and LTE-Advanced

I had earlier put up some MIMO presentations that were too technical heavy so this one is less heavy and more figures.

The following is from NTT Docomo Technical journal (with my edits):

MIMO: A signal transmission technology that uses multiple antennas at both the transmitter and receiver to perform spatial multiplexing and improve communication quality and spectral efficiency.

Spectral efficiency: The number of data bits that can be transmitted per unit time and unit frequency band.

In this blog we will first look at MIMO in LTE (Release 8/9) and then in LTE-Advanced (Release-10)

MIMO IN LTE

Downlink MIMO Technology

Single-User MIMO (SU-MIMO) was used for the downlink for LTE Rel. 8 to increase the peak data rate. The target data rates of over 100 Mbit/s were achieved by using a 20 MHz transmission bandwidth, 2 × 2 MIMO, and 64 Quadrature Amplitude Modulation (64QAM), and peak data rates of over 300 Mbit/s can be achieved using 4×4 SU-MIMO. The multi-antenna technology used for the downlink in LTE Rel. 8 is classified into the following three types.

1) Closed-loop SU-MIMO and Transmit Diversity: For closed-loop SU-MIMO transmission on the downlink, precoding is applied to the data carried on the Physical Downlink Shared Channel (PDSCH) in order to increase the received Signal to Interference plus Noise power Ratio (SINR). This is done by setting different transmit antenna weights for each transmission layer (stream) using channel information fed back from the UE. The ideal transmit antenna weights for precoding are generated from eigenvector(s) of the covariance matrix of the channel matrix, H, given by HHH, where H denotes the Hermitian transpose.

However, methods which directly feed back estimated channel state information or precoding weights without quantization are not practical in terms of the required control signaling overhead. Thus, LTE Rel. 8 uses codebook-based precoding, in which the best precoding weights among a set of predetermined precoding matrix candidates (a codebook) is selected to maximize the total throughput on all layers after precoding, and the index of this matrix (the Precoding Matrix Indicator (PMI)) is fed back to the base station (eNode B) (Figure 1).

LTE Rel. 8 adopts frequency-selective precoding, in which precoding weights are selected independently for each sub-band of bandwidth from 360 kHz to 1.44 MHz, as well as wideband precoding, with single precoding weights that are applied to the whole transmission band. The channel estimation used for demodulation and selection of the precoding weight matrix on the UE is done using a cell specific Reference Signal (RS) transmitted from each antenna. Accordingly, the specifications require the eNode B to notify the UE of the precoding weight information used for PDSCH transmission through the Physical Downlink Control Channel (PDCCH), and the UE to use this information for demodulation.

LTE Rel. 8 also adopts rank adaptation, which adaptively controls the number of transmission layers (the rank) according to channel conditions, such as the received SINR and fading correlation between antennas (Figure 2). Each UE feeds back a Channel Quality Indicator (CQI), a Rank Indicator (RI) specifying the optimal rank, and the PMI described earlier, and the eNode B adaptively controls the number of layers transmitted to each UE based on this information.

2) Open-loop SU-MIMO and Transmit Diversity: Precoding with closed-loop control is effective in low mobility environments, but control delay results in less accurate channel tracking ability in high mobility environments. The use of open-loop MIMO transmission for the PDSCH, without requiring feedback of channel information, is effective in such cases. Rank adaptation is used, as in the case of closed-loop MIMO, but rank-one transmission corresponds to open-loop transmit diversity. Specifically, Space-Frequency Block Code (SFBC) is used with two transmit antennas, and a combination of SFBC and Frequency Switched Transmit Diversity (FSTD) (hereinafter referred to as “SFBC+FSTD”) is used with four transmit antennas. This is because, compared to other transmit diversity schemes such as Cyclic Delay Diversity (CDD), SFBC and SFBC+FSTD achieve higher diversity gain, irrespective of fading correlation between antennas, and achieve the lowest required received SINR. On the other hand, for PDSCH transmission with rank of two or higher, fixed precoding is used regardless of channel variations. In this case, cyclic shift is performed before applying the precoding weights, which effectively switches precoding weights in the frequency domain, thereby averaging the received SINR is over layers.

3) Adaptive Beamforming: Adaptive beamforming uses antenna elements with a narrow antenna spacing of about half the carrier wavelength and it has been studied for use with base stations with the antennas mounted in a high location. In this case beamforming is performed by exploiting the UE Direction of Arrival (DoA) or the channel covariance matrix estimated from the uplink, and the resulting transmit weights are not selected from a codebook. In LTE Rel. 8, a UE-specific RS is defined for channel estimation in order to support adaptive beamforming. Unlike the cell-specific RS, the UE specific RS is weighted with the same weights as the data signals sent to each UE, and hence there is no need to notify the UE of the precoding weights applied at the eNode B for demodulation at the UE. However, its effectiveness is limited in LTE Rel. 8 because only one layer per cell is supported, and it is an optional UE feature for Frequency Division Duplex (FDD).

Uplink MIMO Technology

On the uplink in LTE Rel. 8, only one-layer transmission was adopted in order to simplify the transmitter circuit configuration and reduce power consumption on the UE. This was done because the LTE Rel. 8 target peak data rate of 50 Mbit/s or more could be achieved by using a 20 MHz transmission bandwidth and 64QAM and without using SU-MIMO. However, Multi-User MIMO (MU-MIMO) can be used to increase system capacity on the LTE Rel. 8 uplink, using multiple receiver antennas on the eNode B. Specifically, the specification requires orthogonalization of the demodulation RSs from multiple UEs by assigning different cyclic shifts of a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence to the demodulation RSs, so that user signals can be reliably separated at the eNode B. Demodulation RSs are used for channel estimation for the user-signal separation process.

MIMO TECHNOLOGY IN LTE-ADVANCED

Downlink 8-Layer SU-MIMO Technology

The target peak spectral efficiency in LTE-Advanced is 30 bit/s/Hz. To achieve this, high-order SU-MIMO with more antennas is necessary. Accordingly, it was agreed to extend the number of layers of SU-MIMO transmission in the LTE-Advanced downlink to a maximum of 8 layers. The number of transmission layers is selected by rank adaptation. The most significant issue with the radio interface in supporting up to 8 layers is the RS structure used for CQI measurements and PDSCH demodulation.

1) Channel State Information (CSI)-RS: For CQI measurements with up-to-8 antennas, new CSI-RSs are specified in addition to cell-specific RS defined in LTE Rel. 8 for up-to-four antennas. However, in order to maintain backward compatibility with LTE Rel. 8 in LTE-Advanced, LTE Rel. 8 UE must be supported in the same band as in that for LTE-Advanced. Therefore, in LTE Advanced, interference to the PDSCH of LTE Rel. 8 UE caused by supporting CSI-RS must be minimized. To achieve this, the CSI-RS are multiplexed over a longer period compared to the cell-specific RS, once every several subframes (Figure 3). This is because the channel estimation accuracy for CQI measurement is low compared to that for demodulation, and the required accuracy can be obtained as long as the CSIRS is sent about once per feedback cycle. A further reason for this is that LTE-Advanced, which offers higher data-rate services, will be developed to complement LTE Rel. 8, and is expected to be adopted mainly in low-mobility environments.

2) UE-specific RS: To allow demodulation of eight-layer SU-MIMO, the UE-specific RS were extended for SU-MIMO transmission, using a hybrid of Code Division Multiplexing (CDM) and Frequency Division Multiplexing (FDM) (Figure 4). The UE-specific RS pattern for each rank (number of layers) is shown in Figure 5. The configuration of the UE-specific RS in LTE-Advanced has also been optimized differently from those of LTE Rel.8, extending it for SU-MIMO as well as adaptive beamforming, such as by applying twodimensional time-frequency orthogonal CDM to the multiplexing between transmission layers.

Downlink MU-MIMO Technology

In addition to the peak data rate, the system capacity and cell-edge user throughput must also be increased in LTE-Advanced compared to LTE Rel. 8. MU-MIMO is an important technology for satisfying these requirements. With MU-MIMO and CoMP transmission (described earlier), various sophisticated signal processing techniques are applied at the eNode B to reduce the interference between transmission layers, including adaptive beam transmission (zero-forcing, block diagonalization, etc.), adaptive transmission power control and simultaneous multi-cell transmission. When these sophisticated transmission techniques are applied, the eNode B multiplexes the UE-specific RS described above with the PDSCH, allowing the UE to demodulate the PDSCH without using information about transmission technology applied by the eNode B. This increases flexibility in applying sophisticated transmission techniques on the downlink. On the other hand, PMI/CQI/RI feedback extensions are needed to apply these sophisticated transmission techniques, and this is currently being discussed actively at the 3GPP.

Uplink SU-MIMO Technology

To reduce the difference in peak data rates achievable on the uplink and downlink for LTE Rel. 8, a high target peak spectral efficiency of 15 bit/s/Hz was specified for the LTE-Advanced uplink. To achieve this, support for SU-MIMO with up to four transmission antennas was agreed upon. In particular, the two-transmission-antenna SU-MIMO function is required to satisfy the peak spectral efficiency requirements of IMT-Advanced.

For the Physical Uplink Shared Channel (PUSCH), it was agreed to apply SU-MIMO with closed-loop control using multiple antennas on the UE, as well as codebook-based precoding and rank adaptation, as used on the downlink. The eNode B selects the precoding weight from a codebook to maximize achievable performance (e.g., received SINR or user throughput after precoding) based on the sounding RS, which is used for measuring the quality of the channel transmitted by the UE. The eNode B notifies the UE of the selected precoding weight together with the resource allocation information used by the PDCCH. The precoding for rank one contributes to antenna gain, which is effective in increasing cell edge user throughput. However, considering control-information overhead and increases in Peak-to-Average Power Ratio (PAPR), frequency-selective precoding is not very effective in increasing system throughput, so only wideband precoding has been adopted.

Also, for rank two or higher, when four transmission antennas are used, the codebook has been designed not to increase the PAPR. The demodulation RS, which is used for channel estimation, is weighted with the same precoding weight as is used for the user data signal transmission. Basically, orthogonalization is achieved by applying a different cyclic shift to each layer, but orthogonalizing the code region using block spread together with this method is adopted.

Uplink Transmit Diversity Technology

Closed-loop transmit diversity is applied to PUSCH as described above for SU-MIMO. Application of transmit diversity to the Physical Uplink Control Channel (PUCCH) is also being studied. For sending retransmission request Acknowledgment (ACK) and Negative ACK (NAK) signals as well as scheduling request signals, application of Spatial Orthogonal-Resource Transmit Diversity (SORTD) using differing resource blocks per antenna or an orthogonalizing code sequence (cyclic shift, block spread sequence) has been agreed upon (Figure 6). However, with LTE-Advanced, the cell design must be done so that LTE Rel. 8 UE get the required quality at cell-edges, so applying transmit diversity to the control channels cannot contribute to increasing the coverage area, but only to reducing the transmission power required.