lte-a small cells - isticom.it · why deploying small cells in lte? new competences and new tools...
TRANSCRIPT
Topics
Trends LTE global deployment
LTE technology evolution
Why deploying Small Cells in LTE?
LTE description: Release 8 and Release 9
LTE Release 10 and Release 11
LTE Release 12 and Release 13 Dual connectivity
Small Cells deployment Network Architecture Options
Frequency Usage
Indoor Small Cells
Small Cells products Micro cells, Pico cells, Femto cells, Remote
Radio Heads
Small Cell Interference Management Packet Scheduling Solutions
Enhanced Inter-cell Interference Coordination (eICIC)
Enhanced Coordinated Multi Point (eCoMP)
Small Cell Optimization HetNet Mobility Optimization
Inter-site Carrier Aggregation with Dual Connectivity
Ultra Dense Network Interference Management
Power Saving with Small Cell On/Off
Multivendor Macro Cell and Small Cells
Conclusions and References
2
LTE global deployment
First LTE network in Sweden in Dec. 2009, … 460 LTE network by Dec. 2015
The very first LTE devices supported 100 Mbps, next 300 Mbps, soon 450 Mbps, … also for low cost devices (about 50$)
The mobile data traffic has grown rapidly during the last few years driven by the new smartphones, large displays, higher data rates and higher number of mobile broadband subscribers.
3
LTE technology evolution
4
Why deploying Small Cells in LTE?
New competences and new tools are required Simpler roll out
SON functionalities
Carefully deployment Traffic offload vs increased interference
Co-channel interference between macro and micro BSs
5
LTE description: Release 8 and Release 9
Architecture
Access technique
Physical layer Uplink transmission
Downlink transmission
Terminal capabilities
6
Architettura del sistema LTE
Evolved Packet Core Network (EPC)
Serving Gateway (S-GW)
Packet Data Network Gateway (P-GW)
Mobility Management Entity (MME)
Home Subscriber Server (HSS)
Policy and Charging Rules Function (PCRF)
Evolved UTRAN
eNode B
Evolved Packet System (EPS) tutti i servizi sono offerti su IP
Dati (linee continue)
Controllo (linee tratteggiate)
7
Accesso multiplo nel sistema LTE
A differenza del sistema WCDMA, il sistema LTE usa una tecnica di accesso multiplo basata sulla modulazione Orthogonal Frequency Division Multiplexing (OFDM). In downlink, tratta da uno a molti (si parla di multiplazione), si usa la versione di base dell’OFDM [in alcuni libri è
chiamata OFD Multiple Access]: gli utenti sono individuati assegnando ad ognuno di essi sottoportanti differenti In uplink, tratta da molti ad uno (si parla di accesso multiplo), si usa il Single Carrier – Frequency Division
Multiple Access (SC-FDMA)
Queste tecniche di accesso si basano sull’ortogonalità degli utenti riduzione dell’interferenza incremento capacità di rete
La risoluzione nell’allocazione delle risorse in frequenza è 12 sottoportanti di 15 kHz ciascuna per un totale di 180 kHz: blocco assegnato in uplink e in downlink In uplink sottoportanti contigue per
la trasmissione Single Carrier In downlink sottoportanti scelte
liberamente e assegnate ai vari utenti
NOTA. Single Carrier proposta per la prima volta nel sistema LTE: risolve il problema degli amplificatori di potenza nei dispositivi mobili
8
Accesso multiplo (2): allocazione risorse
Esempio di allocazione delle risorse in downlink con prefisso ciclico breve
Ottimizzazione della trasmissione: nel tempo, favoriti gli utenti con bassa attenuazione del fast fading; in frequenza, eliminate le sottoportanti su cui agisce la selettività del canale
Modulazione uguale su base blocco di risorse
9
Strato fisico: trasmissione dati in uplink
La trasmissione dati d’utente avviene sul Physical Uplink Shared Channel (PUSCH)
Durata di trama: 10ms
Allocazione delle risorse su base tempo – frequenza
Risoluzione di 1 ms in tempo e 180 kHz in frequenza
Lo scheduler nel eNode B definisce l’allocazione delle risorse
Nell’LTE non ci sono risorse dedicate agli utenti
Gli utenti possono trasmettere solo se schedulati dall’eNode B o sui canali di accesso casuale
Gli UE devono riportare all’eNode B lo stato
del proprio buffer dati e la potenza disponibile
10
Strato fisico (2): trasmissione dati in downlink
Trasmissione dati d’utente in downlink avviene sul Physical Downlink SharedChannel (PDSCH)
Risoluzione risorse: 1 ms nel tempo e 12 sottoportanti (180 kHz) in frequenza blocco tempo – frequenza: Physical Resource Block (PRB)
OFDM per separare i flussi degli utenti (multiplazione e non accesso multiplo) bit rate istantaneo dell’utente dipende da quante sottoportanti ha allocate (15 kHz ciascuna)
Allocazione in tempo e in frequenza è eseguita dall’eNode B in base ai CQI trasmessi dai terminali (figura)
L’allocazione delle risorse è trasportata dal Physical DownlinkControl Channel che indica ai terminali quale sottoportante è allocata ad ognuno di essi
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Strato fisico (3): trasmissione dati in downlink
Struttura di trama (figura a sinistra): durata è 10 ms; composta da 10 sottotrame di 1 ms Ogni sottotrama ha due slot di 0.5 ms ciascuno Nello slot 7 simboli (per prefisso ciclico breve) o 6 simboli (per prefisso ciclico lungo) Physical Control Format Indicator Channel (PCFICH) indica il formato del PDCCH Physical Downlink Control Channel (PDCCH) indica l’allocazione dei dati per gli utenti: da 1
a 3 simboli; simboli restanti per il PDSCH Physical Downlink Shared Channel (PDSCH) trasporta: dati
d’utente, dati di broadcast e reference signal (figura a destra)
12
Categorie dispositivi LTE Rel-8
Capabilities per terminali LTE divise in 5 categorie
Categorie definite nelle Release 8 e Release 9 variazioni dovute agli sviluppi futuri previsti nella Release 10 (nuove funzionalità dell’LTE Advanced)
Notevole incremento tra la categoria 1 e 2: argomento nel 3GPP forum
Ricezione multi antenna in dispositivi piccole dimensioni limitato guadagno soprattutto per frequenze inferiori ad 1 GHz
13
LTE Release 10 and Release 11
Carrier aggregation (CA)
Multiple Input Multiple Output (MIMO)
Inter-cell Interference Coordination (eICIC)
Coordinated Multi-Points (CoMP)
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Carrier Aggregation (CA)
Principle: to extend the maximum bandwidth in the uplink and downlink by aggregating multiple carriers.
The carriers to be aggregated are basically Release 8 carriers, necessary for backwards compatibility
Envisaged bands: 800 MHz, 1800 MHz, 2600 MHz
15
PCC = Primary Component CarrierSCC = Secondary Component Carrier
CA: bands for aggregation
In downlink support the intra and inter-band CA: usually each operator does not have more than 20MHz in a given frequency band
In uplink, CA is not attractive: the use of two transmitters simultaneously in the UE is more challenging than two receivers further studies in Rel-11
16
CA: impacts
CA affects the physical layer and the MAC layer: It is unchanged the user plane layers above the MAC and at the core network (except higher data rates).
The MAC layer divides the data between different component carriers: no limitation by multiplexing functionality on the component carriers (CC)
The CA affects the feedback in uplink Difficult to use DTX in uplink (necessary for feedback) since it causes a spectrum with spikes
The power can be an issue (spread over the bands) and amplifier in linear region
17
Multiple Input Multiple Output (MIMO)
Key technology in the LTE Release 8: Transmission modes for one, two and four eNodeB antenna ports have been specified providing peak data rates in excess of 300 Mbps
Advances in the LTE-Advanced
Downlink transmission with up to eight transmit antenna ports peak spectral efficiency increases up to 30 bps/Hz corresponding to 600 Mbps on a 20MHz carrier
Introduction of Multi-User MIMO (MU-MIMO) operation. Multi-User MIMO refers to the transmission where the parallel streams are transmitted to different UEs separated spatially while in Single-User MIMO (SU-MIMO) the parallel streams are sent to single UE.
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MIMO: basic principle
Nt tx antennas, quasi-static channel (i.e. Tb ≫ Tcoh), Nr rx antennas H is the Nr × Nt channel matrix with whose entries hij are complex
channel gains (transfer functions) from the j-th transmit to the i-threceive antenna.
The received signal vector: r = Hs + n = x + n contains the signals received by Nr antenna elements, where s is the transmit signal vector and n is the noise vector.
Consider a singular value decomposition of the channel: H = WLU†, where L is a diagonal matrix containing singular values, and W and U† are unitary matrices composed of the left and right singular vectors, respectively.
19
The received signal is: r = Hs + n = WLU†s + n Multiplication of the transmit data vector by matrix U and
the received signal vector by W† diagonalizes the channel: W†r = W†WLU†Us + W†n; r’ = Ls +n’
RH (rank of matrix H) parallel channels (eigenmodes of the channel) the capacity of parallel channels just adds up.
(.)† = ((.)T)*
MIMO: enhancements in Rel-10
The MIMO needs of reference symbols (RSs) to estimate the channel and apply the matrix inversion
In Rel-8, Rel-9 the MIMO operation is primarily based on cell-specific Common Reference Symbols (CRS) used both for Channel State Information (CSI) measurements as well as the data demodulation
In Rel-10, it is defined another RS pattern to increase the number of antennas: Issues: backwards compatibility; High RS overhead
Basic idea: to decouple RSs for CSI measurements
(namely CSI-RS used for CQI, PMI, RI, with lower
and adaptable periodicity (from 5 to 80 ms)) and
for data demodulation (user-specific and dedicated,
URS or DM-RS), which are flexible, adapted to
the rank of the users
20
MIMO: example of Reference Signals for 8 ports
21
MIMO: Single User and Multiple Users
22
MIMO: summary
Enhancements in MIMO are provided by defining new different Transmission Modes (TM)
The UE will through RRC signalling be informed about the transmission mode to use In the DL there are nine different transmission modes, where TM1-7 were introduced in
R8, TM8 was introduced in R9 and TM9 was introduced in R10
In the UL there are TM1 and TM2, where TM1, the default, was introduced in R8 and TM2 was introduced in R10.
The different transmission modes differ in: Number of layers (streams, or rank)
Antenna ports used
Type of reference signal, Cell-specific Reference Signal (CRS) or Demodulation Reference Signal (DM-RS), introduced in R10.
Precoding type
23From 3GPP site
Inter-cell Interference Coordination (ICIC)
Release 8: Inter-Cell Interference Coordination involves the intelligent coordination of physical resources between various neighboring cells to reduce interference from one cell to another
Inverted Reuse scheme Part of the spectrum is used with reduced power or
not used
Cell-inner users and Cell-edge users
Aim: Concentrate the bulk of the inter-cell interference in a small portion of the total bandwidth, thereby preventing any impact to the majority of users since the interference is now localized to certain sub-carriers and the sub-carriers are orthogonal to each other.
24
Enhanced Inter-cell Interference Coordination (eICIC)
Release 10, HetNets with co-channel interference
Aim: enable cell range extension by using
Almost Blank Subframes (ABS)
25
Coordinated Multi-Point (CoMP)
Aim: to improve network performance at cell edges
Idea of CoMP: depending on a UE’s location, it may be able to receive signals from multiple cell sites, and the UE’s transmissions may be received at multiple cell sites regardless of the system load, in a coordinated fashion
Approaches (Rel-11): Coordinated Scheduling or Coordinated Beamforming (CS/CB)
Dynamic Point Selection (DPS)
Joint Processing/Joint Transmission (JP/JT)
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UE terminal capabilities in Rel-10 and Rel-11
First CA capabilities
Improved CA capabilities
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LTE Release 12 and Release 13
Dual connectivity (DC)
Machine Type Communications (MTC)
Proximity Services (ProSe) or Device-to-Device (D2D)
Unlicensed LTE (LTE-U)
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Dual Connectivity
In Rel-12: a given UE is capable of using radio resources provided by two different network points connected with non-ideal backhaul
Dual connectivity consists of configuring a UE with one Master Evolved NodeB (MeNB) and one Secondary Evolved NodeB (SeNB): signaling overhead towards the CN can potentially be saved
The UE needs in this case to be able to provide physical layer feedback signal to both eNodeBs separately
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Dual Connectivity (2)
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Dual Connectivity (3)
User Plan protocol
31User Plane protocol architecture for Option 2
Dual Connectivity (4): impact on protocols
Master eNode B functions: The MeNodeB retains full control over the dual
connectivity and can always decide to release any configured SCell from the SeNodeB or even the SeNodeB itself, and the SeNodeB shall comply.
The MeNodeB also controls the overall bearer structure, RRC connectivity and measurements at the UE side.
Only MeNodeB can choose to request addition of SCells to the SeNodeB part.
Secondary eNode B functions: The SeNodeB retains control over its own resources
and decides on its own radio configuration part: When MeNodeB requests dual connectivity, the SeNodeB is in control of its radio configuration and MeNodeB will not modify it.
The SeNodeB can also neither request to start dual connectivity nor add an SCell, but can choose to reject such request from MeNodeB.
But like MeNodeB, SeNodeB can at any time request to release an SCell or the dual connectivity itself, and MeNodeB will comply.
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Dual Connectivity (5): signaling
Split bearer – the bearer is served using radio resources from both MeNBand SeNB
In Bearer splitting for DC, HARQ-ACK feedbacks of SCell cannot be transmitted on PCell because of delays in the backhaul. Cells grouped into two groups: MeNB Cell Group (MCG) bearer. The bearer is served using radio resources of MeNB
only. Uplink Control Information sent via PUCCH on PCell or PUSCH in other MCG cells
SeNB Cell Group (SCG) bearer. The bearer is served using radio resources of SeNBonly. UCI sent via PUCCH on Primary SCell (PSCell)
Signaling Radio Bearers (SRBs) are always of the
MCG bearer and therefore only use the
radio resources provided by the MeNB.
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Dual Connectivity (6): scheduling and power control
Scheduling decisions in the SeNodeB and in the MeNodeB cannot be instantaneously coordinated
Uplink power resources of the UE can be exceeded limiting Tx power for MCG and SCG The remaining power can be dynamically allocated on scheduling basis
The priority order for allocating the remaining power to MCG and/or SCG transmissions is based on UCI type. The priority order is the following: HARQ/ACK = SR > CSI > PUSCH without UCI > SRS.
Synchronous Power Control Mode 1, PCM1 (left), and Asynchronous Power Control Mode 2, PCM2 (right).
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Dual Connectivity (7): Flow control between MeNB and SeNB
Flow control mechanism between the MeNBand SeNB over the X2 interface Request-and-forward-based scheme
Aim: to avoid data overflow and underflow in the SeNB To always have data available for transmission in
the SeNB The SeNB (in accordance with 3GPP
specifications) is in charge of requesting data from the MeNB
Received data from the MeNB are buffered in the SeNB until they have been successfully transmitted over the air interface to the UE via the SCell
The data requests from SeNB to MeNB are sent periodically on a per-user basis
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Dual Connectivity (8): benefits and performances
Higher transmission bandwidth
Increased multi-user diversity order and faster interlayer load balancing
36
Small Cell ON/OFF Switching and Enhanced Discovery
Aim: switching off the eNB when the traffic is low. Rel-9 in SON. Cells in OFF (i.e. dormant) state do not transmit any signals and consequently UEs are
not able to detect those cells.
To return an OFF cell back to service, X2 signalling was standardized to allow an eNodeB to request a neighbouring eNodeB to switch on the OFF cell.
Rel-12 On/Off Mechanism
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Small Cell ON/OFF Switching and Enhanced Discovery (2)
Discovery Signal Transmission and Measurement Procedure
Differently from PSS/SSS/CRS, the Discovery Reference Signals are transmitted with a more sparse periodicity for the purpose of cell detection and RRM measurements
The UE performs discovery measurements according to eNodeB-given per- carrier Discovery Measurement Timing Configuration (DMTC)
The network needs to ensure that the transmission times of DRS occasions of all cells on a given carrier frequency are aligned with the DMTC configuration in order to ensure those cells can be discovered.
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Other Small Cell Physical Layer Enhancement
256QAM for LTE Downlink New UE categories Cat11–Cat15 to support for 256QAM with increased peak data rates.
Roughly 200 Mbps per 20 MHz downlink carrier peak data rate.
Power Saving with Small Cell ON/OFF
Over the Air Synchronization between eNodeBs Alternative to GPS synchronization
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UE terminal capabilities in Rel-12 and Rel-13
New Rel-12 UE categories
MTC UE categories Rel-12 Rel-13
40
Small Cells deployment
Area throughput [bit/s/km2] = Bandwidth [Hz] Cell density [cells/km2] Spectral efficiency [bit/s/Hz/cell]
Small Cell Motivation
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Network Architecture Options
Stand-alone BTS
Low transport requirements
Small BTS = RF head
Heavy transportrequirements = direct fiber
42
Small BTS connected to macro
Shared PDCP in macro BTS
Inter-site carrier aggregation
Cloud RAN
Network Architecture Options (2)
Comparison of all-in-one small base station and radio head
43
All-in-one base station Radio frequency head (RF head)
Transport requirements Low requirements. Transport can use radio, copper or fiber
Tough requirements. Direct fiber needed in practice
Feature parity between macro and small cells
Yes possible, but requires extra planning in development
Yes, comes naturally with the common baseband
Mobility between macro and small cells
Inter-site handovers visible also to the packet core
Intra-site handovers are not visible to the packet core
Coordinated Multipoint(CoMP)
Joint Processing not possible but Dynamic Point Selection is possible
Yes, all CoMP optionspossible between macroand small cells
Network Architecture Options (3)
Requirements with small base station and RF head
Comparison of backhaul and fronthaul requirement
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Frequency Usage
Candidate frequencies 700-2600 MHz pool (with macro BS), 3.5 GHz (dedicated to small cells), 5 GHz
(unlicensed)
The small cells can share the same frequency with macro cells or use a dedicated frequency
45
Co-channel with macro cells (in-band)
Dedicated small cellfrequency (out-of-band)
Interference management between macro and small cells
Preferable clear dominance areas needed for small cells. Enhanced ICIC (eICIC) can be utilized
Simple interferencemanagement
Mobility managementbetween macro and small cells
Intra-frequency handover Inter-frequency handovers needed, which requires proper measurement triggering
Capacity Maximizes the overall spectral efficiency
Beneficial for high number of small cells
Indoor Small Cells
Distributed Antenna System (DAS) Passive distributed indoor antennas, coaxial cabling and splitters and a
high power base station.
Passive, active or hybrid
Wi-Fi Traffic offload
Procedures for efficient traffic steering between cellular and Wi-Fi
Femto Cells Typically without advanced schedulers and techniques to coordinate
with macro in order to reduce interference.
Closed Subscriber Groups (CSG), Open Subscriber Groups (OSG)
46
With Femto-gateway Without Femto-gateway
Small Cells products: categories for 3GPP
Deployment scenarios:
Parameters: Maximum output power and the minimum coupling loss (MCL)
47
Small Cells products: Micro Cells
Installations in public areas (above all in outdoor)
Aim: Provide higher capacity in hot spot areas, better coverage and higher data rates
The installation done by the operator to work smoothly together with the macro cell.
Size: typically more than 50 kg; Power: about 10W
2 × 2 MIMO is supported
Support the connection of a large number of simultaneous users (from 500 to 1000)
Synchronization is required: GPS, synchronous Ethernet or backhaul-based IEEE 1588v2
Feature parity between small cell and macro cell is preferred.
Flexible backhaul options are needed to support different deployment options
SON functionalities are required
48
Small Cells products: Pico Cells
Installed in public areas (in indoor premises like shopping malls, train stations or office complexes)
The installation done by operators Size: typically below 5 kg; Power: about 500 mW MIMO is supported; Max capacity: Up to 400 simultaneous users; Subscribers of multiple operators can
be served by network-sharing arrangement between operators. Synchronization: GPS, IEEE 1588v2 or Synchronous Ethernet Feature parity with macro cell is preferred Backhaul typically uses office LAN cabling (Ethernet over copper) Power feed by Power over Ethernet (PoE) (i.e. IEEE 802.3af), IEEE 802.3at (PoE+),
PoE++ (not standardized, yet) or AC/DC adapter Option for integrated Wi-Fi SON functionalities are required
49
Small Cells products: Femto Cells
Aim: to provide coverage for small offices and for home (indoor coverage) Small size and simple installation Typically installed by the end user deployed in uncontrolled manner Location locking Possibility to restrict the service access to a closed subscriber group (CSG) Size: 500 g; Output power Max 100 or 250 mW, Power consumption <13.5 W Peak data rate HSDPA 21 Mbps and HSUPA 5.76 Mbps Limited capacity: Up to 16 simultaneous users Feature parity with macro cell, not required Synchronization: Network time protocol or over the air from macro downlink Backhaul: Ethernet over copper Power feed by PoE+ or AC/DC adapter Some important femto SON: Automatic setup; Downlink listen mode to identify the co-
channel signal (by macrocell); Downlink power level setting based on the received signal level from the co-channel macro and small cells and based on the terminal reports; Periodic monitoring of the surroundings during operation, to detect possible changes
50
Small cell comparison
51
Micro Cells Pico Cells Femto Cells Remote Radio Head
Aim Higher data rate, Better coverage
Capacity increase, Better coverage
Better coverage
Installation Public area, outdoor Public area, indoor Private area (e.g. home) Large indoor spaces
Installation by Operator Operator End Users Operator
Size >50 kg 5 kg 0.5 kg 5 kg
Power 10 W 500 mW 100-250 mW 100 mW-10 W
MIMO 2 x 2 2 x 2 2 x 2
Allowed connections
From 500 to 1000 Up to 400 Up to 16 From 500 to 1000
Synchronization GPS, IEEE 1588v2 or Synch. Ethernet
GPS, IEEE 1588v2 or Synch. Ethernet
Network Time Protocol or over the air from macro BS
GPS, IEEE 1588v2 or Sync Ethernet
Feature parity Preferred Preferred Not required Preferred
Backhaul Flexible Ethernet over copper Ethernet over copper OBSAI over fibre
Power feed Dedicated PoE, PoE+, PoE++ PoE+ or AC/DC adapter
SON functionalities Required Required Some Required
Small Cell Interference Management
Network-based resource partitioning Spatial-domain resource partitioning such as higher order sectorization and coordinated
beamforming
Time-domain resource partitioning (e.g. eICIC and (e)CoMP)
Frequency-domain resource partitioning done on PRB resolution or on carrier resolution
Network-based transmit power control Transmit power control per cell (Adjustment to improve the interference conditions)
UE-based interference mitigation Linear interference suppression by means of linear combining of received signals at the
UE antennas (case M>1), for example, interference rejection combining (IRC)
Non-linear interference suppression where the UE estimates and reconstructs the interfering signal(s) followed by subtraction before decoding the desired signal
52
Packet Scheduling Solutions
3GPP has standardized the interfaces very carefully but not the network-side algorithms. Operating domains: Frequency: Avoid interfered PRBs, Avoid faded PRBs
Time: Quality of Service, eICIC
Power: Power control, Load balancing
Spatial (antennas): CoMP, Beamforming
53
Packet Scheduling Solutions (2)
One Aim: LTE scheduler is to avoid inter-cell interference at the cell edge
Frequency Selective Scheduling (FSS): the scheduler obtains information about the amount of intercell interference in the frequency domain through the channel quality indicator (CQI) reported by UEs
54
Enhanced Inter-cell Interference Coordination (eICIC)
Aim: to increase the pico cell coverage area in the case of co-channel deployment of macro and picocells
eICIC work: The macro cell stops its transmission in some subframes (almost blank subframes, ABS) to minimize interference to the pico cells. Only signaling on ABS such as cell-specific RS, PCFICH,
synchronization channels and paging.
The ABS muting pattern is periodical with 40 subframesfor FDD mode, variable for TDD
55
eICIC (2): measurement restrictions
In order to get accurate interference measurements to eNodeB, the UE feedback reporting is modified with eICIC
Small cell UEs are requested to provide two separate CQI reports: One corresponding to the normal subframes Another one corresponding to ABS
Other changes Time-domain restrictions for RRM measurements for macro
UE to make more accurate handover to pico Configuration of Radio Link Management (RLM)
measurement restrictions is useful for pico users that are typically able to receive service during subframes where the macro uses ABS
Configuration of radio resource control (RRC) messages as specified and are therefore only applicable for connected mode UEs.
56
NOTE: Release 8 and 9 UEs do not support measurement restrictions
eICIC (3): Further eICIC in Rel-11
Interference on ABS is also present due to Common RS (CRS), which is approximately 9% of the Tx eNodeB power The CRS transmission is a constant deterministic sequence for each cell UE can
estimate and cancel the CRS interference by non-linear interference cancellation (IC)
Since cells are assumed to be time synchronized for eICIC, collisions of system information block one (SIB1) can cause problems for the pico users in the cell range extended to correctly receive the SIB1 The network can send the SIB1 via dedicated signalling to the pico user during ABS
57
(f)eICIC (4): factors for improvements
ABS adaptation and load balancing Fast and accurate ABS muting pattern
adjustment and dynamic load balancing maximize the benefits of eICIC
Small cell placement and density Highest eICIC gain observed for cases
with outdoor dense small cell deployment.
Terminal support Highest gain for Release 11 UEs
Offered traffic load Highest gain achieved at full load
condition (interference limited)
58
eICIC(5): Fast Dynamic eICIC
Architecture options: Distributed RRM
RRM functionality in each pico cell and the
inter-eNodeB coordination is done with X2
signalling
Coordination can be slow (several seconds) or fast (milliseconds)
Centralized RRM RRM functions including packet scheduling are located in the macro cell.
The pico cell is just RF head connected with fibre to the baseband hotel.
A tighter inter-cell coordination can increase the performance of the network In all these cases the RRM algorithms for both the macro and pico cells are implemented
in the macro eNode
59
eICIC (6): Slow and fast ABS adaptation
Slow adaptation uses normal subframes and mandatory ABS subframes The number of ABS subframes is changed
only with X2 signaling
Fast ABS uses additionally optional ABS subframes, which can be used for the macro cell or the pico cell transmission It gives more exibility to adapt the
resource allocation to the instantaneous capacity requirements
60
eICIC (7): Slow and fast ABS adaptation
CQI measurement configuration with fast ABS adaptation: the small cell decides which CQI (i.e. ABS or normal) shall be used
The fast decisions in the centralized architecture are made shortly before each optional ABS on whether to configure as ABS or normal transmission
The macro cell decides before each optional ABS on whether to configure as ABS or normal transmission Based on the information exchange with pico cells over the X2 interface
61
Periodic reporting but also event-triggered reporting is allowed
eICIC (8): performances
62
eICIC (9): performance in a real case
UE C is always connected to macro BS, while UE A and UE B are connected to their small cell only when ABS is activated (and RE occurs)
Four cases: no ABS, ABS at 25%, ABS at 50%, ABS at 75%
63
Coordinated Multi-Point (CoMP)
Aim: to improve network performance at cell edges
Idea of CoMP: depending on a UE’s location, it may be able to receive signals from multiple cell sites, and the UE’s transmissions may be received at multiple cell sites regardless of the system load, in a coordinated fashion
Work: For the DL, the transmissions from the multiple cell sites can be coordinated
For the UL, the system can take advantage of reception at multiple cell sites (e.g., through techniques such as interference cancellation).
64
CoMP: three approaches
Coordinated Scheduling or Coordinated Beamforming (CS/CB), The transmission to a single UE is transmitted from the serving cell only (same as in non-
CoMP transmission).
The scheduling and any Beamforming functionality are dynamically coordinated between the cells in order to control and/or reduce the interference between transmissions from different transmission points.
Dynamic Point Selection (DPS) The UE, at any one time, is being served by a single transmission point. But it can
change dynamically from subframe to subframe within a set of possible transmission points
65CoMP with Joint Beamforming
CoMP: three approaches
Joint Processing/Joint Transmission (JP/JT). The transmission to a single UE is simultaneously transmitted from multiple transmission
points, across cell sites.
The multi-point transmissions coordinated as a single transmitter: higher performance but stringent requirement on backhaul communication
66
Enhanced Coordinated Multipoint (eCoMP)
The coordination among cells can avoid inter-cell interference
CoMP study was started in Release 10 and completed in Release 11
Architecture options: Distributed: information exchanged on X2
Centralized: The eNodeBs would provide load and interference information to the centralized element which would then coordinate the scheduling of the individual eNodeBs
67
eCoMP (2)
Amount of exchanged info: In distributed scheduler
No raw info are transmitted, but each eNodeB can access the CSIs of its own UEs and exchange only cell-level metrics between adjacent eNodeBs.
The amount of X2 signalling can be minimized by exchanging only cell-level metrics instead of UE-specific information
Centralized scheduler It can obtain UE-specific CSI information since it does not have direct access to
any local scheduler information.
UE-specific information exchange may increase the amount of signalling.
Scalability: coordination to overcome interference typically requires only local coordination instead of coordination over large areas. Interfering neighbouring cells can change depending on the UE locations,
antenna tilts and network expansions.
The flexible cluster )instead of pre-configured one) to be dynamically defined: liquid cluster
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eCoMP (3)
CoMP scheduler The distributed scheduler can access very fast the local scheduler data and with some
latency to other schedulers
The centralized scheduler can access with some latency to all local scheduler
Distributed architecture is preferable and needn’t any other network node
Anyway, the algorithms need to be designed in such a way that latencies at least up to 10 ms can be tolerated.
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eCoMP (3): CoMP set
Uplink CoMP allows to receive the transmission signal from one UE by several cells (called a CoMP set): Intra-site CoMP: the CoMP set within one eNodeB
Inter-site CoMP: CoMP between eNodeBs
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Cell selection (CS) and joint processing (JP)
Comparison between ICIC and CoMP
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Small Cell Optimization
Need of optimization of RRM in small cell scenarios Mobility: improvement of the reliability of high-speed mobility in the dense small cell
environment.
Data rates: utilization of the macro and small cell resources simultaneously for the maximization of user data rates.
Interference management and capacity: minimization of interference between small cell layers in order to maximize the network capacity.
Power savings: minimization of the total network power consumption.
Multivendor case: operation and optimization of macro and small cell layers from different vendors.
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HetNet Mobility Optimization
Basic LTE handover methods provides good mobility performance for users at lower speed
Mobility state estimation and time-to-trigger (TTT) scaling [Rel. 8] The RRC Connected mode UEs estimate their
mobility state: normal, medium or high RCC parameters are consequently set (e.g.
measurement reports, HO time)
Inter-eNodeB mobility history signalling [Rel. 8]: eNodeBs signal the UE mobility history to the
network (i.e. previously serving cells, time-of-stay per cell and cell type as very small, small, medium, large)
The network estimates handover rate and adjusts mobility parameters or performs a handover to small cells, or mainly be kept at the macro layer.
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HetNet Mobility Optimization (2)
Signalling of UE mobility information [Rel. 12]: The network uses the mobility history stored by
UE to more accurately estimate the UE mobility e.g. RRC Idle – to RRC Connected transition, the global cell identity (GCID), or physical cell identity (PCI), the duration of stay in the 16 most recently visited LTE cells, time spent outside LTE
Target cell-dependent TTT [Rel. 12]: The network can configure UEs to use different
TTTs depending on the target cell Reducing the probability of fast moving users to be handed over to small cells
The network can signal to the UE a list of PCIs with alternative TTT in the measurements
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HetNet Mobility Optimization (3)
For P-M handover, TTT low
For M-M handover, TTT medium
For M-P handover, TTT high
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HetNet Mobility Optimization (4)
Macros and small cells on different carrier frequencies do not suffer interference
The main challenge is for macro-UEs to discover small cells on other carriers in due time without performing unnecessary inter-frequency measurements Periodical inter-frequency measurements every 40 or 80 ms of 6 ms
Methods for enabling inter-frequency measurements Typically, the network first enables inter-frequency measurements for the UEs when the
serving cell signal strength (or quality) drops below a certain threshold (report A2)
Location-aware methods for automatic suspend and resume
Using RF finger printing techniques (e.g. based on collected UE RRM measurements)
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Mobility with Dual Connectivity
A Hand-Over Failure (HOF) event is declared if RLF occurs after TTT expires, during the handover execution time
Two scenarios: generic 3GPP and with streets for European and Tokyo cities
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HOF without DC HOF with DC
Ultra Dense Network Interference Management: UDN characteristics
Very dense deployment of small cells on a set of dedicated carriers (or on a single carrier)
High co-tier interference
A larger number of interfering signals without a clear dominant interference, and only some users are subject to a dominant interfering source (or aggressors) About 30% have Dominant Interference Ratio (DIR) greater than 3 dB
Each cell simultaneously serving a single or few users, while several cells may have no users to serve at certain time Less than the 50% of the cells have scheduled users
Only a subset of users in an UDN can gain from ICIC (i.e. those users with a high DIR) The ICIC mechanism needs to be rather dynamic
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UDN (2): Proactive Time-Domain ICIC
ABS can in principle also be used for UDN but coordinated muting among small cells is challenging. Algorithm: A user is identified as a victim (according to a given criterion)
The victim cell requests the aggressor cell to mute some subframes via X2
The aggressor cell mutes some subframes
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Case of 12 cells
UDN (3): Reactive Carrier-Based ICIC
The ICIC can be conducted in the frequency domain by switching CCs on/off at the small cells in a coordinated manner Benefit and cost are estimated, based on the considered action (i.e. ON or OFF any CC)
A problem with a lot of cases just considering neighbor cells
Only hypotheses that result in a positive value without causing other users to fall below their GBR are considered valid options
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Power Saving with Small Cell On/Off
The network power consumption is a concern when a large number of small cells are added to the existing macro layer
Even if the power efficiency of a single base station improves, it does not compensate the high density of the small cells new system-level solutions for the minimization of the power consumption
One solution: to switch off those small cells that are not needed and switch on the cell again when needed Minimization of inter-cell interference.
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Issue: While switching off the small cells during low load is simple, more intelligence is required to identify when to switch on the small cells again.
Power Saving with Small Cell On/Off (2): proposals
Discovery signal in 3GPP Release 12 The network informs UE about the discovery signal and the
timing information, which allows UE to detect the discovery signal of multiple small cells at the same time.
Pre-configuration A list of preconfigured small cells that should be switched on first
based on the earlier statistical learnings.
Small cell uplink measurements The small cells can measure the uplink interference without
transmitting any data. If there is high uplink interference, it implies that there must be UEs close to that cell.
UE measurements Activation of reference signal transmission in small cells and
requesting UE measurements to find out of those UEs can receive the small cell signal.
Location measurements UE positioning information relative to the small cell locations can
be used to define which small cells would best serve the UEs.
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Multivendor Macro Cell and Small Cells
The carrier aggregation from Release 10 works only within one vendor in practice.
Multiradio RF implementation allows to use single RF unit for all technologies within one band
Also dual-band RF units are available combining for example 800 and 900 MHz bands.
Tighter interworking between macro cells and small cells CoMP works only within one vendor
Inter-site carrier aggregation can also be implemented between vendors over open X2 interface.
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Multivendor Macro Cell and Small Cells (2)
Aim: provide internetworking between frequencies, technologies and cell layers to optimize the complex implementation and improve the system performance
Theoretically, standardized and open interface: S1 interface, X2 interface between macro BSs, X2 interface between macro and small cells
eNodeB algorithms are not standardized and may require coordination Interface between baseband and RF are not open Femto cells commonly use different vendor than the macro cells. Moreover femto
gateways are present SON algorithms are typically vendor specific.
Some SON features are implemented at eNodeBs others are centralized
The network management interfaces are not fully standardized and some part of integration and adaptation is required
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Conclusions
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Conclusions
LTE Trends in terms of deployment and technology evolution
LTE description Release 8 and Release 9
Release 10 and Release 11
Release 12 and Release 13: Dual connectivity
Small Cells deployment Network Architecture Options, Frequency Usage, Indoor Small Cells
Small Cells products Micro cells, Pico cells, Femto cells, Remote Radio Heads
Small Cell Interference Management Packet Scheduling Solutions, eICIC, eCoMP
Small Cell Optimization Mobility, Inter-site Carrier Aggregation, UDN, Power Saving, Multivendor Cells
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References
H. Holma, A. Toskala, J. Reunanen, “LTE Small Cell Optimization: 3GPP Evolution to Release 13”, John Wiley & Sons Ltd, 2016.
4G Americas, “4G Mobile Broadband Evolution: 3GPP Rel-11, Rel-12 and Beyond”, Feb. 2014
4G Americas, “Mobile Broadband Evolution Toward 5G: Rel-12 & Rel-13 and Beyond”, Jun. 2015
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