Tti bundling in fdd and tdd

TTI Bundling in FDD and TD-LTE
Abstract
TTI Bundling has been introduced in FDD and TD-LTE to improve Uplink coverage. Here are the
salient features of TTI Bundling with respect to FDD and TD-LTE. We will first explore the motivation
for TTI Bundling and then compare it with other techniques. Finally, we shall address the
implementation of TTI bundling in FDD and TD-LTE .
Introduction
TTI bundling is a technique used to send a transport block multiple times in consecutive subframes
without waiting for HARQ ACK/NACK messages. Normally, a transport block is converted to multiple
redundancy versions after coding and the first redundancy version is sent in a subframe. Subsequent
transmissions of the transport block are dependent on the HARQ ACK/NACK which is sent 4
subframe durations later or more after the first transmission. In TTI bundling, the different
redundancy versions can all be sent in consecutive subframes without waiting for the HARQ
ACK/NACK feedback and a combined ACK/NACK can be sent after processing all the transmissions
of a transport block.
The motivation for TTI bundling which is illustrated in Figure 1 is the low transmission power of some
handsets, short TTI length, and the long RTT of the HARQ transmissions. TTI bundling is expected
to improve the UL coverage of applications like VOIP over LTE wherein low power handsets are likely
to be involved. This feature has more relevance for TDD over FDD as coverage issues are likely to
be more challenging in TD-LTE. Simulation results reported in publications indicate a 4 dB gain due
to TTI bundling on the UL.
LTE Coverage Improvement by TTI Bundling
ABSTRACT Compared to WCDMA, the LTE radio access has a significantly shorter Transmission
Time Interval (TTI) in order to reduce end-to-end delays. However, if a User Equipment (UE) at the
cell edge is limited by its available transmission power, it may not be able to transmit an entire VoIP
packet during one TTI, since the instantaneous source data rate is too high. Thus TTI bundling has
been recently introduced as a feature of LTE Rel. 8 to improve the uplink coverage. In TTI bundling, a
VoIP packet is transmitted as a single PDU during a bundle of subsequent TTIs without waiting for
the HARQ feedback. HARQ feedback is only expected for the last transmission of the bundle. This
paper studies TTI bundling and compares it to the conventional RLC segmentation. The simulation
results indicate that TTI bundling provides a gain of more than 4 dB in terms of the sustainable path
loss.

Alternative Approach
The alternative to TTI bundling is RLC segmentation wherein a VOIP payload is split into smaller size
RLC PDUs as shown in Figure 2. The smaller RLC PDUs will result in smaller transport blocks which
can be decoded with better accuracy. One drawback of this method is the potential overhead
increase due to RLC segmentation due to multiple RLC headers needed. For a typical VOIP
payload, it has been shown that as we increase the segmentation factor from 1 to 8, the overhead
increases from 14% to 55%. Each RLC PDU which is mapped into a transport block will need a
separate PDCCH assignment message which will contribute to control signal overhead for such a
scheme. There might be retransmissions of each of those transport blocks which will also potentially
increase the control signaling overhead. In addition, since we are transmitting many small transport
blocks, the chances of interpreting a NACK as a ACK also increases proportionately with the increase
in the RLC segmentation size. Hence, RLC segmentation has many disadvantages when we
consider the transmission of a VOIP like payload from a power limited terminal.

Overview of TTI Bundling
TTI bundling is used to achieve successful transmissions from power limited terminals. The process
as shown in Figure 3 is typically triggered by UE informing the eNB about its power limitations at the
present state. This could for example happen at the edge of a cell when the terminal has to send
high power but is limited by the power capability of the terminal. This triggers the eNB to transmit the
various redundancy versions of the same transport block in consecutive subframes or TTIs giving rise
to the name TTI bundling. A single PDCCH allocation is sufficient for the multiple transmissions thus
saving control overhead as compared to the RLC segmentation approach. A single HARQ
ACK/NACK for the combined transmissions is generated after processing the TTI bundle which can
reduce the error rate of the transport block as compared with processing a single redundancy
version. This approach can also reduce the delay in the HARQ process compared to transmissions
of the redundancy versions separated in time using the normal approach.

TTI Bundling Operation
As shown in Figure 4, TTI bundling enables up to 4 redundancy versions of the same transport block
to be sent in 4 consecutive subframes. In TD-LTE systems, the TDD configurations standardized
allow only a maximum of 3 consecutive UL subframes. A single RLC PDU is transmitted as multiple
redundancy versions in consecutive subframes using a single common allocation. The channel
coding used in LTE enables easy generation of the multiple redundancy versions from which the
transmissions in the TTI bundle are generated. A common RLC header is shared across the TTI
bundle and the same HARQ process identity is used for multiple transmissions in the TTI
bundle. Combined processing of the redundant transmissions over multiple subframes leads to a
better probability of detection of the transport block. Thus, with limited power, the UE has a better
chance of a successful transmission with lesser latency using the TTI bundling method.
Summary: TTI bundling is a useful technique for improving coverage of VOIP handsets in LTE
systems. It is applicable to both FDD and TD-LTE deployments and can improve the link budget by
up to 4 dB. Differences in implementation exist between FDD and TD-LTE systems. TTI bundling
helps achieve good latency performance for VOIP even at the edges of cells.
Adaptive transmission of VoIP packets using TTI bundling in LTE uplink
ABSTRACT In long term evolution (LTE) uplink, transmission time interval (TTI) bundling technique is
employed for the voice over internet protocol (VoIP) service to improve the cell coverage
performance. In frequency-division duplex (FDD) mode, LTE uses 4 TTIs or subframes as a bundle
for transmission with a round trip time (RTT) 16ms. Thus the maximum number of transmission
bundles is only three if the delay budget is strictly limited to 50ms in VoIP service. Recently, a
coverage-enhanced scheme can transmit up to five bundles, but lacks time for feedback processing.

Once the unnecessary retransmissions occur, the user equipment (UE) transmit power can be
wasted for extra transmission. To overcome this problem, we propose an adaptive scheme using
block error rate (BLER) versus signal-to-noise ratio (SNR) mapping table, which enables UE
predetermine the maximum number of transmission bundles appropriately. Instead of using five
transmission bundles, our proposed scheme requires no more than five transmission bundles
according to SNR variation scale of the channel. The conducted simulation results show that our
proposed scheme can achieve higher throughput than coverage-enhanced scheme by reducing
unnecessary retransmissions, while it still satisfies the BLER requirements by employing a tolerance
Performance of TTIBundling for VoIP In EUTRAN TDD Mode
ABSTRACT The long term evolution (LTE) of 3GPP radio-access technology aims to develop a
framework towards a high-data-rate, low-latency and packet-optimized radio access technology:
Evolved Universal Terrestrial Radio Access Networks (E-UTRAN). However, low terminal
transmission power, short TTI length and long HARQ RTT give a critical problem on LTE TDD UL
performance in a coverage-limited scenario. To solve coverage problem, this paper presents an
effective coverage enhancement mechanism called TTI bundling to enhance uplink VoIP
performance in LTE TDD mode. HARQ design for different TDD configurations is discussed with TTI
bundling. Performance evaluation for TTI bundling with VoIP traffic is carried out by system
simulation, and the impact from different bundling options with different bundle size and packet delay
budget is investigated. The simulation analysis proves that with proper bundle size and suitable
packet delay budget setting, TTI bundling can enhance the coverage performance for LTE TDD
effectively.
Main Feature
ActivationService::isTTIBundlingForVoIPEnabled
CellL1L2ControlChannelsConf::extraDCI0powerOffsetForTTIBundling
CellL1ULConf::betaOffsetACKIndexForTTIbundling
CellL2ULConf::maxHARQtxTTIbundling
CellL2ULConf::maxNbrOfTTIbundlingUsers
CellL2ULConf::maxULVoIPdataRateForTTIBundling
CellL2ULConf::tTIBundlingFixedMCSvalue
CellL2ULConf::tTIbundlingNotificationRepetitionTimer
CellL2ULConf::ttiBundlingPUSCHPRBZoneSize
CellL2ULConf::ttiBundlingPUSCHPRBzoneStart
CellL2ULConf::uplinkLinkBudgetAlarmClearanceThreshold
CellL2ULConf::uplinkLinkBudgetAlarmTimeToClear
CellL2ULConf::uplinkLinkBudgetAlarmTimeToTrigger
CellL2ULConf::uplinkLinkBudgetAlarmTriggerThreshold
CellRadioConf::minHARQtxWithoutMGcollisionForTTIbundling
RrcMeasurementConf::cpichEcn0OffsetForTtiBVoIPCalls
RrcMeasurementConf::cpichRscpOffsetForTtiBVoIPCalls
RrcMeasurementConf::rsrpOffsetOnNeighborCellForTtiBVoIPCalls
RrcMeasurementConf::rsrpOffsetOnServingCellForTtiBVoIPCalls
RrcMeasurementConf::rsrqOffsetOnNeighborCellForTtiBVoIPCalls
RrcMeasurementConf::rsrqOffsetOnServingCellForTtiBVoIPCalls

New Feature Status
& Definition for TDD & FDD sites.xlsx

TTI Bundling and VoIP Performance in LTE - Part I
One of the benefits of LTE is its superlative performance in the amount of payload that can be delivered in a
short period of time. This is a convoluted way to say: “fantastic throughput”. Yours truly has been testing this
fact, countless times by running a speed test app every night before I go to sleep! (59Mbps in DL is the
maximum I have seen. I would reveal my carrier’s name if it were not for fear of the repercussions …. ;-)
The minimum allocation in LTE, spans 180KHz and 1ms in the frequency and time domains respectively (this
is exactly one resource block in frequency and two resource blocks in time). This resource should be sufficient
to carry a VoIP packet using coded narrow band AMR ( @ 12.2 kbps = 244 bits in 20ms). Since a VoIP packet
carries 20ms worth of speech and our transmission time interval (TTI) is only 1ms, a UE that is engaged in
continuous voice is only active for 5% of the time!
The efficiency of LTE can be exploited to increase the voice capacity of the cell by time-multiplexing many
more users that could be scheduled in the intervening time before the first batch of users have to be scheduled
again. Disregarding many realistic and important constraints, and perfect RF conditions, the theoretical
capacity of voice calls per cell can be estimated as the number of users that can be scheduled in a TTI
multiplied by 20. In 20MHz band there are 100 Resource Blocks and presumably one user can be scheduled per
resource block in a TTI (this is far from the truth due to PDCCH limitations), then the theoretical peak capacity
of LTE is around 2000 calls per cell! Even with a 50% efficiency, 1000 calls per cell is impressive.
There is another way to use the 5% activity factor in LTE that is about improving the Uplink coverage at the
cost of reduced capacity. This technique is known in the 3GPP specs as TTI Bundling. In TTI bundling, the
20ms worth of speech packet is repeated in consecutive frames. Up to four TTIs can be used to send copies of
the same VoIP packet over the air. But why does the uplink coverage improve when copies of a voice packet
are bundled together?

To understand the impact of TTI bundling for uplink coverage, we need to remind ourselves of the uplink link
budget in LTE. In fact we only need to consider how much the eNodeB sensitivity is improved when TTI
bundling is used. TTI-Bundling allows for efficient decoding since it implies a four-fold redundancy in
transmission without any need for retransmissions! This should decrease the required signal to noise ratio at the
cell edge, without appreciable increase in latency. According to RAN1#54 report R1-081856, there is a 4dB
gain in uplink coverage in when 4 TTIs are bundled together (an extra twist to this result is that it is calculated
for 2 RBs which is what is needed for Wide-Band AMR transmissions).
TTI Bundling is activated in the network using Layer 3 signaling. One way to implement the activation or
deactivation of TTI bundling is to consider the UE power head room. This is a strong indicator of how much the
UE is struggling to close the uplink and be heard by the eNodeB at the appropriate Signal to Noise Ratio. So a
simple implementation for TTI bundling algorithm could depend on thresholds for the UEs available power at
any given moment in time.
In part two of this blog, I will look at the effects of delay and retransmission in TTI bundling.

Quality of Service (QoS) in LTE
Background: Why we need QoS ?
There are premium subscribers who always want to have better user experience on their 4G LTE device. These users are
willing to pay more for high bandwidth and better network access on their devices. Not only the subscribers but some
services itself need better priority handling in the network (e.g. VoIP call). To be able to full fill this, QOS plays the key
role. QOS defines priorities for certain customers / services during the time of high congestion in the network
3GPP definition for QoS
In LTE Network QoS is implemented between UE and PDN Gateway and is applied to a set of bearers. 'Bearer' is
basically a virtual concept and is a set of network configuration to provide special treatment to set of traffic e.g. VoIP
packets are prioritized by network compared to web browser traffic.
In LTE, QoS is applied on Radio bearer,S1 bearer and S5/S8 bearer,collectively called as EPS bearer as shown in figure
below.
In orderto comprehendthe conceptof QoS, we must understandthe bearertypesandpropertiesassociatedwitheach
bearerthrough hierarchical chartas shownbelow.Firstthere are twotypesof Bearer,i.e. DedicatedbearerandDefault
bearer.There is at-leastone defaultbearerestablishedwhenUEisattachedto LTE networkwhile dedicatedbeareris
alwaysestablishedwhenthere isneedtoprovide QoStospecificservice (likeVoIP,videoetc).Pleasego throughthe
article DefaultandDedicatedBearerwhichhopefullywill helptoexplainthe conceptinmore detail.

Dedicated bearer can be subdivided into Non-GBR and GBR types.
GBR provides guaranteed bit rate and is associated with parameters like GBR and MBR
- GBR: The minimum guaranteed bit rate per EPS bearer. Specified independently for uplink and downlink
- MBR: The maximum guaranteed bit rate per EPS bearer. Specified independently for uplink and downlink
On the other hand, Non-GBR bearer does not provide guaranteed bit rate and has parameter like A- AMBR and UE-
AMBR
- A-AMBR: APN Aggregate maximum bit rate is the maximum allowed total non-GBR throughput to specific APN. It is
specified interdependently for uplink an downlink
- UE -AMBR: UE Aggregate maximum bit rate is the maximum allowed total non-GBR throughput among all APN to a
specific UE
As you can see,the default bearer can only be non-GBR type. Some other important terms associated with each bearer
type are discussed below:
- ARP: Allocation and retention priority is basically used for deciding whether new bearer modification or establishment
request should be accepted considering the current resource situation.

- TFT: Traffic flow template is always associated with dedicated bearer and while default bearer may or may not have
TFT. As mentioned earlier, dedicated bearer provides QoS to special service or application and TFT defines rules so that
UE and Network knows which IP packet should be sent on particular dedicated bearer. It usually has rules on the basis of
IP packet destination/source or protocol used.
L-EBI: It stands for Linked EPS bearer ID. As I discussed in previous article about dedicated and default bearer,we
know that each dedicated bearer is always linked to one of default bearers. L-EBI tells Dedicated bearer which default
bearer it is attached to.
IP Address/ PDN: Each default bearer is attached to some PDN network and has its own IP address while dedicated
bearer does not need this since it is linked to default bearer.
You can also see one other parameter associated with all bearers i.e. QoS class of identifier (QCI).This parameter
basically defines IP level packets characteristics as shown below
EXAMPLE
Let me try to explain here again with the same example I gave in Default and Dedicated Bearer section
Usually LTE networks with VoLTE implementations have two default and one dedicated bearer
Default bearer 1: Used for signaling messages (sip signaling) related to IMS network. It uses qci 5
Dedicated bearer: Used for VoLTE VoIP traffic. It uses qci 1 and is linked to default bearer 1
Default bearer 2: Used for all other smartphone traffic (video, chat, email, browser etc),assuming qci 9 is used here

This means that Default bearer 1 is associated with IMS PDN and has specific IP address. It has throughput limitations
defined in terms of A-AMBR and UE-AMBR. Since it has qci 5 which means that its IP packets has the highest priority
over other IP packets and maximum delay as 100ms between UE and PGW with packet loss percentage up to 10-6
Default bearer 2 is associated with internet PDN and has specific IP. It has throughput limitations defined in terms of A-
AMBR and UE-AMBR as well. Since it has qci 9 which means that its IP packets has the lowest priority over other IP
packets and maximum delay possible as 300ms between UE and PGW with packet loss percentage up to 10-6
Dedicated bearer will be linked to Default bearer 1 with L-EBI and it also has TFT which basically defines which IP
packets should be allowed to travel on this bearer. It has throughput limitations defined in terms of MBR and GBR. Since
it is using QCI 1, the IP packets traveling on this bearer have the second highest priority. The maximum delay possible to
IP packets on this bearer is 100 ms and the percentage of packet loss will be under 10-2
AKA Digest authentication scheme for VoLTE (IMS)
When a VoLTE client needs to connect to IMS network, it has to authenticate the network while network also
needs to make sure that only the correct user is registered to its network. AKA Digest is one of the scheme to
authenticate VoLTE client to the IMS server
AKA
AKA stands for "Authentication and key agreement". This scheme comes from the legacy 3gpp networks and
has been widely used in LTE, 3G, CDMA and WiMAX technologies. In this mechanism, a secret key is already

known to both user device (USIM, iSIM) and authentication servers (HSS, HLR).
The server will challenge the end user using AKA algorithms and shared key and sends RAND, AUTN values
towards UE. UE will authenticate network and prepares result (RES for network to authenticate UE) with the
help of shared key in UICC and parameters sent by Server.
HTTP Digest
Http Digest is the popular authentication scheme used for authenticating users to access web servers and
other applications which requires security and data integrity. This scheme is much secure than the basic
authentication as it applies hash function to the password before sending it [RFC2617].
HTTP Digest is username / password based authentication procedure. The authentication server provides one
time created " nonce " value to the client. The client uses the nonce value and creates a secure response that
contains the password, username and other parameters to the server. The password which is known both to
server and client, is always fixed
Now For IMS
Now since IMS is a part of 3GPP and on the contrary SIP signaling defines http digest for authentication
[RFC3261]. Therefore in order to use 3GPP AKA with IMS, the parameters from AKA are mapped onto http
digest [RFC3310]. In simple words the parameters / headers used to transport http digest information, will
transport AKA information in identical format
With 3GPP AKA digest, the "nonce" now contains RAND, AUTN. The password now contains the one time
RESPONSE generated by client with help of UICC (USIM, ISIM). Thus the method is even more secure.
Authentication in IMS networks
1) VoLTE Client sends SIP register request to IMS Server. The user is not authenticated at this point.
The SIP register request contains IMS related identities (private identity, public identity, URI, etc)
2) The IMS server (S-CSCF) obtains authentication vector and SQN from HSS that contains a random
challenge RAND, authentication token AUTN, expected authentication result XRES, a session key
for integrity check IK, and a session key for encryption CK
3) The server creates an authentication request, which contains the random challenge RAND, and the
network authenticator token AUTN
4) The authentication request is delivered to the client with "401 UNAUTHORIZED" message
5) The client verifies the AUTN with the ISIM. If the verification is successful, the network has been
authenticated. The client then produces an authentication response RES, using the shared secret K
and the random challenge RAND

3GPP AKA Operation in IMS
I. The authentication response RES is delivered to the server using new regiser sip message
II. The server compares the authentication response RES with the expected response. If the two match,
the user has been successfully authenticated
III. Session keys IK and CK can be used for protecting further communications between the client and the
server
IV. Server sends "200 OK" message to inform the VoLTE client about successful registration
Semi persistent scheduling
Every VoIP packetisreceived /sentevery20ms whenthe useristalkingwhereasinsilence period,discontinuous
transmission(DTX) isusedtoreduce the transmissionrate.Also,inordertosustainvoice quality,silentinsertion
descriptor(SID) packetarrivesevery160ms.The frequentarrival/transmissionof VoIPpacketmeanslarge control
overheadforlowerlayers(L1/L2) inthe radioprotocol stack. To deal withthisissue,semi persistentschedulingplaysan
importantrole.

Schedulingisamechanismwhere UErequests eNBforthe resource allocationduringeachtransmissiontime interval
(TTI).If UE has some data that itneedstotransmitcontinuously,itwillrequesteNBeveryTTIforthe resource allocation.
Thisschedulingtype isdynamicscheduling.The advantage of dynamicschedulingisflexibilityanddiversityof resource
allocationbutas mentioned,thisresultsinhuge L1/L2 loadwhichinturn meansinefficientuse of scarce radioresources.
In case of semi persistentscheduling,eNBcanassignpredefinedchunkof radioresourcesforVoIPuserswithinterval of
20ms. Therefore,UEis not requiredtorequestresourceseachTTI,savingcontrol planoverhead.Thisschedulingissemi-
persistentinthe sense thateNBcanchange the resource allocationtype orlocationif requiredforlinkadaptationor
otherfactors.
TTI Bundling
Withall the hype createdaroundIMS and LTE, operatorshave startedquestioningnetworkvendorsif theyare
supportingRAN specificfeaturesforVoLTE.TTIbundlingisone of the featuresamongmanyothersthat can
helpVoIP (VoLTE) callsinLTE.
TTI BundlingisLTE feature toimprove coverage atcell edge orin poorradio conditions.UEhas limitedpowerin uplink
(only23dBm forLTE) whichcan resultinmany re transmissions atcell edge (poorradio). Re transmission meansdelay
and control planoverheadwhichmaynotbe acceptable forcertainserviceslike VoIP.TounderstandTTIbundlingone
needtohave the basicideaof HybridAutomaticRepeatRequest(HARQ) andTransmissionTime interval (TTI).
HARQ
HARQ isa processwhere dataat mac layerisprotectedagainstnoisywirelesschannelsthrougherrorcorrection
mechanism.There are couple of differentversionsof HARQbutin LTE we have a type knownas
'Incremental RedundancyHybridARQ'.When receiverdetectserroneousdata,it doesn'tdiscardit.Onthe otherhand,
senderwill sendthe same dataagainbutthistime,withdifferentsetof codedbits.The recieverwill combine the
previouslyrecievederroneousdatawithnewlyattempteddatabythe sender.Thiswaythe chancesof successfully
decodingthe bitsimprove everytime.Thiswill repeataslongas the receiverisnotable to decode the data.The
advantage of thismethodis that witheach re-transmission,the codingrate islowered.Whereasinothertypesof
HARQ,it mightuse the same codingrate inevery re-transmission
TTI
TTI is LTE smallestunitof time inwhicheNBiscapable of schedulingany userforuplinkordownlinktransmission.If a
useris receivingdownlink data,then duringeach1ms,eNBwill assignresourcesandinformuserwhere tolookforits
downlinkdatathroughPDCCHchannel.Checkthe followingfigure tounderstandthe conceptof TTI

Now coming to TTI Bundling ...
HARQ isa processwhere receivercombinesthe new transmission everytimewithpreviouserroneousdata.There isone
drawbackhowever,thatitcan resultindelayandtoo much control overheadincase of poor radioconditionsif the
senderhasto attemptmanytransmissions.Forserviceslike VoIPthismeansbadenduserexperience.Well,thereis
anotherway- Insteadof re-transmittingthe erroneousdatawithnew setof codedbits,whynotsendfew versions
(redundancyversions) of the same setof bitsin consecutive TTIandeNBsendsback Ackwhenitsuccessfullydecodes
the bits.I hope the figure belowwillmake itclear.Thiswaywe are avoidingdelayandreducingcontrol planeoverhead
at mac layer
Voice solutions in LTE

The original ideabehindLTEisthat it wouldprovide onlywirelessinternetservices.However,majorrevenue forcellular
operatorscomesfromvoice callsandSMS and therefore Voice inLTEhas become a hottopic.Recently,Igot an
opportunity toworkwithvariousvoice solutions-the experience whichI believe wouldbe usefultoshare here.
LTE doesnothave a 'circuitswitchcore' whichmeansthatwe cannot have voice callsas itis in2G and 3G technologies.
In the initial LTEdeploymentcaseshowever,operatorsare usingtheirlegacynetworksalongwiththeir4G networkfor
voice services.
So far we have heardof the followingavailablevoice solutionswhichIwill discussbriefly.
 CircuitSwitchedFall Back(CSFB)
 SimultaneousVoice andLTE(SV-LTE)
 Voice overLTE (VoLTE)
 Voice overLTE viaGenericAccess(VoLGA)
 Overthe top (OTT)
Circuit Switched Fall Back
An operatorwhodeployedLTEnetwork,alreadyowninga3G or 2G networkcantake benefitfromthe feature called
"Circuitswitchedfall back'.The mainideaisthat 4G smartphonesare goingtohave a radio capabilitiesfor3G/2G
networksaswell.Suchhandsetscanconnectat a time eithertoLTE or 2G/3G . The shortcomingisthatsomeone on
voice call will notbe able touse LTE networkforbrowsingorchatting etc.
CSFB foroperatormeansverylittle investmentsince onlyfew modificationsare requiredin the network.Additional
interface betweenMMEand MSC isrequired(SGs).CSFBsolutionhasalsobeen standardized by3GPPand has gained
large industrial support.

Simultaneous Voice and LTE
SV-LTEis handsetspecificinwhichhandsetiscapabile of usingtworadios(LTEand WCDMA/GSM/CDMA) at one time.
So a usercan use packetservicesfromLTE while voice call canbe made on othernetworkssimultaneouslyunlikeCSFB.
The shortfall here ishighbattery utilization due todual radiooperation.
For CDMA and LTE pair,the SV-LTE isthe standard solutionandbeingwidelyadopted. There are alreadySV-LTE
smartphonesavailable inthe market.Icame across a few available forLGU+ in Koreaand VerizoninUSA.Both operate
LTE networksasan overlaytotheir oldCDMA networks.
SV-LTEis the cheapestoptionforoperatorsasno new modificationisrequiredtothe network.Nevertheless,as
mentioned earlieritisat the cost of highbatteryutilization

Voice over LTE (IMS)
I believethisisgoingtobe the most popularandwidelyadoptedfuturevoice solutionforLTE. Insteadof usinglegacy
networks,VoLTEutilizesIPMultimediaSubsystem(IMS) andprovidesvoice services usingthe applicationlayeronLTE.
IMS is a group of core networkentitiesresponsible forprovidingrichmultimediaservicesoverIPnetwork.VoIPcall,
SMS, MMS, LIVETV are a fewsuch services.IMShasbeeninthe communicationindustryforlongbutwiththe
emergence of 4G networks,itisgainingpopularity again.
Voice over LTE via Generic Access
I think,operatorswill acceptVoLGA asa last optionforvoice capability.ThissolutionusesCScore onlyfromlegacy
networksandalsorequire newnetworkelements.Therefore LTEhandsetsdonotneed3G/2G radiocapabilitiessince
radiopart won't be usedfromlegacynetworks.Goodthingaboutthissolutionhoweveristhatunlike CSFB,LTEhandset
will be able touse voice anddata simultaneously.

Over the top VoIP application
OTT is actuallynotLTE specificbuta genericsolutionthatwe alreadyhave beenusingon3G/WiFi networks.OTT
applicationiscompletelytransparenttonetworkandalsooutof operators'control.I am talkingabout
genericVoIP clientslike Viber,skype,Tangoetc.Theydonot give the real taste of voice flexibilityasinother3GPP
networksandalsolack the QoSfor voice. Nonetheless, these will be widelyusedbythe consumersasan alternative,
because of the fact that it givesthemfull flexibilitytochoose theirownservice.
Semi persistent scheduling
Every VoIP packetis received /sentevery20ms whenthe useristalkingwhereasinsilence period,discontinuous
transmission(DTX) isusedtoreduce the transmissionrate.Also,inordertosustainvoice quality,silentinsertion
descriptor(SID) packetarrivesevery160ms.The frequentarrival/transmissionof VoIPpacketmeanslarge control
overheadforlowerlayers(L1/L2) inthe radioprotocol stack. To deal withthisissue,semi persistentschedulingplaysan
importantrole.
Schedulingisamechanismwhere UErequestseNBforthe resource allocationduringeachtransmissiontime interval
(TTI).If UE has some data that itneedstotransmitcontinuously,itwillrequesteNBeveryTTIforthe resource allocation.
Thisschedulingtype isdynamicscheduling.The advantage of dynamicschedulingisflexibilityanddiversityof resource
allocationbutas mentioned,thisresultsinhuge L1/L2 loadwhichinturn meansinefficientuse of scarce radioresources.
In case of semi persistentscheduling,eNBcanassignpredefinedchunkof radioresourcesforVoIPuserswithinterval of
20ms. Therefore,UEis not requiredtorequestresourceseachTTI,savingcontrol planoverhead.Thisschedulingissemi-
persistentinthe sense thateNBcanchange the resource allocationtype orlocationif requiredforlinkadaptationor
otherfactors.

TTI Bundling
Withall the hype createdaroundIMS and LTE, operatorshave startedquestioningnetworkvendorsif theyare
supportingRAN specificfeaturesforVoLTE.TTIbundlingisone of the featuresamongmanyothersthat can
helpVoIP (VoLTE) callsinLTE.
TTI BundlingisLTE feature toimprove coverage atcell edge orin poorradio conditions.UEhas limitedpowerinuplink
(only23dBm forLTE) whichcan resultinmany re transmissions atcell edge (poorradio). Re transmission meansdelay
and control planoverheadwhichmaynotbe acceptable forcertainserviceslike VoIP.Tounderstand TTIbundlingone
needtohave the basicideaof HybridAutomaticRepeatRequest(HARQ) andTransmissionTime interval (TTI).
HARQ
HARQ isa processwhere dataat mac layerisprotectedagainstnoisywirelesschannelsthrougherrorcorrection
mechanism.There are couple of differentversionsof HARQbutin LTE we have a type knownas
'Incremental RedundancyHybridARQ'.When receiverdetectserroneousdata,it doesn'tdiscardit.Onthe otherhand,
senderwill sendthe same dataagainbutthistime,withdifferentsetof codedbits.The recieverwill combine the
previouslyrecievederroneousdatawithnewlyattempteddatabythe sender.Thiswaythe chancesof successfully
decodingthe bitsimprove everytime.Thiswill repeataslongas the receiverisnotable to decode the data.The
advantage of thismethodis that witheach re-transmission,the codingrate islowered.Whereasinothertypesof
HARQ,it mightuse the same codingrate inevery re-transmission
TTI
TTI is LTE smallestunitof time inwhicheNBiscapable of schedulinganyuserforuplinkordownlinktransmission.If a
useris receivingdownlink data,then duringeach1ms,eNBwill assignresourcesandinformuserwhere tolookforits
downlinkdatathroughPDCCHchannel.Checkthe followingfigure tounderstandthe conceptof TTI
Now coming to TTI Bundling ...
HARQ isa processwhere receivercombinesthe new transmission everytimewithpreviouserroneousdata.There isone
drawbackhowever,thatitcan resultindelayandtoo much control overheadincase of poor radioconditionsif the

senderhasto attemptmanytransmissions.Forserviceslike VoIPthismeansbadenduserexperience.Well,thereis
anotherway- Insteadof re-transmittingthe erroneousdatawithnew setof codedbits,why notsendfew versions
(redundancyversions) of the same setof bitsin consecutive TTIandeNBsendsback Ackwhenitsuccessfullydecodes
the bits.I hope the figure belowwillmake itclear.Thiswaywe are avoidingdelayandreducingcontrol planeoverhead
at mac layer
Voice solutions in LTE
The original ideabehindLTEisthat it wouldprovide onlywirelessinternetservices.However,majorrevenue forcellular
operatorscomesfromvoice callsandSMS and therefore Voice inLTEhas become a hottopic.Recently,Igot an
opportunity toworkwithvariousvoice solutions-the experience whichI believe wouldbe usefultoshare here.
LTE doesnothave a 'circuitswitchcore' whichmeansthatwe cannot have voice callsas itis in2G and 3G technologies.
In the initial LTEdeploymentcaseshowever,operatorsare usingtheirlegacynetworksalongwiththeir4G networkfor
voice services.

So far we have heardof the followingavailablevoice solutionswhichIwill discussbriefly.
 CircuitSwitchedFall Back(CSFB)
 SimultaneousVoice andLTE(SV-LTE)
 Voice overLTE (VoLTE)
 Voice overLTE viaGenericAccess(VoLGA)
 Overthe top (OTT)
Circuit Switched Fall Back
An operatorwhodeployedLTEnetwork,alreadyowninga3G or 2G networkcantake benefitfromthe feature called
"Circuitswitchedfall back'.The mainideaisthat 4G smartphonesare goingtohave a radio capabilitiesfor3G/2G
networksaswell.Suchhandsetscanconnectat a time eithertoLTE or 2G/3G . The shortcomingisthatsomeone on
voice call will notbe able touse LTE networkforbrowsingorchatting etc.
CSFB foroperatormeansverylittle investmentsince onlyfew modificationsare requiredinthe network.Additional
interface betweenMMEand MSC isrequired(SGs).CSFBsolutionhasalsobeen standardized by3GPPand has gained
large industrial support.

Simultaneous Voice and LTE
SV-LTEis handsetspecificinwhichhandsetiscapabile of usingtworadios(LTEand WCDMA/GSM/CDMA) at one time.
So a usercan use packetservicesfromLTE while voice call canbe made on othernetworkssimultaneouslyunlikeCSFB.
The shortfall here ishighbattery utilization due todual radiooperation.
For CDMA and LTE pair,the SV-LTE isthe standard solutionandbeingwidelyadopted. There are alreadySV-LTE
smartphonesavailable inthe market. Icame across a few available forLGU+ in Koreaand VerizoninUSA.Both operate
LTE networksasan overlaytotheiroldCDMA networks.
SV-LTEis the cheapestoptionforoperatorsasno new modificationisrequiredtothe network.Nevertheless,as
mentionedearlieritisat the cost of highbatteryutilization

Voice over LTE (IMS)
I believethisisgoingtobe the most popularandwidelyadoptedfuturevoice solutionforLTE. Insteadof usinglegacy
networks,VoLTEutilizesIPMultimediaSubsystem(IMS) andprovidesvoice services usingthe applicationlayeronLTE.
IMS is a group of core networkentitiesresponsible forprovidingrichmultimediaservicesoverIPnetwork.VoIPcall,
SMS, MMS, LIVETV are a fewsuch services.IMShasbeeninthe communicationindustryforlongbutwiththe
emergence of 4G networks,itisgainingpopularity again.
Voice over LTE via Generic Access
I think,operatorswill acceptVoLGA asa last optionforvoice capability.ThissolutionusesCScore onlyfromlegacy
networksandalsorequire newnetworkelements.Therefore LTEhandsetsdonotneed3G/2G radiocapabilitiessince
radiopart won't be usedfromlegacynetworks.Goodthingaboutthissolutionhoweveristhatunlike CSFB, LTEhandset
will be able touse voice anddata simultaneously.

Over the top VoIP application
OTT is actuallynotLTE specificbuta genericsolutionthatwe alreadyhave beenusingon3G/WiFi networks.OTT
applicationiscompletelytransparenttonetworkandalsooutof operators'control.I am talkingabout
genericVoIP clientslike Viber,skype,Tangoetc.Theydonot give the real taste of voice flexibilityasinother3GPP
networksandalsolack the QoSfor voice. Nonetheless, these will be widelyusedbythe consumersasan alternative,
because of the fact that it givesthemfull flexibilitytochoose theirownservice.
Parameters
This section describes the parameters introduced by the TTI Bundling feature and
parameters affected by activating the feature.

Introduced Parameters
The followingtable describesthe parametersintroducedbythe TTI Bundlingfeature:
Parameter Description
featureStateTtiBundling
Activates or deactivates the licensed feature TTI Bundling. The value of the
attribute is irrelevant when no valid license key is installed for the feature.
keyIdTtiBundling The license key ID for the TTI Bundling feature.
licenseStateTtiBundling
The license status of feature TTI Bundling, ENABLED or DISABLED. The
value is ENABLED when a license key is installed.
serviceStateTtiBundling Indicates if the feature TTI Bundling is operable or inoperable.
TtiBundlingId The value component of the RDN (Relative Distinguished Name).
Affected Parameters
The following table describes the parameters affected by the TTI Bundling feature:
Parameter Description
QciProfileOperatorDefined:
ServiceType or
QciProfilePredefined:
ServiceType
Indicates the service that the bearer is used for VoIP. It shall be set to
VOIP for one bearer if the UE shall use TTI Bundling.
DRX parameter:
OnDurationTimer
The TTI Bundling feature can affect DRX: the DRX parameter
OnDurationTimer. If the value of OnDurationTimer is less than 4 in the
MOM it will be set to 4 for UEs that are configured to use TTI Bundling.
New Counter Validation
The following table lists the important counters associated with the TTI Bundling
feature:
Counter Description
pmTtiBundlingUeSum
Sum of all sample values recorded for number of UEs using TTI
Bundling.
pmTtiBundlingUeSamp Counts the number of times the corresponding pmTtiBundlingUeSum has

been incremented.
pmTtiBundlingUeMax
Maximum sample value of number of UEs using TTI Bundling.
pmVoipQualityUeUlOk Number of UE that are satisfied with their VoIP quality.
pmVoipQualityUeUlNok Number of UE that are not satisfied with their VoIP quality
pmVoipQualityUeUlLowSampl
Number of VoIP quality measurements with low number of samples while
the VoIP satisfaction of a UE is between 50% and 99%

Tti bundling in fdd and tdd

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