Basic LTE Call Flow before it can receive or transmit data. These steps can be categorized in cell search and cell selection, derivation of system information, and random access. The complete procedure is known as LTE LTE Initial Access LTE LTE a terminal must perform certain steps
uccessful e!ecution of the cell search and selection procedure as well as ac"uiring initial
system information is essential for the #E before taking further steps to communicate with the network. $or this reason, it is important to take a closer look at this fundamental physical layer procedure. but I strongly recommend you to try to have some big picture of the whole process. %henever you have some issues or something for you to work, try to ask your self &%here is the current issue located in the whole picture '&.
Step A: Initial synchronization: Step A-1: Primary Synchronization Signal The #E first looks for the primary synchronization synchronization signal ()* which is transmitted in the last +$symbol of the first first time slot of the first subframe subframe (subframe (subframe * in a radio frame. This This enables the #E to ac"uire the slot boundary independently from the chosen cyclic prefi! selected for this cell. /ased on the downlink frame structure (Type 0, $*, which is shown in $igure 1, the primary synchronization signal is transmitted twice per radio frame, so it is repeated in subframe 2 (in time slot 00*. This enables the #E to get time synchronized on a 2 ms basis, which was selected to simplify the re"uired inter3fre"uency and inter34AT measurements.
Query_1: How does UE know to look for the PSS synchronization signal?
Well, UE doesn't need to worry much for this. As, the synchronization signal are always sent only on the center 62 sub carriers irrespectie of the channel bandwidth !".2#,$,#,"%,2%&. herefore, UE will loo( for the central sub carriers, i.e at the last )*+ symbol of the "st time slot and again at the last )*+ symbol of the ""th slot. With this UE synchronizes at the slot leel.
Step A-2: Secondary Synchronization Signal After the mobile has found the 2 ms timing, the second step is to obtain the radio frame timing and the cells5 group identity. This information can be found from the . In the !time domain, the is transmitted in the symbol before the ) . The also has 2 ms periodicity, which means it is transmitted in the first and si!th subframes (subframes and 2*.
Query_2: How does UE know to look for the SSS synchronization signal?
)nce, when the - is identified, is always send at the slot before the - is present. /n other words, immediately precedes the -.
0et's see how the UE deries the 1ell /+ using these two signals *rom - -34/1A0 0A4E5 1E00 /+E/4 is deried. /t carries the alue of %, " and 2. *rom -34/1A0 0A4E5 1E00 /+E/4 75)U- is deried. /t can ta(e the alue to % to "68.
*ormula Cell ID= (3*PHYSICAL LAYER CELL IDENTITY GROUP) + PHYSICAL LAYER CELL IDENTITY
Step A-3: Downlink Reerence Signal The #E is thus able to become fully synchronized with the radio cell because the reference signals are transmitted in well3defined resource elements. In every si!th subcarrier in the fre"uency domain a reference symbol from the generated reference signal pattern is transmitted. In the time domain, every fourth +$- symbol transmits a reference symbol . A resource block contains four reference symbols.
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Step !: !roadcast o essential system inormation Step !-": #aster inormation $lock $rom the -I/, #E gets the following information6 7hannel bandwidth in terms of 4esource /locks $8 (ystem $rame 8umber* )9I79 configuration (used for 9A4: A7;<8A7;* Query_: How does the UE read !"#?
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he /9 is transmitted on physical channel !9113:913:-913& and it always occupies the central 82 sub carriers in the *re;uency domain irrespectie of the channel bandwidth.
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he first transmission of the /9 is scheduled in sub:frame number % of radio frames for which the * mod < = %
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repetitions are scheduled in sub:frame % of all other radio frames
Step !-%: Si!1 i) Cell Access Related Information - PLMN Identity List, PLMN Identity, TA Code, Cell identity & Cell Status ii) Cell Selection Information - Minimum Receiver Level iii) Scedulin! Information - SI messa!e ty"e & Periodicity, SI# ma""in! Info, SI $indo% len!t
Step !-&:Si!2 i) Access #arrin! Information - Access Proaility factor, Access Class #arin! List, Access Class #arin! Time ii) Semi static Common Cannel Confi!uration - Random Access Parameter, PRAC' Confi!uration iii) (L freuency Information - (L *AR+CN, (L # and%idt, additional emmission
After the aboe process the UE is synchronized with the networ( in the +ownlin( direction and hae read /9" and /9 2. ow, it needs to synchronize in the Uplin( direction. he UE cannot start utilizing the serices of the networ( immediately after downlin( synchronization unless it is synchronized in the uplin( direction too.
ow, 5A- !5andom Access -rocedure& is initiated
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here are two types of 5A- 1ontention based 5Aon:contention based 5A-
Typical =7ontention /ased= 4A79 )rocedure is as follows 6 i) (* -- N$ RAC' Preamle .RA-RNTI, indication for L/0L1 messa!e si2e) ii) (* 3-- N$ Random Access Res"onse .Timin! Advance, T4C-RNTI, (L !rant for L/0L1 messa!e) iii) (* -- N$ L/0L1 messa!e iv) Messa!e for early contention resolution
Typical =7ontention $ree= 4A79 )rocedure is as follows 6 i) (* 3--N$ RAC' Preamle Assi!nment ii) (* -- N$ RAC' Preamle .RA-RNTI, indication for L/0L1 messa!e si2e) iii) (* 3--N$ Random Access Res"onse .Timin! Advance, C-RNTI, (L !rant for L/0L1 messa!e)
'ontention $ased RAP In contention based, multiple #E=s attempt to connect to the network at the same time. The e8/ is intelligent enough to tackle this situation because every #E should be uni"ue to the network.
The #E=s can always send the same )reamble I to the network, thereby resulting on collisions. This kind of collision is called &7ontention& and is known as &7ontention based& 4A79 )rocess. The network would go through additional process to resolve these contention and hence this process is called &7ontention 4esolution& step.
tep 06 In the first message the #E provides an indication to the network about it=s resource re"uirement. This carries the )reamble I, 4A348TI Query_$: How does UE gets or selects these %ara&eters:
a. ost of the information is passed on to the UE through /92 ' click here( to know &ore a)out S"#2 %ara&eters* i. UE A1 layer has to select the -reamble se;uence !7roup A or 7roup 9& ii. UE will configure itself with the ma> retires it will try for sending 5A- !if it doesn't receie 5A5& iii. Also, after eery retry, how much power leel has to be increased for transmitting the 5Ai. UE A1 layer constructs the 5A- message and passes it to the UE -34 layer. UE -34 layer will transmit this message through -5A13 . )nce the UE has transmiited the 5A- on -5A13, it will start loo(ing for 5A5 immediately after $ sub:frames. his number i.e. $ sub:frame is specified by $7--.
Query_+: How long should UE &onitor the fra&es for ,-,?
his sub:frame number is again specified in /92 and is (nown as window length? so, after the $ sub:frames as mentioned aboe, UE will start loo(ing for 5A5 in the sub:frames as mentioned by the Window length. /f by that time UE doesn't receie 5A5, it will go bac( to transmit 5A-
tep >. The e8/ conveys the resources reserved for this #E along with the Timing Advance (TA*, )reamble I and T3748TI (a number generated by e8/ and asks the #E to send the 447 connection* tep ?. #E sends the 447 connection 4e"uest using resources given by the e8/. It also sends the identifier (74I* to the e8/ which is used to resolve the 7ontention. tep @. The e8/ runs an algorithm and generates 7348TI which will be a permanent I for the #E till the connection is alive. The e8/ sends the #E identifier. In this step, the #E which has received the I continues while other #E=s will back off and try again.
Scenario: -ultiple #E=s attempt to access the network6
0. o, the #Es initiates 4A79 with same )reamble se"uence, 4A348TI >. Therefore, the #Es will receive the same T37348TI and resource allocation from e8/
?. All #Es would send msg ? (447connection4e"uest* message through the same resource allocation to the 8etwork @. +nce, when msg? is transmitted, two Timers are started6 a. T? 6 Transmission of 447connection4e"uest b. 7ontention 4esolution Timer6 broadcasted in I/>. If the #E doesn=t receive msg@ (7ontention 4esolution message* within this timer, then it go back to tep 0 i.e. transmitting 4A). If there is a 9A4: 8A7; for msg? (447connection4e"uest* and it has to be re3transmitted then this 7ontention 4esolution Timer will be re3 started @uery6 ow the big ;uestion 3ow should the e9 behaeB ". )ne he signals act as interference to each other and e9 decode neither of them. /n this case, none of the UE would hae any response !3A5@ A1C& from e9 and all UE will go bac( to tep ". 2. econd he e9 would successfully decode the message from only one UE and fail to decode from others. he decoded UE will get 3A5@ A1C from e9 $. hird e9 receies msg$ !551connection5e;uest& from both the UE's. 3ere, e9 will send msg< !1ontention 5esolution& with A1 15/ !1ontention 5esolution /dentity& to both the UE's. his 15/ will carry a reflection of the 551connection5e;uest as generated by one of the UE. he A1 layer of the UE will match the 15/ !as receied from msg<& with the 15/ embedded in the 551connection5e;uest. /f it matches, then the UE will proceed to decode 551connectionetup and the other UE's will bac( off and return to tep", i.e start the 5A procedure again.
'ontention Resol(tion process is again o two types: 0. -A7 based 7ontention 4esolution B 7348TI on )779 B uses the 779 logical channel B used in 9+ scenarios BThe rule is6 if the #E has a valid 7348TI and is going for 4A procedure then it will be a -A7 based 7ontention 4esolution procedure
>. L0 based 7ontention 4esolution B 74I (7ontention 4esolution Identity* on L379 based B 7ontention 4esolution is addressed to T3748TI B uses 7779 logical channel BThe rule is6 if the #E doesn=t has a valid 7348TI and is going for 4A procedure then it will be L0 based 7ontention 4esolution procedure Query_6: Exactly when and Where a UE transmit RACH ? you need to refer to $7-- specification $6.2"" : able #.8.":2. +id you open the specification now B /t shows e>actly when a UE is supposed to send 5A13 depending on a parameter called D-5A13 1onfiguration /nde>D. *or e>ample, if the UE is using D-5A13 1onfiguration /de> %D, it should transmit the 5A13 only in EE number *!ystem *rame umber&. /s this good enough answer B +oes this mean that this UE can transmit the 5A13 in any time within the specified the * B he answer to this ;uestion is in Dub *rame umberD colulmn of the table. /t says D"D for D-5A13 1onfiguration /de> %D. /t means the UE is allowed to transmit 5A13 only at sub frame number " of eery een *.
Query_7: How does Network knows exactly when UE will transmit the RACH ?
/t is simple. etwor( (nows when UE will send the 5A13 een before UE sends it because etwor( tells UE when the UE is supposed to transmit the 5A13. !/f UE fails to decode properly the networ( information about the 5A13, etwor( will fail to detect it een though UE sends 5A13&. *ollowing section will describe networ( informaton on 5A13.
Which 551 essage contains 5A13 1onfiguration B /t is in /92 and you can find the details in $7-- $6.$$".
Query_8:Exactly when and where etw!r" transmit RACH Res#!nse We all (nows that etwor( should transmit 5A13 5esponse after it recieed 5A13 -reamble from UE, but do we (now e>actly when, in e>actly which subframe, the networ( should transmit the 5A13 5esponse B he following is what $7-- $6.$2" !section #.".<& describes. Once the Random Access Preamble is transmitted and regardless of the possible occurrence of a measurement gap, the UE shall monitor the PDCCH for Random Access Response(s) identified b the RA!R"#$ defined belo%, in the RA Response %indo% %hich starts at the subframe that contains the end of the preamble transmission &' plus three subframes and has length ra! Responseindo%*i+e subframes /t means the earliest time when the networ( can transmit the 5A13 response is $ subframe later from the end of 5A13 -reamble. hen what is the latest time when the networ( can transmit it B /t is determined by ra:5esponseWindowize. his window size can be the number between % and "% in the unit of subframes. his means that the ma>imum time difference between the end of 5A13 preamble and 5A13 5esponse is only "2 subframes !"2 ms& which is pretty tight timing re;uirement.
Query_$: Why/when UE send another PRACH? / When/How soon do I have to send the next PRACH?
#ac5off Indicator "rovide te ans%er to tis uestion6 #ac5off Indicator is a s"ecial MAC sueader tat carries te "arameter indicatin! te time delay et%een a PRAC' and te ne7t PRAC'6 .As "er 1861/9)6 +or e7am"le, if te #I field value is 9:, #ac5off Parameter value is 1/: ms6 Tis means (* can send PRAC' any time in et%een : and 1/: ms from no%6 you %ould notice tat #I .#ac5off Indicator) field is made u" of ; its, im"lyin! tat it can carry te value from :<9=6
#I sueader sould al%ays e at te e!innin! of te %ole MAC eader6 If you see more carefully, you %ould notice tat #I sueader is so%n %it >dotted> rectan!le6 It means tat tis is o"tional, im"lyin! tat te net%or5 send or does not send #I de"endin! on te situation6 If you see even more carefully, you %ould notice tat #I sueader does not ave any corres"ondin! "ayload "art6 It means ?#ac5off Indicator? information is carried directly y te MAC eader0sueader and it doesn>t use any "ayload field6
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What is an antenna port and their mapping? The LTE standard defnes what are known as antenna ports. These antenna ports do not correspond to physical antennas, but rather are logical entities distinguished by their reerence signal sequences. Multiple antenna port signals can be transmitted on a single transmit antenna !-"# port $ and %E-"# port &, or e'ample(. !orrespondingly, a single antenna port can be spread across multiple transmit antennas %E-"# port &, or e'ample(.The LTE standard defnes what are known as antenna ports. These antenna ports do not correspond to physical antennas, but rather are logical entities distinguished by their reerence signal sequences. Multiple antenna port signals can be transmitted on a single transmit antenna !-"# port $ and %E-"# port &, or e'ample(. !orrespondingly, a single antenna port can be spread across multiple transmit antennas %E-"# port &, or e'ample(.
The )*++ T# ). LTE standard defnes antenna ports or the downlink. /n antenna port is generally used as a generic term or signal transmission under identical channel conditions. 0or each LTE operating mode in the downlink direction or which an independent channel is assumed e.g. #1#2 3s. M1M2(, a separate logical antenna port is
defned. LTE symbols that are transmitted 3ia identical antenna ports are sub4ect to the same channel conditions. 1n order to determine the characteristic channel or an antenna port, a %E must carry out a separate channel estimation or each antenna port. #eparate reerence signals pilot signals( that are suitable or estimating the respecti3e channel are defned in the LTE standard or each antenna port. 01* shows the antenna ports defned in the LTE standard in "eleases 5,6 and $.
The way in which these logical antenna ports are assigned to the physical transmit antennas o a base station is up to the base station, and can 3ary between base stations o the same type because o di7erent operating conditions( and also between base stations rom di7erent manuacturers. The base station does not e'plicitly notiy the %E o the mapping that has been carried out, rather the %E must take this into account automatically during demodulation 01* (. /s ar asThe way in which these logical antenna ports are assigned to the physical transmit antennas o a base station is up to the base station, and can 3ary between base stations o the same type because o di7erent operating conditions( and also between base stations rom di7erent manuacturers. The base station does not e'plicitly notiy the %E o the mapping that has been carried out, rather the %E must take this into account automatically during demodulation 01* (.
Let us consider antenna ports used or +8#!9 allocations since they probably ha3e the most 3ariations. 1nitially, the 56$$ :#/;s LTE demodulator supported only analysis o +8#!9 transmitted on /ntenna +orts $, $ and (, $, , (, or $, , , )(. These ports are considered !-"# antenna ports, and each port has a di7erent arrangement o !-"#
resource elements. :arious confgurations are defned that use these !-"# antenna ports, including - or <-port T' 8i3ersity and -, )-, or <-port #patial Multiple'ing.
Then beamorming support was added and single-layer +8#!9 allocations transmitted on +ort & could be analy=ed. The LTE demodulator has since been enhanced to support the LTE "elease 6 which added Transmission Mode 5--8ual-Layer >eamorming i.e. beamorming ? spatial multiple'ing(--where +8#!9 is transmitted on /ntenna +orts @ and 5 note that single-layer beamorming in "el 6 can also use port @ or port 5 in addition to port &(. 1n "el $ o the standard, the new transmission mode 6 TM6( added up to 5-layer transmissions using +orts @-<. TM6 is supported by the LTE-/d3anced demodulator.
/s +orts $-) are indicated by the e'istence o !-"#, so +orts & and @-< are indicated by the %E-specifc "eerence #ignal %E-"#(. The ollowing is a table that summari=es the 3arious +8#!9 mappings that can be used along with the corresponding reerence signal and antenna ports.
1n a M1M2 or T' 8i3ersity confguration, each !-"# antenna port must be transmitted on a separate physical antenna to create spatial di3ersity between the paths. #ingle-layer beamorming, on the other hand, is accomplished by sending the same signal to each antenna but changing the phase o the each antenna;s signal relati3e to the others. #ince the same %E-"# sequence is sent rom each antenna, the 56$$ :#/ can compare the recei3ed %E-"# sequence with the reerence sequence and calculate the weights that were applied to the antennas to accomplish the beamorming.
Multi-layer beamorming adds some comple'ity to beamorming by transmitting as many %E-"# sequences as there are layers to allow demodulation o each layer;s +8#!9 data. The %E-"# sequence or each antenna port is orthogonal to the others, either in timeArequency domain or in the code domain. This can be thought o as beamorming o each layer independently. B-layer beamorming is an e'tension o dual-layer beamorming and supports up to 5 data layers with the ability to beamorm each layer separately.
