Sunday, March 30, 2014

Cell update with cause "Re-entering service Area"


We often see a UE in Cell_FACH/Cell_PCH/URA_PCH state sending Cell update with cause “Re-entering service Area”, In this blog we will try to understand in what all scenarios this can happen. 

Before we go ahead it is required to understand below terminology
1. Out Of Service Area
2. In Service Area  

Out Of Service Area detection: 
If the UE finds that the serving cell does not fulfil the cell selection criteria,
UE initiates the measurements of all neighbour cells and if it does not find any new suitable cell for a predefined period, UE is considered to be "out of service area".
UE initiates cell selection procedures for the selected PLMN after this  period. 
In Cell_PCH/URA_PCH this period is 12 seconds while in Cell_FACH this period is 4 seconds. 

In Service Area detection: 
When a suitable cell is found (cell <re>selection criteria is met ) , the UE considers it as having detected "in service area".

Cell Update with cause “Re-entering service Area” might be sent to the same cell (on which UE was previously camped on) or to the different cell.
So "In Service Area" detection does not always mean that UE has detected the same cell.

Coming back to the possible scenarios when UE can send Cell update with cause “Re-entering service Area” 

Scenario  1 :
UE enters into Cell_FACH/Cell_PCH/URA_PCH state  ----> starts timer T305------>T 305 expires and UE detects “Out Of 
Service Area”-----> starts timer T307------now if before T307 expiry UE detects "In Service area" -------> UE sends cell update with cause "re entering service Area"

Scenario 2 :
UE in CELL_FACH state and detects out of 
service Area --->starts timer T317----->before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "re entering service Area"

Scenario 3 :
UE in Cell_PCH/URA_PCH state  and detects “Out Of 
Service Area”--->starts timer T316----->Before expiry of T316 OR if T 316 expires and UE detects "In Service Area" ----->UE sends cell update with cause "re entering service Area"

Scenario 4:
UE in Cell_PCH/URA_PCH state and detects “Out Of 
Service Area”--->start timer T316----->T 316 expires and UE still “Out Of Service Area”----->UE starts T317 and move to CELL_FACH state -----before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "reentering service Area"

I hope this blog is helpful, I would definitely like to include any suggestion or correction in case needed. 

Ref specs : 25.133, 25.331, 25.304

Wednesday, July 24, 2013

UL Channelization Code in UMTS

One day I stumbled upon a question  "How NodeB come to know about the channelization of an UL  Data channels that UE transmit? Does it communicated by UE in some message on some channels or what?

Generally the channelization code and SF are fixed for control channels in UL and DL both directions but when it comes to data channels there are certain things which are defined and some are to be derived by Network.

Network(RNC) only communicates the MIN Spreading Factor to be used in UL in physical channel configuration. For example in case of HSUPA RNC communicates the MIN SF to be used in both BearerSetup/Reconfig and RL_Setup/Reconfig which could be 2SF2, 2SF4, 2SF2 + 2SF4.

In UL direction there is no dearth of coding resources as its being selected by UE and it does not have to share this resource. UE always selects higher SF and do the TFCI/ETFCI selection based on its transmit power. So network(NodeB) is not even aware of the SF in use to decode a particular channel and this is necessary to know the SF and Channelization code to decode a channel.

So first of all NodeB derive the Spreading Factor using the received data rate from the UE.

TBD....

RRC states and RRC State transitions

We would try to find the answer for below questions:

What are various RRC States and why?
How and when to move a UE from one state to another state?
Is it worth putting extra burden of signaling on UE and Network?
What is Paging and Cell update?

RRC States: 

UE can be either in connected mode or in idle mode in UMTS network. While being in connected mode a UE can stay in one of these 4 RRC States/Modes which are:
  • CELL_DCH  
  • CELL_FACH
  • CELL_PCH
  • URA_PCH
When a UE sends a request to a Network for connection establishment then the Network has the prerogative to select appropriate RRC state for the UE based upon the certain parameters like Cause, Resources,  Qos, etc.   
A UE can only be admitted into states where UL and DL data transfer can be possible like CELL_DCH or CELL_FACH, However a UE can also be released from CELL_PCH state(supported only in R5 and above) other than CELL_DCH and CELL_FACH. 

An UMTS Network always takes below parameters into account to ensure best results:

  • Limited Power, (Most impotently UE power)
  • Limited Channel Resource( Code in case of UMTS)
  • Qos (Service demanded by UE)

Different RRC States have been introduced for better resource utilization, for better connectivity, for best Qos with less possible power uses.  So to achieve all this, Network implements algorithms which take all the above points into account and directs a connected UE to move from one state to another state.

In this post I would like to talk about CELL_DCH, CELL_FACH and CELL_PCH states only and will take 3GPP Rel 6 into consideration.



RRC States:

CELL_DCH:
As the name suggests CELL_DCH  is the dedicated state where UE gets the access to the costly dedicated and data intensive resources in the network. The Transport and Physical channels which can be allocated by the network to the UE in CELL DCH state are:

Transport Channels:
  • DCH (UL/DL)
  • HS-DSCH (DL)
  • EDCH (UL)
Physical Channels:
  • DPDCH/DPCCH
  • HS-PDSCH/HS-SCCH/HS-DPCCH
  • E-DPDCH/E-DPCCH/E-RGCH/E-HICH/E-AGCH
All above DCH channels are used for high data rate services(CS AMR is a real time service and can only be in DCH) so consumes more Network and UE resources like Battery and processing resources in UE and Power, Code, Processing resources in Network. If we talk specifically about the PS connection the traffic behavior is bursty in nature like if we access an internet page and go idle, in this scenario having DCH resource allocated for a long time is not a good idea. To deal with this scenario network can either have a low resources intensive state or may put UE in Idle, But putting UE in idle can cause more problem as network has to bring the UE back to the network as soon as the data requirements comes for the UE.
And that is why UMTS supports a low data rate state with shared resources called CELL_FACH

An example scenario:

if((UE_Data_Rate< 3kbps) && (for Time T))
{ Put UE in FACH } 

CELL_FACH:
UE having only low data rate PS connection can be moved from DCH to FACH (obviously as CS calls cannot be established in FACH STATE) and also A UE can be directly admitted into FACH STATE based on the UE admission cause(eg Registration), However it also depends on admission control algorithms sometimes.  Suppose a UE just wanted to do location update/Routing area update then allocating and de-allocating DCH resource is pointless, such UEs can be admitted in FACH, can setup a PS RAB in FACH and also can be moved to DCH if required to transfer high volume of PS Data or wanted to make CS Call.  The transport and physical channels are as follows in CELL_FACH State.

Transport Channels:
  • RACH (UL)
  • FACH (DL)
Physical Channels:
  • PRACH, AICH
  • SCCPCH
The capacity of data transfer of FACH channel is generally 32kbps and can also be increased of decreased slightly. FACH and RACH are shared channels with very low capacity. All the UEs in the CELL_FACH state have only these channels available (till Rel 6, In later releases 3GPP has also added HS-DSCH and EDCH in CELL-FACH state).
  • The UE is known on cell level according to the cell where the UE last made a cell update.
  • Use C-RNTI assigned in the current cell as the UE identity on common transport channels unless when a new cell is selected
  • UE performs cell reselection and upon selecting a new UTRA cell, initiate a cell update procedure
If an UE is being idle for a longer period of time with PS connection then it's a point of concern for network because there is a certain limit on the number of UE supported in this state and also congestion. To deal with this scenario again network has two option either it can put the UE into IDLE or put it in a power/resource saving connected mode call CELL_PCH. Putting UE back to idle is tough task as network has to do fresh setup for the UE if the data requirement comes and may result into the flooding of signaling messages and high latency, If supported network will put this UE in CELL-PCH state.

CELL_PCH:
Network does not provide any resource to the UE to do any UL or DL data transfer in CELL_PCH. In this state UE can only  listen to paging requests sent by network in DL or if it has to send data in UL then it has to come to CELL_FACH rather transient CELL_FACH state and send CELL_UPDATE_IND to the network with cause UL_DATA_TRANSFER. We can say that while in CELL_PCH state UE uses CELL_FACH as stepping state momentarily without the consents of network. 
  • UE monitors the paging occasions according to the DRX cycle and receive paging information on the PCH
  • The UE is known on cell level according to the cell where the UE last made a cell update in CELL_FACH state
  • UE mobility is performed through cell reselection procedures. The UE performs cell reselection and upon selecting a new UTRA cell, it moves to CELL_FACH state and initiates a cell update procedure in the new cell.
  • Network uses URNTI to identify a UE is CELL_PCH state wich is SRNTI + SRNC-ID a unique identifier of RRC Connection in a PLMN.
  • Paging Type 1 sent (on PCCH) is used to page a UE in CELL_PCH state.
Transport Channels:
  • PCH (DL)
Physical Channels:
  • PICH, SCCPCH
Paging a UE in cell PCH_STATE is more efficient than paging a UE is Idle state. IU-PS Network connection is maintained with CN and network also preserves the UE context at various back-end nodes but UE does not have any radio resources at hand .



State Transitions:






TBD..

Tuesday, July 23, 2013

UARFCN: UTRAN Absolute Radio Frequency Channel Number

UARFCN: Absolute Radio Frequency Channel Number is an unique number representing a full 5 MHz carrier in a particular band.

UARFCN are assigned in both in UL and DL which are basically an identifier to the frequency in UL and DL which is used to modulate the baseband signal.


Operating Band
UL Frequencies
UE transmit, Node B receive
DL frequencies
UE receive, Node B transmit
I
1920 - 1980 MHz
2110 -2170 MHz
II
1850 -1910 MHz
1930 -1990 MHz
III
1710-1785 MHz
1805-1880 MHz
IV
1710-1755 MHz
2110-2155 MHz
V
824 - 849 MHz
869-894 MHz
VI
830-840 MHz
875-885 MHz
VII
2500-2570 MHz
2620-2690 MHz
VIII
880 - 915 MHz
925 - 960 MHz
IX
1749.9-1784.9 MHz
1844.9-1879.9 MHz
X
1710-1770 MHz
2110-2170 MHz
XI
1427.9 - 1452.9 MHz
1475.9 - 1500.9 MHz

As per 3GPP, UTRA FDD is designed to operate in the above paired bands.


Carrie frequency has channel raster of 200kHz, it means one carrier frequency is at least 200kHz apart from another carrier frequency in a particular band.
UE bands are defined. A UE supporting band I will be able to scan all the DL carrier frequency in the Band I.

The channel raster is 200 kHz, which for all bands except Band II means that the centre frequency must be an integer
multiple of 200 kHz. In Band II, 12 additional centre frequencies are specified.


The UARFCN is defined as follows:
NU = 5 * (FUL - FUL_Offset), with FUL_low ≤ FUL ≤ FUL_high

ND = 5 * (FDL - FDL_Offset), with FDL_low ≤ FDL ≤ FDL_high

NU and ND are the UARFCN for the uplink and the downlink.

For each operating band, FUL_Offset, FUL_low, FUL_high, FDL_Offset, FDL_low and FDL_high are defined in 3GPP TS 25 101.


Band
DL to UL Frequency Separation (MHz)

Center Frequency Range (MHz)

UARFCN Equation
UARFCN Range Test Set "DL Channel" Range
I
(IMT-2000)
190 2112.4 - 2167.6,
increment = 0.2
5 * (center freq in MHz)
10562 - 10838
10562 - 10838
II
(U.S. PCS)
80 1932.4 - 1987.6,
increment = 0.2
5 * (center freq in MHz) 9662 - 9938 9662 - 9938
1932.5 - 1987.5,
increment = 5
5 * ((center freq in MHz) - 1850.1 MHz) 412, 437, 462, 487, 512, 537, 562, 587, 612, 637, 662, 687 412, 437, 462, 487, 512, 537, 562, 587, 612, 637, 662, 687
III
(DCS/PCS)
95 1807.4 - 1877.6,
increment = 0.2
5 * ((center freq in MHz) - 1575 MHz) 1162 - 1513 1162 - 1513
IV 400 2112.4 - 2152.6,
increment = 0.2
5 * ((center freq in MHz) - 1805 MHz) 1537 - 1738 1537 - 1738 *
2112.5 - 2152.5,
increment = 5
5 * ((center freq in MHz) - 1735.1 MHz) 1887, 1912, 1937, 1962, 1987, 2012, 2037, 2062, 2087 1887, 1912, 1937, 1962, 1987, 2012, 2037, 2062, 2087 *
V
(US Cellular)
45 871.4 - 891.6,
increment = 0.2
5 * (center freq in MHz) 4357 - 4458 4357 - 4458 #
871.5, 872.5, 876.5, 877.5, 882.5, 887.5 5* ((center freq in MHz) - 670.1 MHz) 1007, 1012, 1032, 1037, 1062, 1087 1007, 1012, 1032, 1037, 1062, 1087 #
VI
(Japan 800)
45 877.4 - 882.6,
increment = 0.2
5 * (center freq in MHz) 4387 - 4413 4387 - 4413 +
877.5, 882.5 5 * ((center freq in MHz) - 670.1 MHz) 1037, 1062 1037, 1062 +
VII 120 2622.4 - 2687.6,
increment = 0.2
5 * ((center freq in MHz) - 2175 MHz) 2237 - 2563 2237 - 2563
2622.5 - 2687.5,
increment = 5
5 * ((center freq in MHz) - 2105.1 MHz) 2587, 2612, 2637, 2662, 2687, 2712, 2737, 2762, 2787, 2812, 2837, 2862, 2887, 2912 2587, 2612, 2637, 2662, 2687, 2712, 2737, 2762, 2787, 2812, 2837, 2862, 2887, 2912
VIII 45 927.4 - 957.6,
increment = 0.2
5 * ((center freq in MHz) - 340 MHz) 2937 - 3088 2937 - 3088
IX 95 1847.4 - 1877.4,
increment = 0.2
5 * (center freq in MHz) 9237 - 9387 9237 - 9387 *
X 400 2112.4 - 2167.6,
increment = 0.2
5 * ((center freq in MHz) - 1490 MHz) 3112 - 3388 3112 - 3388 *
2112.5 - 2167.5,
increment = 5
5 * ((center freq in MHz) - 1430.1 MHz) 3412, 3437, 3462, 3487, 3512, 3537, 3562, 3587, 3612, 3637, 3662, 3687 3412, 3437, 3462, 3487, 3512, 3537, 3562, 3587, 3612, 3637, 3662, 3687 *
XI
48
1478.4 - 1498.4, increment = 0.2
5 * ((center freq in MHz) - 736 MHz)
3712 - 3787
3712 - 3787
XII
30
730.4 - 743.6, , increment = 0.2
5 * ((center freq in MHz) + 37 MHz)
3837 - 3903
3837 - 3903
730.5, 731.5, 736.5, 737.5, 742.5, 743.5 5 * ((center freq in MHz) + 54.9 MHz) 3927, 3932, 3957, 3962, 3987, 3992 3927, 3932, 3957, 3962, 3987, 3992
XIII
31
748.4 - 753.6, , increment = 0.2
5 * ((center freq in MHz) + 55 MHz)
4017 - 4043
4017 - 4043
748.5, 753.5 5 * ((center freq in MHz) + 64.9 MHz) 4067, 4092 4067, 4092
XIV
30
760.4 - 765.6, , increment = 0.2
5 * ((center freq in MHz) + 63 MHz)
4117 - 4143
4117 - 4143
760.5, 765.5 5 * ((center freq in MHz) + 72.9 MHz) 4167, 4192 4167, 4192
XIX
45
877.4- 887.6
5 * ((center freq in MHz) - 735 MHz)
712 to 763
712 to 763
877.5, 882.5, 887.5
5 * ((center freq in MHz) - 720.1 MHz)
787, 812, 837
787, 812, 837
XX
41
793.4 - 818.6
5 * ((center freq in MHz) + 109 MHz)
4512 to 4638
4512 to 4638
XXI
48
1498.4 - 1508.4
5 * ((center freq in MHz) - 1326 MHz)
862 to 912
862 to 912



Sunday, June 23, 2013

HSUPA Code Allocation

HSUPA is the 3gpp release 6 featue which is basically enhancement in up-link data rate. There are multiple aspects of this feature but we will only discuss code allocation at phy in this blog post.

Here are the new physical channels added in HSUPA in UL and DL directions:


UL(UE to UTRAN) : 

  • E-DPDCH
      Its an uplink Data channel which carries UE data to the NodeB. Could be 
      maximum up to 4.  the max code can be used like this 2SF2 + 2SF4.
      Two EDPDCH can use Spreading Factor 2 and two can use Spreading
      Factor 4.


  • E-DPCCH
      It's an UL control channel which is used to carry control information related
      to the EDPDCH Data channels. There could be only 1 EDPCCH. And the
      channelization code and Spreading Factor is fixed for this channel.


DL(UTRAN to UE):

E-AGCH This channel carries Absolute grant, Fixed SF and Fixed Channelization code

E-RGCH: This channel is used to carry Relative grant

E-HICH: This channel is used to carry HARQ ACK/NAK information

Code and Spreading Factor for  E-DPDCH is not fixed but code for rest of the channels are fixed.

TBD ....