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..

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:
N_U = 5 * (F_UL - F_{UL_Offset}), with F_{UL_low} ≤ F_UL ≤ F_{UL_high}

N_D = 5 * (F_DL - F_{DL_Offset}), with F_{DL_low} ≤ F_DL ≤ F_{DL_high}
N_U and N_D 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