Fast Dormancy and Smartphones

Always-On:
 Accessing Internet and using applications or games are the most common habits of the people around the world. Smart phone are stuffed withe application and these applications needs to be connected to the internet 24/7 .

Lets dive into the details of these stuffs one by one, how all this things happen and how such a small device can manage to be connected to the network 24/7. There are so many good things or features have been introduced on both side UE and Network to make this happen. This always on requirement of cell phones creates so many problem for the network and themselves. Here are the some of them:

• Battery Life of the Cell phone
• Signaling Congestion in network.
• Resource wastage in Network

Below the some of the popular cell phones and how they initiate connection on startup:

• Blackberry and iPhone 

       Both UEs does GPRS Attach when they are power on  and trigger PDP Context Activation                when requested by application.

• Android 

     Android based all the UEs does GPRS Attach and PDP Context Activation while power on.

• Nokia 5800 /Some HTC UEs 

       Only ask for Attach or PDP only while triggered by application.

Smart phones applications mainly uses below two techniques to interact to the network or to access data from internet:

• Poll :In this technique of communication data request is initiated by client application and responded immediately by the server application somewhere in the network.

• Push : In this style of communication Server directly pushed the data to to the client. client just send periodic keep alive packets to the server.

Fast Dormancy:
There biggest challenge associated with such data hungry smart phones are network congestion and battery consumption of phones. Smart phones and network has imlemented FAST Dormancy to save battery life of the phones and to reduce congestion in the Netwok at some extent.

Every application sends or receives a certain amount of data in some defined or unprecedented manner, and this requirement demands to be in connected mode but if cell phone would have to monitor a dedicated channel regularly then its battery will drain very fast and this will also results into a resource crunch for dedicated channels in the serving network. As per 3GPP a UE can be in 4 connected mode in the network with data bearers active.

• CELL_DCH
• More data rate, more power
• CELL_FACH
• Lower than dch in power and data rate(Shared channel)
• CELL_PCH
• No data at all just signalling.
• URA_PCH
• Same as Cell PCH but at slightly larger area

Fast dormancy that has also been taken into account in Rel8 by 3GPP for analysis and standardization. Smart phones sends Signalling Connection Release Indication(SCRI) to the network as soon as the data transfer is finished or UE becomes dormant. If network support this feature then in response to this message network can make various decisions like may think of putting UE in CELL_FACH or CELL_PCH or URA_PCH or in IDLE. (Will have a separate blog on Why support FACH and PCH). It has also been seen in some proprietary implementations of mobile manufacturer that the  UE may move to IDLE after sending SCRI or may send SCRI in CELL_PCH or FACH.

If network does not support fast dormancy then UE will keep on sending this message to network (which will be ignored) which also cause unnecessary power consumption in UE. Currently some Samsung phones have the feature to switch off the fast dormancy feature.

To disable fast dormancy in Samsung Galaxy S.ace dial *#9900#.

Study and  enhancements are going on in  Fast dormancy feature and in CELL_FACH/CELL_PCH state of RRC to lower the impact on Battery life of the UEs, Network performance and Network congestion.

HSDPA Code allocation

Let talk about the basic resource allocation strategy in HSDPA.
HSDPA resource allocation strategy is based on the sharing of the available resources like code, power etc. In this particular post we will talk about the channelization code resources and its uses  in HSDPA Network.

Under one channelization code tree there could be maximum 15 codes (of SF16) that can be used for HSDPA users (HS-DSCH transmission). A service provider can opt for desired implementation if the same carrier is being shared between HSDPA and non HDSPA users.

An example scenario has been taken for illustration where 12 codes has been reserved for HS-DSCH transmission.

Code reservation can be dynamic or static but code allocation is always dynamic.
In static code reservation strategy, desired number of codes are reserved at the time of cell setup and can be  allocated or deallocated to the desired UEs as per the data requirement.
 But In dynamic code reservation strategy, codes are reserved dynamically as per HSDPA users availability in the cell which is good implementation from resource uses point of view.

Now will will take a look how channelization codes are reserved and allocated by Network.

Channelization Code Tree:

in the above scenario codes shown in red are reserved for HS-DSCH transmission and rest 4 green codes are left free for common/control or Non-HSDPA users.

• Once reserved these code cannot be allocated to non HSDPA users. 
• One or all twelve codes can be allocated to a UE being served in a TTI
•  Reserved twelve codes can be used to serve more than 12 UEs by time and code multiplexing but in different TTI.
•  Codes are allocated in a particular order. Pls refer to 3GPP 25.213  for Code group formation.

Code Allocation:

Node-B manages these resources autonomously at per TTI basis (2ms TTI). Reserved codes are allocated and deallocated to the UE as per data rate requirements and CQI basis.

Code sharing strategies:

Time Multiplexing

•  Every TTI, available resources are allocated to different set of UE(s), according to priorities.
•  Sequential allocation.

Code multiplexing:

•  Multi-code transmission, (multiple codes in same TTI)
•  Assign available codes to multiple UEs in same TTI
•  Max number of simultaneous UEs served in same TTI, is equal to number of HS-SCCH.
•  There are 4 HS-SCCH channel allocated to a cell so only 4 UE could be served per TTI.

Note:

• UE can monitor up to 4 HS-SCCH. (UE need to find its allocated code, modulation information and other control information from its corresponding HS-SCCH)
• If one UE has to be served per TTI then only one HS-SCCH transmission is required.

Snapshot of code allocation:

It’s a snapshot of 5 TTI in which 12 codes resources has been used to serve 15 UEs.

• Vertical plane represents the code and Horizontal plane represents the TTI
•  Different colors are used to represents different UEs, so there are 15 UEs served.
•  Four or less UEs have been served in a TTI ( four HS-SCCH assumed)
•  Total 15 UEs have been served in 10 millisecond period using 12 codes.

Note:
     Code reservation is performed by Radio Resource Manager in RNC which basically manages all the radio related resources. Whenever a new HSDPA RAB is allocated or de-allocated the RRM checks if it can optimize the use of code ie whether it can reserve or release some codes from the pool and then if yes it reserves/releasea and notify NodeB about it. Now nodeB has updated set for dynamic allocation or de-allocation.