Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W16/00—Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
  • H04W16/24—Cell structures
  • H04W16/32—Hierarchical cell structures
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W52/00—Power management, e.g. Transmission Power Control [TPC] or power classes
  • H04W52/02—Power saving arrangements
  • H04W52/0203—Power saving arrangements in the radio access network or backbone network of wireless communication networks
  • H04W52/0206—Power saving arrangements in the radio access network or backbone network of wireless communication networks in access points, e.g. base stations
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/12—Arrangements for detecting or preventing errors in the information received by using return channel
  • H04L1/16—Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
  • H04L1/1607—Details of the supervisory signal
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L2001/0092—Error control systems characterised by the topology of the transmission link
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W84/00—Network topologies
  • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
  • H04W84/04—Large scale networks; Deep hierarchical networks
  • H04W84/042—Public Land Mobile systems, e.g. cellular systems
  • H04W84/045—Public Land Mobile systems, e.g. cellular systems using private Base Stations, e.g. femto Base Stations, home Node B
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
  • Y02D30/00—Reducing energy consumption in communication networks
  • Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks

Definitions

• the invention relates to femtocell (a.k.a. small cell); in particular, to a composite cell including a parent femtocell and child femtocells.
• femtocell a.k.a. small cell
• the invention can be easily applied to any type of cellular basestations.
• Femtocell a.k.a, small cell
• RNC Radio Network Controller
• Femtocell is characterized by its small size and low power. However, these good characteristics also cause some challenges in real life deployments. Two of those challenges are:
• a handset can be either in connected mode or in idle mode (here we include RRC states like CELL_PCH, URA_PCH, CELL_FACH as part of the idle mode, and CELL_DCH as connected mode as are typically treated in the 3GPP standard).
• RRC states like CELL_PCH, URA_PCH, CELL_FACH as part of the idle mode, and CELL_DCH as connected mode as are typically treated in the 3GPP standard.
• a handset in idle mode is required to periodically perform cell reselection to find a better cell to camp on.
• a handset When a handset registers itself with a particular basestation, it is usually treated as "camp on" to that basestation (or a.k.a. cell). The criteria to kick off the cell reselection are discussed in 3GPP standard. In a nutshell, if a handset sees the signal strength it receives from the registered cell (the cell it is currently camped on) degrades to a certain level and other conditions described in 3GPP standard, it shall follow the rules specified in 3GPP standard to perform cell-reselection. Cell reselection involves exhaustive search over cells in multiple RF frequencies and usually consumes quite some power. If conducted frequently, it drains the battery power quickly.
• a handover is triggered when a handset is in connected mode and the network starts to see the signal quality from the handset degrade to a certain level.
• the network asks the handset to conduct intra-frequency, inter-frequency or even inter-RAT measurement and reports back to the network for handover decision.
• the network selects a handover target, it informs the handset and sends handover message to the target basestation with all the information of the handset that is being handed over. This process is time and resource consuming. In a larger cell when handset handover does not happen often, this overhead is not an issue. However, with femtocell, if pre-cautions are not taken, the system can be overloaded by these requests.
• these secondary scrambling codes are used in a beamformed or sectorized cell where separate sectors do not have to be orthogonal to each other.
• the secondary downlink scrambling codes can be applied with the exception of those common channels that need to be heard in the whole cell and/or prior to the initial registration. Only one scrambling code should be generally used per cell or sector to maintain the orthogonality between different downlink code channels. With adaptive antennas the beams provide additional spatial isolation and the orthogonality between different code channels is less important. However, in all cases the best strategy is still to keep as many users as possible under a single scrambling code to minimize downlink interference. If a secondary scrambling code needs to be introduced in the cell, then only those users not fitting under the primary scrambling code should use the secondary code. The biggest loss in orthogonality occurs when the users are shared evenly between two different scrambling codes. Please refer to FIG. 1.
• FIG. 1 illustrates how different secondary scrambling codes are applied to different beams.
• the big circle represents the signals that are scrambled by the primary scrambling code.
• common channels like the CPICH (common pilots) and CCPCH (common control) channels are scrambled with primary scrambling code and are not beamformed.
• CPICH common pilots
• CCPCH common control
• One thing to note is that, it is very difficult to design a lot of beams while still keeps a good spatial distance between the beams.
• One way to ensure less overlapping between the beams is to reduce the beam numbers as shown in FIG. 2.
• the above beam patterns are not common as most basestation adopt sectorized design which practically puts 3 basestations together in one location, each overlooking 120 degree of the circumference. Beam forming while can effectively increase the downlink capacity by allowing more code trees within the same sector; it does not increase the uplink capacity/throughput as all handsets are still processed together by the same baseband processor.
• the design of the composite cell required each cell (parent or child) transmit their P-CPICH and P-CCPCH using its own primary scrambling code which cause interference inside the composite cell.
• the invention provides a composite cell to solve the above-mentioned problems occurred in the prior arts.
• a scope of the invention is to use femtocell to achieve space division multiple access similar to beamforming but in a much clever way that can truly increase both the downlink and uplink capacity/throughput and eliminate the deployment problem mentioned above (too many cell-selection and handset not able to camp on the femtocell when larger cell signal is too strong).
• Another scope of the invention is to achieve that all femtocells in the composite cell shall transmit using the same primary scrambling code used by the parent femtocell, and the information carried on the P-CCPCH (a.k.a, the BCH channel) shall all be the same.
• a mobile a.k.a. UE
• it shall treat all signals from the femtocells in the composite cell as if it is a multi-path component being transmitted from the same source. This results in better performance as the total received signal power is constructively added instead of interfering with each other.
• Another scope of the invention is to provide a composite cell which can reduce co- channel interference, excessive cell reselection, excessive measurement report request, and downlink dead zone created by regular femtocell deployment. It is self-organizing and can facilitate data offload and enhance RACH/FACH performance. It also guarantees that the femtocell is deployed within the composite cell instead of unknown remote location.
• the composite cell is a co-channel deployment technology using the concepts of SFN, SDMA, SSC (Secondary Scrambling Code), DAS, ICIC, SON, DSPS (distributed signal processing system), IaCIC (Intra-cell interference coordination), network assist intra-cell handover, pilot flashing, UE (user equipment) location polling, asymmetric child femtocell coverage, and multi-tier SFN.
• the composite cell consists of a parent femtocell overlooking multiple child femtocells. It requires no change at the UE and in the existed 3GPP standard. There is no complicated neighboring cell information list for the femtocells in the composite cell.
• Another scope of the invention is to extend the composite cell ideal into work in 3 GPP LTE and LTE-A system. It is easy to form an LTE composite cell.
• the broadcast, pilot, and common channels of the child femtocells are converted to be the same as the parent femtocell.
• Parent femtocell and all child femtocells will transmit these channels in a synchronized fashion.
• the parent femtocell controls the formation of the composite cell and can reject any child femtocell from joining the composite cell.
• the parent femtocell and the child femtocells are connected either using a wired line such as Ethernet, fiber optic cable, etc, or through a wireless method such as WiFi or 3G/4G mesh network.
• the parent femtocell in the LTE composite cell is in charge of all scheduling decisions for the child femtocells.
• the LTE Composite Cell shall employ elCIC to avoid the interference between the control and data channels.
• the composite cell uses a co-channel deployment technique and includes a plurality of child femtocells and a parent femtocell.
• the parent femtocell is used for overlooking the plurality of child femtocells. All common channels on the plurality of child femtocells use the same PSC (primary scrambling code) as the parent femtocell.
• the composite cell uses concepts of SFN, SDMA, SSC (secondary scrambling code), DAS, ICIC, SON, DSPS (distributed signal processing system), IaCIC (Intra-cell interference coordination), network assist intra-cell handover, pilot flashing, UE (user equipment) location polling, asymmetric child femtocell coverage, and multi-tier SFN.
• the composite cell is self-organizing and facilitates data offload and enhance RACH/FACH performance.
• the composite cell reduces co-channel interference, excessive cell reselection, excessive measurement report request, and downlink dead zone created by regular femtocell deployment.
• FIG. 1 illustrates how different secondary scrambling codes are applied to different beams.
• FIG. 2 shows reducing the beam numbers to ensure less overlapping between the beams.
• FIG. 3 shows an embodiment of the composite cell structure.
• FIG. 4 shows an embodiment of the UE communicating with the parent femtocell and the child femtocells in the composite cell.
• FIG. 5 shows an embodiment of the UE communicating with the child femtocells in the composite cell.
• FIG. 6 shows an embodiment of the composite cell structure.
• FIG. 7 shows an embodiment of the handover in the composite cell.
• FIG. 8 shows an embodiment of the RACH random access procedure.
• FIG. 9 shows an embodiment of the FACH IaCIC random access procedure.
• FIG. 3 shows an embodiment of the composite cell structure. As shown in FIG. 3, the triangle is the "parent femtocell", while the stars means the "child femtocells" covered under the "parent femtocell”.
• a "composite cell” as it consists of multiple regular basestations (a.k.a cells) under the same parent femtocell. Note that each of the femtocells can be individually beamformed to support areas that requires better coverage under the "parent femtocell”.
• the composite cell should be setup as follows:
• Each of the "child femtocells" (stars) is assigned a unique primary scrambling code and is transmitting a PCPICH, PCCPCH, S-CCPCH, and broadcasting its PRACH preamble signatures accordingly using the unique primary scrambling code.
• the "parent femtocell” (triangle) is assigned a unique primary scrambling code as well and transmitted PCPICH and PCCPCH, S-CCPCH, and broadcasting its PRACH preamble signatures accordingly.
• Each of the femtocells is assigned a unique secondary scrambling code from the secondary scrambling code set of the parent femtocell.
• the femtocells inside the composite cell shall transmit the S-CPICH (secondary common pilots) scrambled by the secondary scrambling code assigned by the parent femtocell.
• S-CPICH secondary common pilots
• All femtocells shall publicize themselves as “reserved cell” as defined in 3GPP or use any other methods that shall prohibit any handset to camp on them.
• any handset that comes into the range of the composite cell if it is in idle mode, it can only camp on the parent femtocell, not the femtocells in the composite cell. If it is in connected mode, it can only be handover to the parent femtocell initially.
• the handset is listening to the PCCPCH of the parent femtocell, and it communicates with the parent femtocell through the RACH/FACH channels. It will not be able to communicate with the femtocells in the composite cell directly in any way. It can use the RACH/FACH to initiate dedicated connections such as DPCH or high-speed data connections HSPA.
• any CELL_DCH mode connections (such as DPCH or HSPA) with any handset served by the composite cell must be initiated with the parent femtocell.
• the parent femtocell shall perform estimation on the handset's moving speed based on the perceived channel variation or the perceived Doppler effect of the signal. If the result of the estimation of the handset moving speed is lower than a threshold (equivalent to 5Km/hr), the parent femtocell shall continue with the following steps to "offload" the handset's connection to a certain femtocell in the composite cell.
• the parent femtocell shall request an intra-frequency measurement report from the handset.
• the handset upon request shall perform the intra-frequency measurement of the same frequency band and report the measurement back to the parent femtocell.
• the parent femtocell inspects the report from the UE and searches for any cell ID corresponding to the primary scrambling code of the femtocells inside the composite cell and form a set of detected femtocells in the composite cell.
• the parent femtocell selects the strongest detected femtocell in the composite cell from the handset measurement report and set the strongest detected femtocell as the "target cell".
• the parent femtocell shall issue a handover request to the target cell and get ready to transfer handset related information to the target cell.
• the information may include but not limited to the following information regarding the handset being transferred to the target cell: UL scrambling code, HSDPA/HSUPA buffer contents, unfinished HARQ transmissions/receptions, and UL and DL DPCH OVSF code numbers.
• the parent femtocell shall issue an RRC message to the handset to request the handset to switch to the secondary scrambling code that is assigned to the target cell during the composite cell setup phase. This switch should happen after the handset information transfer from the parent femtocell to the target cell has been finished.
• the parent femtocell shall perform the following power control steps of:
• the parent femtocell shall inform the handset by RRC signaling to get its downlink reference signal from the S-CPICH (secondary common pilot channel) both for HSDPA and DL DPCH.
• S-CPICH secondary common pilot channel
• the parent femtocell can also command the handset having a DL-DPCH connection to use dedicated pilot on the DPCCH as the reference signal. After switching to the secondary scrambling code, the handset is effectively "served" by the target cell while still “camped on” the parent femtocell.
• the measurement command If the measurement command is commanded by the target cell, it shall select the next target cell as the femtocell having the strongest signal in the report and inform the parent femtocell to switch the secondary scrambling code of the handset to the code assigned to the new target cell.
• Current target cell shall calculate the power offset information between the current target cell and the new target cell and adjust the OLPC SIR target as necessary (i.e. if DL DPCH connection exists).
• Current target or the parent femtocell upon receiving the handset handover request from the current target cell shall command the handset to switch to the secondary scrambling code of the new target cell;
• parent femtocell shall inform the current target cell a service access point of the new target cell, the current target cell shall prepare handset related information and send the handset related information to the new target cell, and parent femtocell shall command the handset to switch to the secondary scrambling code of the new target cell after the information of the handset is sent from the current target cell to the new target cell. Timing synchronization
• Physical channel timing synchronization between femtocells and parent femtocell in the same composite cell is important to ensure that the handset is agnostic to the switching between the primary and secondary scrambling codes. To the handset, it is as if the handset is still camped on and served by the same composite cell when switching between the primary and secondary scrambling codes while for the cells inside the composite cell, the handset is almost treated as being "handovered" from the parent femtocell to the target cell when the handset switches from the primary to the secondary scrambling codes. Therefore, it is of upmost importance that the physical channel timing is synchronized between the child femtocells and the parent femtocell inside the same composite cell.
• each femtocell shall have a network monitor that listens to the broadcast signal of the parent femtocell such as CPICH or PCCPCH and use a time/frequency locking mechanism such as phase lock loop to lock on to the signal timing of the parent femtocell;
• the parent femtocell shall have a complete Uu stack to communicate with the handsets and a complete Iu-h, or Iu, or IMS-subsystem stack to communicate with the core network nodes such as the femtocell gateway, MSC, SGSN/GGSN, etc.
• Each femtocell in the composite cell shall have a connection with the parent femtocell in order to transfer the handset information back and forth from the parent femtocell.
• One example of such interface would be a combination of the Iu-b like interface and the Iu-r.
• One other candidate would be the Iu-rh interface. Communications between the femtocells inside the composite cell should be possible in order to facilitate handset transition from one femtocell to another directly.
• connections should have similar protocol stack as the Iu-r or Iu-rh.
• the femtocell shall start the procedure to tear down the DPCH/HSPA connections with the core-network and the handset. After the connection is terminated, the handset will automatically switch back to the idle mode and start listening to the paging channel and broadcast channel on the primary scrambling code which belongs to the parent femtocell. No communications are provisioned between the target cell and the parent femtocell at the moment.
• all femtocells in the composite cell shall be transmitted using the same primary scrambling code used by the parent femtocell, and the information carried on the P-CCPCH (a.k.a, the BCH channel) shall all be the same.
• the secondary synchronization channels (S-SCH) are the same from all femtocells in the composite cell.
• a mobile a.k.a. UE
• it shall treat all signals from the femtocells in the composite cell as if it is a multi-path component being transmitted from the same source. This results in better performance as the total received signal power is constructively added instead of interfering with each other.
• the random access signatures shall be monitored by RACH receivers at the femtocells.
• This is different from the prior art, where all random access preambles and messages are received and processed only by the parent femtocell.
• the problem with prior art is that the parent femtocell is usually further away from the UE than the nearby femtocell.
• the UE needs to send RACH preamble and the message with higher power which cause interference to the child femtocell in the composite cell.
• the invention allow child femtocells to also listen to the RACH transmissions from the UE and respond to the UE accordingly which allow the UE's RACH transmission to be detected and processed at much lower transmitting power to reduce overall noise rise in the composite cell.
• the potential problem with all femtocells in the composite cell to detect and receive RACH preamble and messages is when multiple femtocells detected the preamble and try to send either an ACK (preamble received and cell is ready for message) or a NACK (preamble received but cell is not able to process the message) through their AICH channels. Since the UE treats the different signals from different cells as the same signal but different multipaths, it will try to detect and combine these multipaths without knowing that the signal contents are different. In the end, the differences in the information (ACK or NACK) will be combined together to form a final decision. The stronger signal from the nearby femtocell will eventually surface through the soft combining process.
• the signal from the nearby femtocell will "drown out” the signal from the far basestation. If ACK is decoded, the UE will start transmitting PRACH message, which will be received by the basestation that ACKed the UE and be ignored by the basestation that sent a NACK. If NACK is decoded, the UE will back off and retransmit later.
• the impact to inconsistent ACK/NACK over the AICH from multiple femtocells should be minimal. As if a nearby femtocell tells the UE not to transmit the message, while a far femtocell tells the UE to transmit, it is better the UE does not transmit the message as it will cause interference to the nearby femtocell.
• FIG. 4 shows an embodiment of the UE communicating with the parent femtocell and the child femtocells in the composite cell.
• the UE When the UE first camps on the composite cell, it receives the broadcast information on PCCPCH which is broadcast by all femtocells in the composite cell. To the UE, it is registered to the composite cell. To the composite cell, this UE is also camped on the composite cell as a whole. When the UE is in the IDLE mode, there is no physical connection. The network will page the UE by sending a paging signal from all the femtocells in the composite cell.
• the composite cell shall share the same cell id, PLMN id, and Routing area code, Location area code, etc. And to the UE, the composite cell is like a single femtocell to it.
• each femtocell shall either send ACK or NACK over the AICH according to whether there is another RACH procedure ongoing with the femtocell detected the RACH preamble.
• these AICHs shall be treated as multipath component coming from the same source. Even if the information may not be the same, the UE can still decode the one having stronger signal.
• paging is done though PICH then followed by the S-CCPCH.
• both random access procedure and paging are only allowed with the parent femtocell but not the child femtocells.
• paging is allowed through the child femtocells. There are two possibilities as follows.
• Paging information is sent to all the femtocells in the composite cell and broadcast at the same time.
• the UE treats the paging from different femtocells as multipath from the same source. This can be used with the UE that is in CELL_PCH or URA_PCH RRC states.
• Another possibility is to send paging only to the femtocells that detected the RACH preambles if the paging is in response to a RACH message on the uplink. This can be used with UE that is in CELL_FACH mode and is continuously monitoring the FACH and RACH.
• Paging and FACH operations across different femtocells must be coordinated since S-CCPCH can only scrambled by the primary scrambling code and if the information contents are different from different femtocells, it will cause significant interference to each other. Coordination can be done according to space division methods if the child femtocells are far from each other and cannot hear each other's signals. This is sometimes called the hidden cell (node) in a networking jargon. So, one child femtocell is hidden from the other. If this is the case, Paging and FACH from femtocells that are "hidden" from each other can be different.
• the parent femtocell is responsible for grouping child femtocells into mutually exclusive "hidden” groups. Then, each group can freely broadcast S-CCPCH with independent information content. This is similar to the paging area concept but used in the composite only. Optimization of the "hidden” groups is not detailed here.
• the UE Since all femtocells are transmitting the same broadcast signal and the same synchronization channel and the same pilot pattern, the UE will think it is connecting to the composite cell as a whole. When initiating a RACH procedure, the UE will think it is connecting to the composite cell.
• the UE Before the UE establishes a dedicated connection (DPCH) with any femtocell in the composite cell, it needs to go through a random access procedure. It is most likely the same child femtocell communicating with the UE in the CELL_FACH, CELL_PCH or the IDLE mode will be the serving cell for the dedicated channel and HSPA channels in CELL_DCH mode.
• the serving femtocell After receiving higher layer exchange information over RACH/FACH, the serving femtocell shall direct the UE to start DL DPCH reception over a selected secondary scrambling code. On the uplink direction, only the serving femtocell will try to descrambling the UE having CELL_DCH connection with it.
• the desired serving femtocell is not the femtocell communicating with the UE during the CELL_FACH mode or during the random access handshaking phase, then a "virtual" handover between the femtocells must take place so the backhaul information will be redirected to the dedicated mode serving femtocell.
• This "virtual" handover gets its name because albeit it dictates a complete handover operation at the backhaul, the UE is NOT aware of this change and still think itself as communicating with the composite cell.
• the UE When the UE is in the IDLE mode, it performs periodic measurements to ensure the signal strength from the femtocell it is camped on is above certain quality. Once the quality degrades, it performs cell reselection procedure in which the UE conducts measurements to search for femtocell with stronger signal and, if it is found, the UE camps on to another stronger femtocell.
• This IDLE operation will happen less when the UE is in the composite cell, as when the UE moves within the composite cell, all PCPICH and PCCPCH are transmitted using the same scrambling code which is a signature of the composite cell. The UE received signal for the same scrambling could come from any femtocell in the composite cell.
• the UE When the UE is in the connected mode, it has an active link with one of the femtocell within the composite cell. Assuming this is a child femtocell, when the UE is moving away from the current serving child femtocell, its signal quality will degrade as the channel is connected using the secondary scrambling code which is only transmitted by the serving femtocell. When the UE senses its signal degrades, likewise, its uplink connection to the serving femtocell will weaken.
• the serving femtocell will either ask the parent femtocell (if the centralized control is implemented) or send directly to the femtocells that are neighboring to the serving femtocell to kick of a multi-cell measurement of the UE' s uplink signal to see if any of the femtocells asked to do the search can receive a better signal quality from the UE.
• the multicell search operation can be done by turning on the neighboring femtocell' s power control preamble detection to detect the UL-DPCCH of the UE under search.
• the target UE's UL scrambling code shall be send to these "neighboring" femtocells involved in the search prior to the PCP detection and SIR measurement. Once detected, a SIR measurement report shall be generated for the UE by the femtocell based on the UL-DPCCH. If the search results in a stronger signal from femtocell other than the serving femtocell. The search result can either be passed back to the parent femtocell (for the centralized control) or back to the serving femtocell (for the distributed control) for further processing.
• the serving femtocell or the parent femtocell shall decide whether to do a "virtual" handover of the UE to the femtocell with better signal reception.
• the "virtual" handover involves a complete handover operation between the femtocells involved, and a new secondary scrambling code assignment to the UE in order to continue the connection with the femtocells being handover to.
• FIG. 5 shows an embodiment of the UE communicating with the child femtocells in the composite cell.
• Timing synchronization needs to be accurate to the chip level in order for the transmission to be treated as "single network" or as from the same transmitter. Timing synchronization can be achieved over-the-air. It means that each child femtocell shall "lock" their clocks to the timing of the parent femtocell. Once frequency and time are synchronization, frame synchronization can be done.
• a composite cell is provided.
• the composite cell can reduce co- channel interference, excessive cell reselection, excessive measurement report request, and downlink dead zone created by regular femtocell deployment. It is self-organizing and can facilitate data offload and enhance RACH/FACH performance. It also guarantees that the femtocell is deployed within the composite cell instead of unknown remote location.
• the composite cell is a co-channel deployment technology using the concepts of SFN, SDMA, SSC (Secondary Scrambling Code), DAS, ICIC, SON, DSPS (distributed signal processing system), IaCIC (Intra-cell interference coordination), network assist intra-cell handover, pilot flashing, UE (user equipment) location polling, asymmetric child femtocell coverage, and multi-tier SFN.
• the composite cell consists of a parent femtocell overlooking multiple child femtocells. It requires no change at the UE and in the existing 3GPP standard. There is no complicated neighboring cell information list for the femtocells in the composite cell. All common channels on the child femtocells are transmitting using the same PSC (primary scrambling code) as the parent femtocell.
• FIG. 6 shows an embodiment of the composite cell structure.
• Composite cell is a technology adopting many advanced concepts. It reduces interference coming from femtocells in a co-channel deployment scenario. It reduces excessive cell reselection and measurement report caused by the UE moving across the femtocells in the field significantly. It allows parent femtocell to command the UEs to establish voice and data connections with any children femtocell for data offloading. It eases the challenges created by the femtocell coming and leaving the network. It enhances common channel performance through interference coordination. It enhances dedicated channel capacity and data throughput. It is self- organizing. It reduces downlink dead zone created by the regular femtocell deployment.
• the composite cell has minimal-to-no impact to the existed UTRAN settings.
• the parent femtocell handles all activities created by the children femtocells. It can distribute the management of the femtocells. It also greatly simplifies the efforts needed to manage the femtocells. Femtocells are much more manageable in hundreds, not in thousands. No need to change macro layer settings. No need to adjust intra-frequency measurement reporting thresholds to make camping on femtocell layer easier when HCS is used. No need for Macro layer to dynamically change CELL INFO LIST (neighboring cell list) to facility hand in to femtocell layer. Operators don't need to worry about femtocell randomly turned on/off.
• the composite cell is for co-channel deployment. Co-channel deployment is efficient since the UE performs less inter frequency handover. It is prone to create interference to the existed network. Need to carefully plan or need technologies like composite cell manage interference.
• the composite cell is for inter-frequency deployment. Femtocell is often treated as a new layer in the HCS. Safer approach as it creates minimal disturbance to the existed network. Ii is a waste of highly valuable frequency resource if an independent femtocell layer is created. Ii involves complicated adjustment of the HCS settings.
• Unlimited number of the child femtocells can be added to the composite cell. For example, 15 SSC reused based on SDMA.
• SFN is applied to P- CCPCH and P-CPICH. Since BCH is homogeneous, >20 interfering FAP power can be converted into the constructive signal enhancement to the parent femtocell and the frequency of the femtocell reselection and measurement performed by the UE in the idle mode can be significantly reduced and the downlink "dead-zone" created by the femtocell to the nearby macrocell UE can be also reduced.
• IaCIC is used on S-CCPCH. It can enhance FACH and PCH performance.
• DSPS distributed signal process system
• Network assist intra-cell handover can significantly reduce the frequency of the connected mode UE measurement report. Its Mechanism is similar to the SDMA/beam forming which is a well proven concept.
• the child femtocells use secondary scrambling codes for HSDPA and DPCH. No complicated neighboring cell information list for femtocells in the composite cell. Parent femtocell maintains all child femtocells neighboring information in a centralized fashion. Neighboring femtocell information list on the BCH is the same for all femtocells in the composite cell containing only other non child femtocell information. It can eliminate the need for child femtocells to maintain individual neighboring cell information list.
• the parent femtocell When accepting a new child femtocell, the parent femtocell shall organize the following information: the updated child femtocell neighboring information, the intra-cell paging area optimization, the new child femtocell power optimization, and the new child femtocell secondary scrambling code information. If the join request of a femtocell is rejected, it shall not be allowed to transmit inside the child femtocells.
• FIG. 7 shows an embodiment of the handover in the composite cell.
• Inter-cell handover Handover between the child femtocells and other larger cells is called "inter-cell handover".
• the UE hands out follows the usual procedure. During a hand in, the UE hands over to the parent femtocell first. The parent femtocell initiates a cell-wide hunt for the UE. The parent femtocell switches the UE to appropriate the child femtocell if found.
• Intra-cell handover Handover within the child femtocells is called "intra-cell handover".
• the UE sees strong PCCPCH & PCPICH throughout the child femtocells.
• Parent femtocell doesn't need measurement report from the UE to make HO decision.
• cell reselection won't be triggered inside the child femtocells.
• the UE does not know anything about "intra-cell handover”.
• the UE switches from SSC to PSC when moving from one child femtocell to another child femtocell.
• the UE switches from SSC to PSC when moving from one child femtocell to the parent femtocell.
• FIG. 8 shows an embodiment of the RACH random access procedure.
• FIG. 9 shows an embodiment of the FACH IaCIC random access procedure.
• the UE selects RACH preambles from received BCH. All child femtocells are configured to the received RACH. Once detected, both detected preamble signature and RACH messages are forwarded to the parent femtocell. The parent femtocell uses this information to construct the UE location information. Then, the parent femtocell forwards the information to the core network for further processing.
• the parent femtocell can do one of the folio wings: sending the FACH information to all femtocells detected the UE for S-CCPCH DL IaCIC; if the parent femtocell receives ACK-ed RACH, the parent femtocell can decide to broadcast S-CCPCH only from the parent femtocell based on the RACH message SIR reported.
• PICH transmission should be coordinated similar to FACH. All paging information is routed through the parent femtocell. The intra-cell paging area optimization is performed by the parent femtocell periodically and/or when a new child femtocell is accepted.
• Pilot flashing is a technique to reduce the intra-cell interference of the child femtocells caused by enabling the secondary scrambling code (SSC).
• the children femtocells with strong parent femtocell signal shall not transmit common channels, such as P-CCPCH, S- CCPCH, AICH, PICH, and P-CPICH, only when the parent femtocell signal is strong. It can reduce interference to the FUE served by the child femtocells.
• the child femtocells only transmit pilots and common channel when needed, for example, to implement IaCIC.
• Downlink PCPICH is turned on shortly before the P-CCPCH, S-CCPCH, AICH or PICH to allow the UE to conduct channel estimation.
• the amount of timing advance needed for the UE to perform channel estimation is ⁇ 1 radio frame (10 ms).
• the parent femtocell When the radio link configuration request is sent through RACH/FACH, the parent femtocell identifies the serving child femtocell is applicable. The parent femtocell is selected as the serving femtocell, if the parent femtocell also receives RACH and sends AICH on the DL. If the parent femtocell didn't sent AICH prior to the request and the parent femtocell receives RACH message(s) from multiple child femtocells.
• the UE shall select the child femtocell with the best RACH message SIR as the serving child femtocell for the UE.
• the UE establishes DL using the scrambling code assigned (PSC or SSC) and sends UL DPCH to complete the setup. It dedicates channel under ILPC. If the UL DPCH power is higher than the allowed UL power per the UE, the serving child femtocell shall instruct the UE to switch back to PSC.
• the parent femtocell is responsible for the folio wings:
• the inter-cell handover from other femtocell determines which child femtocell to be handover to.
• the child femtocell is responsible for the folio wings:
• Dilemma is created by the femtocell in an area with good macro signal.
• the femtocell is mainly deployed to increase the capacity/throughput and offload data traffic.
• the femtocell downlink creates dead zone to the nearby (within a couple of meters) macro UE. Problems for the UE in the idle mode:
• the parent femtocell initiates measurement request to the neighboring child femtocell of the serving child femtocells;
• the parent femtocell makes HO decision based on the reported SIR and the UE can be handout to the parent femtocell or the child femtocells
• scenario 1 (a) the parent femtocell monitors link quality
• the parent femtocell initiates the cell-wide UE hunt and sends measurement request to the UE looking for other cells;
• the parent femtocell provides a UE search list for each child femtocells
• the parent femtocell makes HO decision whether to handover the UE to the child femtocells.
• the child femtocell should have the DL coverage "smaller" than the UL coverage. It means that the child femtocell should be able to detect the UE before the UE detects the child femtocell.
• the child femtocell declares that the UE is detected when the UL pilot SIR from the UE is over certain threshold.
• the UE detects the child femtocell when it is able to pick up PCPICH from that child femtocell.
• Asymmetric cell coverage is implemented using the UE UL power capping and the child femtocell DL power limit. When the UE is served by the parent femtocell, it should be handover to a child femtocell as soon as it is detected.
• the UE location polling is a method for the parent femtocell to gain location proximity of a UE. It can be used to detect the location of the UE in the idle mode.
• the composite cell broadcasts the paging information with unknown cause to a UE known to be within the femtocell range.
• the UE receives the paging and sends a UL RACH in response.
• the UE location is known if any of the child femtocells picks up the UL RACH from the intended UE.
• Benefits of the UE location polling are that it does not require UE to perform any RF measurement, nor does it rely on AGPS to calculate the UE location. In addition, it is fast and does not add work load to the UE, so that the battery consumption can be reduced. Multi-tier SFN
• the SFN is implemented to convert the child femtocell common channels such as PCCPCH, PCPICH into the same as the parent femtocell.
• adding the child femtocell into the SFN does not help improve the PCCPH/PCPICH performance. Instead, it could cause interference to data users on the secondary scrambling code of that UE. It is best to turn off or reduce the child broadcast and pilot channel power when the parent signal is strong.
• the child femtocell common/broadcast/pilot channels power should depend on the received parent femtocell common signal strength. It can be divided into different tiers, the child femtocell in different tier allocate different % of its total power to the broadcast/common/pilot channels.
• Multi-tier SFN is implemented as follows: the child femtocell during SON shall measure RSCP of the parent femtocell, based on the received RSCP power, it shall receive a guideline from either parent femtocell or HMS to inform what % of its total power should be used on PCCPCH/PCPICH and other DL signaling channels.
• the parent femtocell When the RRC connection request/setup is sent through RACH/FACH, the parent femtocell identifies the serving femtocell. The parent femtocell is selected as the serving femtocell, if the parent femtocell also receives RACH and sends AICH on the DL. If the parent femtocell didn't send AICH prior to the request and the parent femtocell received RACH message(s) from multiple child femtocells, it shall select the child femtocell with the best RACH message SIR as the serving child femtocell for the UE. The UE establishes DL using the scrambling code assigned (PSC or SSC).
• PSC scrambling code assigned
• the UE sends UL DPCH to the serving femtocell to complete the setup and dedicates channel under ILPC. If the UL DPCH power is higher than the allowed UL power per the UE, the serving child femtocell will inform the parent femtocell to conduct the intra-cell handover procedure. The parent femtocell informs the serving child femtocell to instruct the UE to switch back to PSC or another SSC. Therefore, the intra-cell handover occurs.
• LTE requires a lot of interference coordination between eNodeBs to reduce co-channel interference.
• Femtocell creates large amount of X2 signaling messages between eNodeBs.
• the signaling passing between eNodeBs includes HI (high interference indicator) and OLI (overload indicator). These indicators are sent for each resource block.
• Existed methods to reduce interference include ICIC and elCIC.
• ICIC is optional in Rel8 LTE and elCIC is part of heterogeneous network (HetNet) in RellO LTE- A.
• the parent femtocell and child femtocells are connected either using wired line such as Ethernet, fiber optic cable, etc, or through wireless method such as WiFi or 3G/4G mesh network;
• the parent femtocell and the child femtocells are connected either using wired line such as Ethernet, fiber optic cables, etc, or through wireless method such as WiFi or 3G/4G mesh network;
• Benefits of doing so are similar to WCDMA to avoid interference from the common channel of the child femtocells and reduce handover signaling.
• Parent femtocell in LTE composite cell is in charge of all scheduling decision for the child femtocells. A couple of different implementations are used for user scheduling.
• the LTE composite cell shall employ elCIC to avoid interference between control and data channels.
• ICIC is a method to decrease interference between neighboring macro basestations. It can lower the power of a part of the subchannels in the frequency domain. These subchannels can only be received close to eNodeB. These subchannels do not interfere with the same subchannels used in neighboring femtocells. Data can be sent faster on those subchannels to mobile devices close to the femtocell. It is used between macro eNodeB s. The RF planning is well managed and not suitable between eNodeB s and femtocells. elCIC for LTE-A
• HetNet HetNet
• macro cells are complemented with pico cells inside their coverage area (hotspots in shopping centers, at airports, etc.) macro cells emit long range high power signals, the pico cells only emit a low power signal over short distances to mitigate interference between a macro cell and several pico cells in its coverage area.
• the elCIC coordinates the blanking of subframes in the time domain in the macro cell. No interference in those subframes from the macro cell so data transmissions can be much faster.
• overall system capacity is increased as each pico cell can use the empty subframes without interference from the other pico cells.
• Macro cell capacity is diminished as it can't use all subframes. Methods have to be put in place to quickly increase or decrease the number of subframes assigned for exclusive use of in pico areas when traffic patterns change.
• the low power nodes such as RRUs/RRHs, pico eNodeB s, home eNodeBs, and relay nodes, are deployed in the macro coverage cell.
• LPNs low power nodes
• CRE Cell range expansion
• Picocells share more network load, and their coverage is extended.
• the femtocells belong to a closed subscriber group (CSG) where the access and services are restricted to the authorized subscribers.
• CSG closed subscriber group
• the macrocell subscribers enter the areas covered by the femtocells, the femtocells strongly interfere with them.
• Method 1 the parent femtocell is in charge of all the UE scheduling inside the composite cell.
• the child femtocell scheduling command is passed down from parent femtocell every sub-frame. Most effective interference coordination as the parent eNodeB controls all transmissions happen inside the composite cell. Backhaul may not be able to handle the heavy traffic. In this case, the child eNodeB shall perform now scheduling function and all the UE reported channel state information and link quality reports are passed to the parent femtocell immediately to enable scheduling.
• Method 2 the parent femtocell is in charge of assigning frequency bandwidth to all child femtocells.
• the parent femtocell decides which the UE is served by which the child eNodeB or by itself.
• the parent femtocell assigns available bandwidth to different child eNdoeBs based on the UE QoS provisioning and each child eNodeB 's loading.
• all child femtocells report HII (High Interference Indication) and 01 (Overload indication) to the parent eNodeB to enable the interference coordination.
• the parent femtocell can choose to implement elCIC like scheduling scheme which will blank out a few sub frames for a given group of the child eNodeBs.
• the parent femtocell can also implement COMP (Coordinated Multipoint) to signal the multiple child eNodeBs to transmit/receive the same data to/from the same UE at the same time.
• COMP Coordinatd Multipoint

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