Automatic Neighbor Relation (ANR) in LTE

Manually adding neighbor cells in network is indeed a very hectic process in GSM & WCDMA Network. While  networks are becoming more and more complex, it is required to find an automatic and a more optimized way of adding neighbor cells.

ANR comes under the umbrella of Self Organizing Networks ( SON) features. ANR relies on UE to detect unknown cells and report them to eNB. There are two major types:

i) UE based ANR
ii) ANR with OAM Support

UE based ANR

·                     No OAM support is required.

·                     UE detects PCI of unknown cell when it needs to do measurement (as configured by network)

·                     In case of inter-frequency or inter-RAT measurements, eNB needs to configure measurement gaps/or DRX so UE can detect PCI to different frequencies as well.

·                     UE reports the unknown PCI to eNB via RRC-Reconfiguration message.

·                     eNB request UE to report Eutran Cell Global ID (ECGI).

·                     UE reports ECGI by reading BCCH channel.

·                     eNB retrieves the IP address from MME to further setup the x2 interface. 

ANR with OAM Support

·                     OAM support is required

·                     Every new eNB registers to OAM and download the table with information of PCI/ECGI/IP related to neighbors

·                      Neighbors also update their own table with new eNB information

·                     Now like "UE based ANR", UE will detect unknown PCI and report it to the eNB

·                     eNB doesn't request for ECGI and does not need support from MME

·                     eNB setups x2 interface with the help of mapping table created in second step above

SC-FDMA

The LTE uplink transmission scheme for FDD and TDD mode is based on SC-FDMA (Single Carrier Frequency Division Multiple Access).

This is to compensate for a drawback with normal OFDM, which has a very high Peak to Average Power Ratio (PAPR). High PAPR requires expensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal and also drains the battery faster.

SC-FDMA solves this problem by grouping together the resource blocks in such a way that reduces the need for linearity, and so power consumption, in the power amplifier. A low PAPR also improves coverage and the cell-edge performance.

Still, SC-FDMA signal processing has some similarities with OFDMA signal processing, so parameterization of downlink and uplink can be harmonized.

OFDMA

OFDMA : Orthogonal Frequency division multiple access.

LTE uses OFDM for the downlink –that is, from the base station to the terminal. OFDM meets the LTE requirement for spectrum flexibility and enables cost-efficient solutions for very wide carriers with high peak rates. OFDM uses a large number of narrow sub-carriers for multi-carrier transmission.

The basic LTE downlink physical resource can be seen as a time-frequency grid. In the frequency domain, the spacing between the subcarriers, Δf, is 15kHz. In addition, the OFDM symbol duration time is 1/Δf + cyclic prefix. The cyclic prefix is used to maintain orthogonality between the sub-carriers even for a time-dispersive radio channel.

One resource element carries QPSK, 16QAM or 64QAM. With 64QAM, each resource element carries six bits.

The OFDM symbols are grouped into resource blocks. The resource blocks have a total size of 180kHz in the frequency domain and 0.5ms in the time domain. Each 1ms Transmission Time Interval (TTI) consists of two slots (Tslot).

In E-UTRA, downlink modulation schemes QPSK, 16QAM, and 64QAM are available

System Architecture Evolution(SAE)

System Architecture Evolution (aka SAE) is the core network architecture of 3GPP's future LTE wireless communication standard.

SAE is the evolution of the GPRS Core Network, with some differences.

The main principles and objectives of the LTE-SAE architecture include :

A common anchor point and gateway (GW) node for all access technologies

IP-based protocols on all interfaces;

Simplified network architecture

All IP network

All services are via Packet Switched domain

Support mobility between heterogeneous RATs, including legacy systems as GPRS, but also non-3GPP systems (say WiMAX)

Support for multiple, heterogeneous RATs, including legacy systems as GPRS, but also non-3GPP systems (say WiMAX)

Key Features of LTE

·         Multiple access scheme : Downlink: OFDMA  & Uplink: Single Carrier FDMA (SC-FDMA)

·         Adaptive modulation and coding

·         DL modulations: QPSK, 16QAM, and 64QAM

·         UL modulations: QPSK and 16QAM

·         Rel-6 Turbo code: Coding rate of 1/3, two 8-state constituent encoders, and a contention-free internal interleaver.

·         Bandwidth scalability for efficient operation in differently sized allocated spectrum bands

·         Possible support for operating as single frequency network (SFN) to support MBMS

·         Multiple Antenna (MIMO) technology for enhanced data rate and performance.

·         ARQ within RLC sublayer and Hybrid ARQ within MAC sublayer.

·         Power control and link adaptation

·         Implicit support for interference coordination

·         Support for both FDD and TDD

·         Channel dependent scheduling & link adaptation for enhanced performance.

·         Reduced radio-access-network nodes to reduce cost,protocol-related processing time & call set-up time

Paging Success Rate in 2G

The paging success rate measures the percentage of how many

paging attempts that have been answered, either as a result of the

first or the second repeated page.

PSR = Time of Paging Responses / Time of Paging

Possible reasons for poor Paging Performance could be:

 Paging congestion in MSC

 Paging congestion in BSC

 Paging congestion in Base Transceiver Station (BTS)

 Poor paging strategy

 Poor parameter setting

 Poor coverage

 High interference

LTE Throughput Calculation

Solution:

1 RB = 12 sub-carriers

1 Sub-carrier = 7 OFDM symbols in time domain = 7 Resource elements

Thus total OFDM symbols in 1 RB = 12 X 7

With 64 QAM 1 symbol = 6 bits

Thus total bits in 1 RB = 12 X 7 X 6 transmitted in 0.5 msec

With 20 MHZ BW, 100 RB are transmitted

Thus total bits in 100 RB = 12 X 7 X 6 X 100 transmitted in 0.5 msec

Thus in 1 msec, total bits transmitted = 12 X 7 X 6 X 2 X100

In 1 sec, total transmitted bits = 12 X 7 X 6 X 2 X 100 X 1000 = 100.8 Mbps

This is possible with Single antenna configuration

But with 4 x 4 MIMO

Maximum throughout = 4 X 100.8 = 403.2 Mbps

LTE – Resource Block

›       Resource Block

–        1 Slot (0.5ms) and 12 sub-carriers.

–        1 User is Scheduled every TTI (1ms), which means a minimum of 2 consecutive resource blocks in time at every scheduling instance.

2G Timer parameter

T3105 :Time between repetition of physical information to MS. Will be repeated NY1 times.

T3101 : Guard time for Immediate Assign. Started as 'IMM_ASS_CMD' is sent to BTS and stopped by reception of 'EST_IND' from BTS.The allocated channel will be released at timer expiry.

T3260 : The timer is started when AUTHENTICATION REQUEST is sent from MSC. The timer is stopped when AUTHENTICATION RESPONSE is received in MSC. The connection is released with a CLEAR COMMAND at timer expiry.

T3212 : Time between periodic update of MS. SET by the MML command RLSBC. The value is sent to the MS in system information. The MS will do periodic update, if it is residing in the same location area at a time interval exceeding T3212 . If the MS is roaming to a new location area, update will be done anyway and the time to next periodic update will start counting from zero again.

RLINKT (T100)  : Time before an MS disconnects a call due to failure in decoding Slow Associated Control Channel (SACCH) messages. The parameter is given as number of SACCH periods (480ms).

T3101 : Guard time for Immediate Assign. Started as 'IMM_ASS_CMD' is sent to BTS and stopped by reception of 'EST_IND' from BTS.The allocated channel will be released at timer expiry.

T3105 : Time between repetition of physical information to MS. Will be repeated NY1 times.

T3109  : Guard timer for channel release indication when clearing mobile station. Started by sending DEACT_SACCH to BTS and stopped by reception of 'REL_IND' from BTS. It is controlled by RLINKT.

T3111 : Delay for connection release. Started as REL_IND is received from BTS. When expired RF_CHAN_REL is sent to BTS.

RLINKUP : Time before a BSC disconnects a call due to failure in decoding UPLINK Slow Associated Control Channel (SACCH) messages. The parameter is given as number of SACCH periods (480ms).

T3126 : This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during an immediate assignment procedure or on receipt of an IMMEDIATE ASSIGNMENT REJECT message, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the immediate assignment procedure is aborted.

T203 : Timer that supervises the maximum time without frames being exchanged on a A-bis link (LAPD).

RACH in 2G

Random Access (RACH) Success :
Random Access Channel (RACH) is used by the MS on the “uplink” to request for allocation of an SDCCH. This request from the MS on the uplink could either be as a page response (or incoming call) or due to user trying to access the network to establish a call. Availability of SDCCH at the RBS will not have any impact on the Random Access Success.

The number of times an MS tries to access the network (repeated access in the event of no response from the BS in the form of immediate assignment or immediate assignment reject on AGCH) is decided by the BSS parameter MAXRET (maximum number of retransmissions) and the randomness in the time interval between each of these access request is defined by the parameter TX.

Random Access Success Rate = (CNROCNT)/ (CNROCNT+RAACCFA) * 100

RAACCFA: Failed Random Access
CNROCNT: All accepted Random Access

Root cause analysis of poor Random Access Success
  Fish Bone diagram for root cause analysis of poor Random-Access Success Rate

Reason for Poor RACH Failure.

Poor BSIC Plan
Poor BCCH plan
Poor Coverage / Spillage
Phantom RACH
ACCMINand CRO
Faulty Antenna / Cable.
MAXRET and TX
 

GSM

What is GSM?

GSM (Global System for Mobile communications) is an open, digital cellular technology used for transmitting mobile voice and data services.

What does GSM offer?

GSM supports voice calls and data transfer speeds of up to 9.6 kbps, together with the transmission of SMS (Short Message Service).

GSM operates in the 900MHz and 1800MHz bands in Europe and the 1900MHz and 850MHz bands in the US. GSM services are also transmitted via 850MHz spectrum in Australia, Canada and many Latin American countries. The use of harmonised spectrum across most of the globe, combined with GSM’s international roaming capability, allows travellers to access the same mobile services at home and abroad. GSM enables individuals to be reached via the same mobile number in up to 219 countries.

Terrestrial GSM networks now cover more than 90% of the world’s population. GSM satellite roaming has also extended service access to areas where terrestrial coverage is not available.

In India GSM operates in 900MHz and 1800MHz Band and 1900MHz band are reserve for Military purpose.

900MHz Bandwidth is 25MHz, Channel Bandwidth is 200KHz, & ARFCN is 125
1800MHz Bandwidth is 75MHz,  Channel Bandwidth is 200KHz, & ARFCN is 374.

TDMA Frame

TDMA (Time Division Multiple Access) refers to a digital RF link where multiple phones share a single carrier frequency by taking turns. Each phone gets the channel exclusively for a certain time slice, then gives it up while all the other phones take their turn. TDMA is also used sometimes to refer specifically to the standard covered by IS-136, which is a source of confusion because GSM also uses a TDMA air interface, as does IDEN, and neither of those systems are compatible with IS-136.

Rake receiver

Rake receiver is the digital section of a CDMA receiver which permits the phone (or cell) to separate out the relevant signal from all the other signals. The relevant signal will be encoded with a known Walsh Code and a known phase of the Short code, and the rake receiver can do this because the Walsh codes are orthogonal and the Short code is orthogonal to itself at different offsets. The rake receiver is capable of receiving multiple signal sources and adding them together using multiple fingers, each of which has the ability to use a separate phase of the short code and long code and a separate Walsh code if necessary. Different fingers might track multiple signals from the same cell (arriving at slightly different times due to multipath) or might track separate cells due to soft handoff.

Chips in CDMA

Chip in the context of CDMA is distinct from bit and refers to binary digits transmitted over the RF link. The chip rate in IS-95 is 1.2288 MHz (thus allowing adequate guard bands to permit the carriers to be spaced 1.25 MHz apart). Each bit is represented by many chips, and if a majority of the chips get through then the bit can be reconstructed properly. The number of chips representing each bit varies depending on the bit rate. When using an 8K Vocoder (such as EVRC) there are 128 chips for each bit. Chips as such don't contain data because both the sender and receiver know the spreading pattern used to create them from a bit, and as such are not directly subject to the laws of Information Theory. Though there are many phones simultaneously using a single frequency to transmit full chiprate, that means that the channel is not saturated unless the bitrate approaches the bandwidth of the carrier.

2G HO Algorithm

2G Handover Optimization

Classification by Reason

Timing advance (TA) Emergency HO

Triggering condition

_ The actual TA > TA HO Thrsh.

Object cell selection

_ The cell must be of the highest priority in the candidate cell sequence and

meet the following restrictions.

Restriction

_ The service cell cannot be the object cell.

_ HO is not allowed when TA Thresh. of the neighboring cell with the same

BTS is smaller than that of the service cell.

Bad Quality HO

Triggering condition

_ UL receiving quality >=UL receiving quality thrsh. of the service cell.

_ OR DL receiving quality >=DL receiving quality thrsh. of the service

cell.

Object cell selection

_ The cells must be of the highest priority in the candidate cell

sequence and meet the following restrictions.

Restriction

_ Handover to the neighboring cell with the highest priority. If there is no

neighboring cell, handover to the service cell, and the channel at

different TRX is preference.

_ Rx Level (n) > Rx Level (s) + Inter Cell HO Hysteresis + BQ HO Margin

Signal Level Rapid Drop HO

Triggering condition

_ Due to downlink signal level drop

_ Triggered upon detecting rapid level drop during MS busy mode

_ Object cell selection

_ The neighboring cell with the highest priority and whose priority is higher

than that of the service cell in the candidate cell group.

_ Restriction

_ The service cell cannot be the object cell.

Interference HO

Triggering condition

_ UL receiving quality>=Service cell UL receiving quality Thrsh. AND

UL receiving level>=Service cell UL receiving level Thrsh.

_ OR DL receiving quality>=Service cell DL receiving quality Thrsh.

AND DL receiving level>=Service cell DL receiving level Thrsh.

Object cell selection

_ The cells must be of the highest priority in the candidate cell

sequence and meet the following restrictions.

Restriction

_ The service cell that is not in the penalty time for intra-cell handover.

_ The neighboring cell with the receiving level higher than the inter

layer HO Thrsh.

Load HO

Triggering condition

_ The load HO switch of the service cell is enabled.

_ The system signaling flow is not larger than the Load HO system flow Thrsh.

_ The service cell traffic is larger than the Load HO Thrsh.

_ The DL receiving level is in the load HO zones.

Object cell selection

_ The service cell cannot be the object cell.

_ The traffic of the neighboring cell must be lower than its load HO receiving

thrsh.

_ The neighboring cell with the receiving level higher than the inter layer HO

Thrsh.

Restriction

_ It is not available with SDCCH.

_ Load HO just occur within the same BSC.

Edge HO

Triggering condition

_ The DL receiving level < Edge HO DL RX_LEV Thrsh.

_ OR The UL receiving level < Edge HO UL RX_LEV Thrsh.

_ Satisfying P/N rule.

Object cell selection

_ The service cell cannot be the object cell.

_ The neighboring cell with the highest priority and whose priority is

higher than that of the service cell.

Layer HO

Triggering condition

_ The layer of the object cell is lower than that of the service cell.

_ The DL level of the object cell is higher than the inter layer HO thrsh.

_ Satisfying P/N rule.

Object cell selection

_ The service cell cannot be the object cell.

_ The neighboring cell with the highest priority and whose priority is

higher than that of the service cell

PBGT HO         

Triggering condition

_ The layer and level of the object cell are the same as those of the

service cell.

_ The DL level must be the result of the following formula.

Object cell selection

_ The service cell cannot be the object cell.

_ The neighboring cell with the highest priority and whose priority is

higher than that of the service cell.

Restriction

_ It is not available with SDCCH.

 Fast-Moving HO

Triggering condition

_ In Fast Moving Watch Time, the mobile phone moves through P cells

of N.

_ The layer of N cells must be less than four (none Umbrella cell).

Object cell selection

_ The neighboring cell with the highest priority and meet the following

condition.

_ The layer of the object cell must be no less than four, that is, the

Umbrella cell.

_ The receiving level of the object cell >= the inter layer HO thrsh. +

inter layer HO hysterisis.

MIMO ( Multi Inpit Multi Output )

MIMO Systems can provide two types of gain:

Spatial Multiplexing Gain and transmit Diversity Gain

In Spatial Multiplexing Gain , maximum transmission rate  can be achieved by sending different  data streams at different antennas

Whereas  in Diversity Gain, maximum quality(QOS) can be achieved  by sending same data streams to different antennas.

There is a tradeoff between  these two gains as spatial diversity can be achieved under good radio conditions where as transmit diversity is done under poor radio conditions to achieve good quality.

System designs are based on trying to achieve either goal or a little of both

LTE ARCHITECTURE

Mobility Management Entity (MME)

–MME is a controller at each node on the LTE access network. At UE in idle state (idlemode), MME is responsible for tracking and paging procedure which includes retransmission therein.

–MME is responsible for selecting SGW (Serving SAE Gateway) which will be used during initial attach EU and the EU time to do intra-LTE handover.

–Used for bearer control, a different viewR99/4 whichis still controlled by the gateway

Serving SAE Gateway (SGW)-Set the path and forwards the data in the form of packets of each user-As an anchor / liaison between the UE and the eNBat the time of the inter handover-As a liaison link between the 3GPP LTE technology with the technology

(in this case the 2G and 3G)

Gateway Packet Data Network (PDN GW)

-Provides for the UE 's relationship to the network packet-Provide a link relationship between LTE technology with technology

non 3GPP (WiMAX) and 3GPP2 (CDMA 20001X and EVDO)

The MME Function

• NAS signalling;
• NAS signalling security;
• AS Security control;
• Inter CN node signalling for mobility between 3GPP access networks;
• Idle mode UE Reachability (including control and execution of paging retransmission);
• Tracking Area list management (for UE in idle and active mode);
• PDN GW and Serving GW selection;
• MME selection for handovers with MME change;
• SGSN selection for handovers to 2G or 3G 3GPP access networks;
• Roaming;
• Authentication;
• Bearer management functions including dedicated bearer establishment;
• Support for PWS (which includes ETWS and CMAS) message transmission.

RSRP and RSRQ

In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbor cells. In LTE network, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).

RSRP is a RSSI type of measurement. It measures the average received power over the resource elements that carry cell-specific reference signals within certain frequency bandwidth. RSRP is applicable in both RRC_idle and RRC_connected modes, while RSRQ is only applicable in RRC_connected mode. In the procedure of cell selection and cell reselection in idle mode, RSRP is used.

RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal. It is defined as (N*RSRP)/(E-UTRA Carrier RSSI), where N makes sure the nominator and denominator are measured over the same frequency bandwidth;

The carrier RSSI (Receive Strength Signal Indicator) measures the average total received power observed only in OFDM symbols containing reference symbols for antenna port 0 (i.e., OFDM symbol 0 & 4 in a slot) in the measurement bandwidth over N resource blocks. The total received power of the carrier RSSI includes the power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc.

The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision. In the procedure of handover, the LTE specification provides the flexibility of using RSRP, RSRQ, or both.