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