Long Code in CDMA

Long code in CDMA is a chip sequence which is 2⁴⁰ chips long, which repeats every 41.4 days. Its primary purpose is to assist in spreading the signal, to make spread spectrum work more efficiently. The Long code used on the reverse link is usually modified using the phone's ESN when in a call. See Long Code Mask.

CDMA 1X- KPI

 Below are the main KPI’s of 1X services

1. AFR (Access Failure Rate)

2. CSSR (Call Setup Success Rate)

3. CDR (Call Drop Rate)

4. HOSR (Handoff Success rate)

5. Blocking

6. FFER (Forward Frame Error Rate)

7. RFER (Reverse Frame Error Rate)

 Access Failure Rate: - Whenever network fail to assigned services to MS during Call Setup this is considered as access failures. It’s the ratio of origination call failure number to Total number of originated calls.

AFR=No of Failed originated call*100 / (No of failed attempt + No of Successful call attempt)

 An access attempt failure can occur at any point in the process:

• During origination Access probes exhausted (not received by BTS)

• Access probes exhausted (seen by system but ACK not receive by Mobile Station)

• Acknowledgement received by Mobile Station but Channel Assignment Message not seen.

• Channel Assignment Message seen at mobile but mobile station does not acquire Forward Traffic Channel.

• Mobile station acquires Forward Traffic Channel but system does not acquire Reverse Traffic Channel.

• System acquires Reverse Traffic Channel but Service Connect Message is not seen at mobile station

 Reason of Access failures: (AFR)

 If the mobile does not hear acknowledgment from the BTS within certain time, this could mean either:

1       The BTS did not hear the mobile.

• Maybe the mobile collided with another mobile transmitting at the same time.

• Maybe mobile was too weak to overcome the existing reverse noise level at the BTS.

• In either case another probe should solve the problem, provided PI is set reasonably and additional probes are allowed (RF Team).

2      The BTS is acknowledging but the mobile cannot hear the acknowledgment.

 • If the mobile can’t hear the BTS acknowledging, Ec/Io is likely quite poor. If so, check whether this is due to weak signal (poor coverage) or pilot pollution (RF Team).

 One Dreaded Possibility during this process is: Reorder.

Mobile beeps and displays “Call Failed – System Busy” results Access fail.

If this problem happens frequently, the BTS traffic overload must be relieved.

Here are some steps to try:

•Investigate BTS TX hardware to ensure everything is working correctly and properly calibrated, particularly the BTS Cards/RRU i.e. Radio Frequency Subsystem unit of BTS (BSC and O&M Team).

•Check the antenna VSWR status, which should not exceeds more than 1.3 (BSC and O&M Team).

•To free up more forward power for traffic channels. Basically optimize the carrier level power allocation threshold. Like SCH_Setup_Threshold, Call_Setup_Threshold, Soft_HO_Threshold, T_Setup_RF, T_HO_RF, power_setup_size etc.(RF Team)

3. After hearing the BTS acknowledgment.

The mobile will stop probing and wait for further instructions on the paging channel.

If the mobile does not hear the Channel Assignment Message within 12 seconds, the mobile will beep and display “Call Failed”.

Possible causes:

• The BTS did not transmit the Channel Assignment Message. (BSC and Switch Team)

• Check system logs to see if this was not transmitted.(BSC Team)

4. The BS (BSC and BTS include) transmit the Channel Assignment Message, but the mobile did not hear it.(BSC & O&M& Tx Team) Check from OMC end, is there any Abis media alarms between BSC and BTS physical link. E.g. ES and SES alarms on Abis media, continuous media fluctuation of Abis link. If any hardware alarm persist in BTS media communication card .

CALL SETUP SUCCESS RATE

CSSR: Definition: Ratio of Successful call attempt to total attempt CSSR= (No of Successful call)*100/ Total Call Attempt OR (100-AFR)%

CALL DROP RATE

Call Drop: The release of traffic channel made by mobile station or base station without the permission of users. In other words, a call drop is a process of an abnormal release. If the radio link fails after the mobile sends the “Service Connect Complete” Message then it will be considered as a dropped call.

Call Drop Rate: In a specific (assigned) period, all the call drop times divided by call origination success number. It is an important indicator to evaluate the CDMA system.

CDR =No of drop calls*100/No of successful originated Calls

Factor affecting Call Drops

Improper neighbors defined in sector neighbor list. (RF Team)

• Radio Capacity limitation Walsh Code, Channel Element or Power Blocking (RF Team)

• Check the RF parameter. Like RSSI, Ec/Io, FER etc. (RF Team)

• Check the Abis (E1/T1 fluctuation) media stability. (Tx Team)

• Check the hardware alarms of BTS(GPS , RRU, Optical fiber, other BTS cards). (BSS Team)

• BTS fluctuation. (BSS Team)

HANDOFF SUCCESS RATE

HOSR : It is the ratio of successful handoff count to total no of handoff attempt in the network.

 HOSR= No of successful handoff*100/Total no of handoff attempts.

Handoff Fail Analysis: Handoff fails due to mainly below mentioned reason:

1. Neighbor list and priority not properly defined. (RF Team)

2. Need to check the Handoff parameter at BSC end. (BSC & RF Team)

3. Need to check the blocking of neighbor cells. (RF Team)

4. Need to check the E1/T1 alarms at the sites. (Tx Team & BSC Team)

5. GPS failure at sites (BSS Team)

6. Hardware Issue at BTS (BSS team)

Blocking

Block call: If the system responds that no services is available at the time for any origination or termination call attempt then this call attempt is called as block call.

 Blocking= No of block call attempt*100/ Total no of call attempted.

Mainly three types of Blocking:

1. Power Blocking

2. Walsh code blocking

3. Resource Blocking ( CE blocking)

4. Abis/backhaul Blocking

1.Power Blocking:-Blocking happens when the sector processing the mobile call request does not have sufficient forward power resources to support the call, a condition called power blocking.

 Resolution: Parameter level optimization-Power step size, HO threshold, Call setup threshold, SCH threshold. (RF Team)

2. Walsh code blocking: If Walsh code blocking happens then it is recommended to change the radio configuration Like Dynamic change RC3(63codes)RC4(128 codes) (BSC, RF Team)

3. CE Blocking: Call will be block from origination or termination on a cell when there are no traffic equipped channel elements to service these calls. (BSC and RF Team).

-Due to shortage of CE at BTS level,

-Due to PP blocking

4. Abis Blocking:

-Check Abis Utilization

- Check Alarms at abis link

- Shortage of E1s If abis/backhaul blocking is there then we need to add more E1’s at immediate basis. And this we need to monitor the E1 utilization report on regular basis .(BSC and Tx Team)

FORWARD FRAME ERROR RATE

Definition: The frame error rate is defined as the ratio of the number of bad frames received over a period of time to the total number of frames received in that same duration. FFER= No of bad frame on FL*100/Total no of frame transmitted on FL Optimization Steps for FFER: The optimization steps suggested for FFER will be identical to those recommended to manage drop calls. This is because typically most drop calls will be preceded by a period of high FFER. The converse is also true, that is, an area with high FFER will also be an area with a high likelihood for drops.

REVERSE FRAME ERROR RATE

Definition: The frame error rate is defined as the ratio of the number of bad frames received over a period of time to the total number of frames received in that same duration. The RFER may always be computed precisely by the network because the information is captured right at the network. All the frames sent by the mobile are captured by the network, which subsequently evaluates whether these frames are received in error or not.

RFER= No of bad frame on RL*100/Total no of frame received on RL

Optimization Steps for RFER: As with FFER, all of the recommendations for optimizing drop call performance also apply to managing RFER performance. In addition to this there may be problem in the reverse link that could result in performance degradations only in that link.

a. External Interference on Reverse Link Carrier Only.

b. Loss of Reverse Link Diversity.

Frame IN CDMA

Frame is the name of a CDMA digital voice packet duration. Frames are 20 milliseconds long. IS-95 transmits 50 frames per second, with each frame containing sufficient information to reproduce 20 milliseconds of sound. It should be pointed out that it may not require the whole 20 milliseconds to transmit the frame. The IS-95 codecs can generate "half-rate", "quarter-rate" and "eighth-rate" packets if the sound in that 20 milliseconds is sufficiently simple to require fewer bits to represent. A half rate packet only requires 10 milliseconds to transmit. An eighth rate packet only requires 2.5 milliseconds to transmit

Coding gain in CDMA

Coding gain in CDMA refers to the ability to use digital techniques and redundancy inherent in the chip sequence to reproduce the bit sequence without requiring much absolute power on the RF. Generally speaking, the more coding gain, the less absolute power is needed to get the signal through. CDMA uses very sophisticated error correction methods (such as the Viterbi FEC Encoder/Decoder) to increase the coding gain.

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.