Problem of performance in new network often occurred by cellular telecommunication operator either GSM or CDMA. Like a dropcall when we are talking, hand off failure or slow call set up is something currently met. Many factors are contributed to this event, both due to radio side design and due to PN planning, inaccurate traffic forecasting, bad link budget plan, and last is due to minimum network design. In this article, some things will be described about many thing that affect CDMA performance and its solution.
Terminology of CDMA
Before we discuss it, some terminologies that will be used in this analysis must be understood. We use terminology of mobile station (MS) to declare terminal in globally whether it is mobile or not. And base station often used to state entity of BTS.
Eb/I0 or Eb/N0 is comparison among energy per bit of information signal towards interference noise. The important thing is comparison among signal output that is wanted toward unwanted signal output. Bigger value of Eb/I0 better performance resulted.
FER (frame error rate) is comparison between frame error and frame that is accepted well. It is a parameter that is used to measure problem of voice quality and service range. Value of FER is represented in percentage, such as 2% means is only 2 of 100 frames that is sent allowed to be error. FER in good CDMA system has a low value both for BTS direction to MS (forward) terminal to BTS (reverse).
Cell Coverage or coverage means as an area that is still in service area of cell base station. Communication that is connecting either forward direction or reverse direction must be in the same condition.
Processing Gain is comparison between signal carrier bandwidth (W) and information signal that is sent, in this case, vocoder rate or rate set ® is used. Rate set that is used in CDMA is vocoder rate or rate set ®. Rate set that is used in CDMA is vocoder 9.6 kbps and 14.4 kbps. Comparison between W/R for vocoder 9.6 kbps is 21.072 dB and for vocoder 14.4 kbps is 19.311 dB, where W is 1.228 MHz. Processing gain will affect many thing in CDMA system, such as coverage and voice quality.
Mean opinion score (MOS) is representation of voice quality that is done by comparing between one vocoder to others according to audience opinions in a room which is free of voice interference with the same treat, same people, and same condition. Weigh is carried out by giving value from one to five, where one is the worst quality and five is best. Sample of voice quality for PSTN telephone and PCM have MOS value about 4,1.
Parameter of Cellular Network Performance
Basically performance of cellular system either CDMA or BSM can be measured by seeing some parameters of network Quality of Service (QoS). Cellular operator in advance countries had a test toward network performance periodically before getting a service complaint from customer. The following is some models of parameter that are used to measure network performance in Singapore for three operators like SingTel Mobile, Starhub Mobile and M1 Mobile. This parameter also called as Key Performance Index (KPI).
Call success ratio is a ratio of call success based on amount of call success toward total amount of call.
Service coverage is coverage that based on signal strengthen and network ability to keep maintaining signal strong at 100dBm or be better during call period.
Voice Quality is voice quality based on network ability to provide voice quality level which can be accepted well with MOS method and it is a complement of service coverage.
Call Drop-out or Drop call. This parameter is based on network uncertainty when line is broken during the call by MS terminal at 100 seconds.
Phenomenon and Analysis of CDMA Radio Network
Causal factor of fourth QoS above is chained one to other. So that, in analyzing CDMA system, all must be integrated. There is trade-off among coverage area, system capacity and voice quality that are affecting so when one of performances rose up, so those two will be down. Otherwise, in call success ratio, it is not only affected by signal output Eb/I0 in ratio side, but also network dimensioning side.
Coverage Area
MS terminal will be served by CDMA system if value of Eb/I0 it needs is enough. Power margin is needed to handle the change of environment condition that causes Eb/I0 go down under the level. Eb/I0 that is required in coverage boundary of a cell is determined by pilot signal Eb/I0, which is needed by each terminal added with power margin. The low of Eb/I0 price is because pilot signal that received by MS is low. The solution is by increasing value of power signal in base station or if attenuation is very high by adding cells (base station).
The usage of rate set in the system will influence coverage width that can be provided. As said before that rate set will be used is to determine processing gain, which finally affects the arrival of Eb/I0 in MS terminal side. The difference of processing gain between rate set 9.6 kbps and rate set 14.4 kbps is very significant, where the system uses rate set 9.6 kbps around 1.76 dB that is much better. This could be for a place or location that system used rate set 9.6 kbps receives the service, but when it is replaced with rate set 14.4 kbps, possibly cannot access it. For your information, TELKOMFlexi uses both rate set in its network.
Cell in CDMA system has shrink characteristic when a burden gets closer to maximum threshold, which can be provided by cell transmitter. This causes subscribers who are in coverage boundary which starting shrink will not get service when it is happened. So that subscriber must be overflow to neighbor cell, which is having light in weight burden. People said as soft capacity, which was experienced by CDMA system. Figure 1 indicates the change of service coverage due to phenomenon of cell shrinking.
Figure 1. The Change of Scope due to phenomenon of cell shrinking (cell breathing). 1)
Voice Quality
Voice quality that is received by subscriber, affected by vocoder set that is used and FER that is happened in network. Vocoder 14.4 kbps is used to get higher voice quality. But this quality will be down if FER value increases. For example, CDMA system that is using vocoder 14.4 kbps and 1% FER has value of MOS 3.94, and when FER is down to be 2%, so MOS’s value is 3.89.
If value of FER is constant and type of vocoder is changed, there will be a difference of voice quality between vocoder 9.6 kbps and vocoder 14.4 kbps. Voice quality in vocoder 9.6 kbps of 1% FER roughly is equivalent with vocoder 14.4 kbps of 3% FER. So that vocoder 14.4 kbps gives voice quality that is better than vocoder 9.6 kbps at the same FER value. To find the same voice quality, system of vocoder 9.6 kbps needs Eb/I0, which is higher than 14.4 kbps. Figure 2 points out the connection between MOS and FER. As a note, the curve here is just an illustration.
Figure 2. Approximation between MOS and FER connection2)
System Capacity
Subscriber capacity that can be served by one carrier frequency of CDMA system, which is affected by some factors such as coding rate that is used, Eb/I0 level needed by each MS and interference of other cell if multiple cell is available in the system. Generally subscriber capacity in CDMA system is written as the following:
N = ((W/R) x Gs x Gv)/((Eb/I0)(1+f))
where:
W : bandwidth of CDMA carrier frequency 1,228 MHz
R : rate of vocoder, 9.6 kbps or 14.4 kbps
Eb : energy per bit
Io : density of interference spectral energy
Gs : gain of antenna sector
Gv : gain of voice activity
‘f’ : interference percentage of other cell.
Vocoder rate (R) that is used affecting subscriber capacity, in which smaller the coding rate is used bigger subscriber, can be provided. The system that is using coding rate at 9.6 kbps will have bigger capacity than system that is using coding rate at 14.4 kbps. And Eb/I0 will be affected by signal output that subscriber received and interference in both internal and external system. Like in frequency of 1900MHz band, CDMA system has the same frequency with DECT system. If CDMA system is in the same location with DECT, noise floor in CDMA system will be up so the value of Eb/I0 is also increased.
Difference between CDMA IS-95 and CDMA 2000 1x is that the use of pilot in reverse channel in CDMA 2000 1x. Pilot channel in this reverse facilitates MS terminal to manage coding and measure signal of base station so it reduces Eb/I0. That is one of factors why CDMA 2000 1x has almost doubled capacity better than CDMA IS-95. But if there are some IS-95A terminals that operate in CDMA 2000 1x network, capacity of CDMA 2000 1x cannot reach optimally, because this terminal will increase Eb/I0.
Network Optimizing
Network performance in field will be affected by charging change. It could be happened before 8 AM where network load is still not busy and in good performance, and when it is 5 PM, the traffic becomes higher, and network performance is getting worse. Right hand rule is in effect, which is the network that gives bad performance in light load, so it is worst in overloaded condition.
Pilot Signal
MS needs Eb/I0 which is available to get into service coverage of base station. So that, Eb/I0 signal must be rose up when low that is by enhancing transmission of base station, so all MS in in service coverage can be provided. But if attenuation cannot be handled, the solution is by gaining base station.
Interference and Its Solution
Interference in forward link direction is shown with the high level of FER value due to Eb/I0 is low and along with high energy that received by MS terminal. Terminal receives high energy because it measures total signal which is available in all carrier signal band, so high level of FER in MS indicates a lot of interference in base station direction to MS.
At least there are four types of interference which affect to CDMA system. First, interference dues to traffic channel of forward direction from BTS itself (home base station). This interference is because all traffic channel from base station are sent to MS terminal. The solution is by bounding traffic channel that can be used by the cell. In this part, CDMA network programmer must concern about subscriber projection that can access or be served by a base station in top condition, so interference can be avoided.
Second, interference from base station to MS is due to overhead transmission which is overloaded from neighbor BTS). People often called it as pilot pollution, because pilot signal in field is the highest signal compared to other overhead signal. The solution is by reducing overhead signal via re-arrange transmission energy, especially for overhead channel from nearer BTS.
Interference from base station to the third MS that is due to traffic of transmission from other base station. This interference signal is total amount of signal in traffic channel from other base station to MS terminal in other cell. The solution of this problem is by rearranging antenna without disturbing the scope of neighbor cell.
Last interference that is occurred in base station directs to MS is interference that comes from non-CDMA signal. Other system is in frequency band of CDMA system. Model of case in this interference is DECT system that has work frequency same with CDMA system in 1900MHz. If both systems is in same location and frequency, they will decrease the performance.
And bad of reverse-link scope is pointed out with FER in reverse-link direction because the low of Eb/I0 value and high power that is received by MS terminal. This is because of power control that is continuously tried to close with reverse-link by adding transmission of base station. There are three sources that cause reverse link interference. The interferences are due to traffic channel transmission by other customer in the same one cell, interference of traffic channel by other user from other cell, and last are due to non-CDMA signal. The solution is almost same to handle interference in forward direction.
Allocation of PN Offset
Unlike GSM system where difference between one and other cell is determined by its carrier frequency. In CDMA system, the difference of them is on PN offset. If allocation of this PN offset is not properly, so it will cause the ambiguity of cell identification that provides MS terminal. PN offset must be made as well so nearer cells are not disturbed.
Index of PN offset that is available in CDMA system is 512 as unique value. It is 64 chips differences between PN offset index, so total period is 32768 chips. One chip is about 0.814 micro second. When a pilot signal moves from a cell to MS, it will cause a delay. Minimum of delay between one and other offset allowed to be 64 chip = 4.09 x d (kilometer). So it results interval of one index of PN offset is about 15.6 kms.
If signal of two cells that has neared PN offset by coincidence gets propagation delay to an MS, so ambiguity of PN offset detection will be happened. MS is difficult to make system acquisition due to it does not know which cell is being active. In figure below, two pilot signals of two cells that have nearer PN offset index are received in 64 chips by MS. Receiver system in MS will be confused to choose which cell is function to be an active cell and which cell is a candidate. When signal that is from active cell regarded as candidate cell or neighbour cell, handoff command cannot be done; so when it is busy, drop call will be happened.
Figure 3. Ambiguities of cell identification due to pilot pollution 3)
PN offset Optimizing
New CDMA operator, which has no experience before, usually traps to paradigm of GSM cellular network. Or also because of champagne that CDMA does not need radio planning, so without thinking allocation of PN offset, install base station without thinking about the effect. In CDMA, indeed; radio design is not needed but it needs to design PN offset allocation for each cell to avoid pilot pollution. If it is happened what operator has to do? The answer is reusing by cells in the network. The last goal of this effort is to make the same or neared PN offset not to affect among cells by settling the distance.
Conclusion
Radio designing in cellular telecommunication like CDMA is not always worked out at once and after that the operator is doing nothing. But the designing is a continuous improvement in which the effort is kept monitoring and doing improvement due to network performance is always changed, which because nature change, traffic charging habitual, and also the change within the network itself.
Reference
1. Website www.reteumts.com/page_4.htm tentang cell breathing, 2003.
2. Ketchum, J., M. Wallace, and R. Walton, “CDMA Network Deployment of 8 Kbps and 13 Kbps Voice Services,” Proc. Of International Conf. On UPC, IEEE, 1996.
3. Dong Seung Kwon, “Rapporteur’s report for Study Question 3.1 Planning, Implementation and Operation for CDMA Technology and WLL System Implementation aspects”, ETRI, Korea, September 07, 2003.
4. IDA Cellular Network Performance Measurement System Second report, Singapore, July 2000.
Hazim Ahmadi, author is joint with R & D Center PT TELKOM since 1996. he is working in Wireless Access laboratory to handle cellular technologies such as GSM and CDMA. Some activities he is dealing with are interference in CDMA with DECT, the usage of SIM card (RUIM) in CDMA, ISMSC for cross network sms, and analysis of CDMA terminal. He is also active in some organizations such as PHS Mou Group, CDMA Development Group, IEEE and Mobile Comm International magazine. The last publication was represented in International Mobile Data Services Seminar in Seoul, South Korea in August 2003, bringing up the paper of CDMA network data optimization.
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