Challenges of poor performance in cellular mobile systems

Transcription

1 Challenges of poor performance in cellular mobile systems Mamatha Shankarappa Ambo University, Ethiopia Abstract In today s lifestyle, cellular wireless communications has taken a fast entry into majority of the world s population. Generally speaking, most of the time, urban areas have good network signal when compared to the rural areas. This paper focuses on few such critical challenges the customer is facing due to poor performance in a non-optimized cellular system. Challenges the customers are facing is discussed in terms of call drops, types of call drops and reasons for call drops, handover failures and reason for the same. Though the focus is on thorough understanding of different performance challenges some remedial actions are also suggested. Keywords- Call drops, handover failure, RF optimisation, Handover priorities I. INTRODUCTION The current cellular system service demands to fulfil more efficiently, the needs of customers and the service providers are always challenged with a lot of performance issues [1-9]. It could be technical, financial, new technological upgrades, competition and sustainability issues. The ease to this demand made many telecommunications giants to enter into the business of cellular wireless communication and coming up with lot of alternatives[4]. As further moved, the need for better technologies and reliability of services, integration and cost effective practical solutions have become a necessity for service providers [5]. This paper focuses on the critical challenges from the customer perspective. change their service provider without changing their mobile number. This made the service providers to stand on their toes. Hence this paper also considers some of the suggestions for the stated challenges of call drops and handover failures. II. NETWORK ARCHITECTURE GSM Network Architecture Today GSM (Global system for mobile communication) is the most popular cellular network service by gaining more than 85% of its market share. In GSM the whole network or geographical area is divided into different cells and clusters. The Fig.1 below shows a partial network architecture of GSM and Fig.2 shows the representation of cells and clusters. Cells are basically grouped into clusters for the purpose of efficient frequency reuse and also to avoid co-channel interference. Even though the performance of cellular communication systems depends significantly on the mobile radio channel, the need to tackle the problems of poor network coverage, traffic congestion, call drops, increased failure to access and engage communications channels, poor quality of service, poor access to radio resources and latency in the current cellular system is high and if attended seriously, it not only benefits the customers but also the vendor for providing a satisfactory service[3]. High quality of service has become an inevitable competitive advantage for service providers to retain their customers because many countries with GSM networks today provide a facility called MNP(Mobile number portability). MNP facilitates the customers to Fig: 1 GSM network Architecture All mobile stations are initially connected to RBS (Radio Base station) also called as BTS (Base transceiver station) via unguided media. BTS contains Transceiver (TRX),power couplers, radio equipment, antenna and much more, which are responsible for the transmission and reception of several radio frequency (RF) signals and is then connected to BSC (Base station controller). BSC is in turn connected to Network switching system through transcoder where MSC (mobile switching centre) resides. BSC usually handles radio resource management and handovers of the calls 36

2 from one Cell to the other cell that it has equipped. BSC is then connected. Inter BSC Handover is done by MSC. MSC is connected to all the other functional nodes and databases including VLR (visitor location register)/hlr(home Location register). Call supervision and routing are also the important responsibilities of MSC. As there is a limitation of frequency spectrum, frequency reuse technique is adopted for the efficient use of frequency is shown in Fig.2. obtaining signals. Additionally, the weather and volume of traffic in network may impact the signal strength. Most Common type of call drop is due to network congestion but there are also other types of call drops which are from BSS(Base station system) so they can be classified as a. Call drop due to network congestion b. RF call drop i. RF call drop failure during downlink ii. RF call drop failure during uplink c. Call drop during the process of handover d. Call drop due to LAPD protocol failure Call drop due to network congestion: FIG 2: FREQUENCY REUSE IN CLUSTERS III. CRITICAL CHALLENGES Performance challenges that are often encountered might be due to Cell Dragging, Dropped Call, Ping- Ponging, System Busy and Handover boundary. Here the critical challenges of poor performance are discussed in terms of i. Call drops: various types and reasons for dropping of calls ii. handover failures High demand for cellular services and limited capacity are major causes for congestion in GSM cellular systems, peak hours contribute more congestion. By congestion, it means that in some cells, there will be no more capacity left. More specifically, in a congested cell, there will be no more available data channels to use by additional mobile hosts in the cell. However, the control channels for signalling (or paging) may still be accessible by all mobile hosts (Kuboye, 2006). Normally Network congestion occurs when a node or link is burdened with more data than its capacity which in turn affect the quality of service. The important effects due to network congestion are loss of packet, blocking of new calls and queuing delay. All these contribute to a stable state with low throughput known as congestion collapse. A. Types and reasons of Call Drops Dropping of a call could happen due to many reasons. It may be due to poor signalling, configuration, power, resource congestion. As per the various analysis report, maximum call drops occur at the air interface only because it is not possible to control the air interface behaviour in the current network architecture. However in an optimized network, lack of radio resource is very rare. Rather, call duration, call arrival rate, improper settings of radio parameters, hardware equipment faults should be dealt with more attention. If the cells are maintaining good signals in their coverage area then there may be other factors which affect the performance, thereby making it either stronger or weaker, it may cause complete disturbance or interference. As Rohit et al in their paper gives the example, a high-rise building or building with thick walls may prevent a mobile phone from being used. Many underground areas, such as tunnels and subway stations, hills and mountains lack in Congestion collapse is a situation packets reach when little or no useful communication is happening due to congestion in traffic channelled network. It normally occurs at choke points in the network where the total incoming bandwidth to a node or link is becoming much more than its outgoing bandwidth. LAN and WAN connection points are most likely the choke points. Pragyan Verma et al says A DSL modem is the most common small network example, with the range of 10 to 1000 Mbit/s of incoming bandwidth and at the most 8 Mbit/s of outgoing bandwidth. The reasons for Network congestion calls 1. Heavy call traffic in peak hour. 2. Increase of signalling load. 3. Network congestion through traffic Management. 4. Overloading of network equipment. 5. Wrong configuration in mobile network. 6. Installed software is incompatible with hardware. 7. Mobile user is moving at a high speed. 8. Imbalance between uplink and downlink 37

3 RF call drop i. RF call drop failure during downlink: As per the GSM specifications, MS (Mobile station) has a timer S (T100) which is allotted with an initial value called RLT (radio link timeout) through BCCH channel (Broadcast control channel). When severe interference occurs during downlink process, MS cannot correctly decode the SACCH (slow associated control channel) which transfers system information message, so that timer S is decreased by 1 and, it should be noted the same timer S is increased by 2 whenever MS is able to decode the SACCH message. S will not exceed the initial value defined by RLT. When S value reaches zero, MS will release a radio resource connection abruptly, which will result in call drop. ii. RF call drop failure during uplink: The link failure in the system can be also due to uplink failure. When the site is unable to decode a SACCH message correctly due to uplink interference, the timer is decreased by 1.when the site correctly decodes a SACCH message, the timer is increased by 2. When the timer value is zero, the site stops transmitting downlink SACCH and starts the rr_t3109 timer (rr_t3109>t100). When T100 of MS is timeout, MS returns to idle mode and call drop occurs. The site releases the radio channel when the rr_t3109 timer is timeout and BSC will send a Clear request message to MSC. Either uplink failure or downlink failure will stop sending SACCH to the opposite end which results in radio link time out and call drops[2]. Reasons for RF Call drop 1. Unreasonable radio parameter settings. 2. Intranetwork interference. 3. Existence of weak coverage area and weak radio signal. 4. Equipment hardware faults such as low output power of the power amplifier, large difference among different carrier transmission power, carrier transmitter fault, combiner fault and splitter fault. 5. Faults in antenna feeding system. 6. Weak battery power. Call drop during the process of handover: When the MS receives handover command or assignment message but fails to handover the call to the destination cell and does not return to the originated cell, it results in call drop failure during handover process. Here MS is unable to send handover transfer a complete message while maintaining the originated cell channel or neither does it return to the original cell channel and sends handover failure. Rather, MS is disconnected from the network. At this moment the handover control timer of BSC will be timed out and will notify the MSC to clear the release and count this exception event as handover failure call drop. Reasons of Handover Failure Call drop The following are reasons of handover failure call drop: 1. Existence of interference such as intra-network interference due to unreasonable frequency planning and other external interference. 2. Hardware Equipment fault, such as clock fault in destination cell or in source cell, low output power of the power amplifier, large difference among different transmitter s transmission power, transmitter fault, combiner fault, and divider fault. 3. Unreasonable radio parameter settings. Call drop due to LAPD protocol failure: The LAPD is a signalling layer 2 protocol which is defined in CCITT Q.920/921 and existed between BTS (Base transceiver station and BSC (Base Station Controller). LAPD works in the Asynchronous Balanced Mode (ABM). This mode is totally balanced. Each station may initialize, supervise, recover from errors, and send frames at any time. When an LAPD link is broken, the call-in-progress on the carrier will be interrupted. Reasons of LAPD call drop 1. Site transmission problems, such as transmission interruption or unstable transmission (intermittent). 2. Site-side hardware fault, such as unreliable E1 cable, CMM (Control and Maintenance module) fault, and backplane connection fault, etc. 3. BSC-side hardware fault such as LAPD processing board fault may result in LAPD Call drop. B. Handover Failure As stated already in cellular system, the whole geographical area is split into many small cells to make use of frequency reuse technique efficiently. This situation leads to handover. Handover is a process which happens when the customer moves from one cell to another when the phone is in the active mode. The same is also called as handoff. It is important, also very critical, and leads to a call drop, when performed incorrectly. A simple technique is employed in cellular networks to assign channels for both, handover, as well as news calls, without any priority for NPS (nonprioritized scheme). Here both will have the same probability for channel allocation. However a forced termination of on going call seems to be more awkward from the customer point of view than blocking a new call attempt so it becomes necessary to introduce methods for decreasing the probability of handover failure as well as new call blocking. However more attention is given to GSM handover here. Basic types of GSM handover: There are four basic types of call handover in GSM only networks 38

4 Intra-BTS handover: When, due to any reason of interference, if the mobile phone has to change the frequency channel for better quality of service as per the instructions of BTS, this handover occurs. Here the mobile remains in the same BTS by changing only the channel of frequency. Inter-BTS Intra BSC handover: when the mobile moves out of the coverage area of one BTS and enters into another BTS area but controlled by the same BSC this handover will happen. Here the BSC is able to perform the handover and it assigns a new channel and slot to the mobile, before releasing the old BTS by communicating with the mobile station. Inter-BSC handover: When the mobile moves out of all cells controlled by one BSC, a more involved form of handover has to be performed, handing over not only from one BTS to another BTS but also from one BSC to another BSC. This form of the handover is controlled by the MSC. Inter-MSC handover: This form of handover occurs when the call has to be handed over across two different networks. Here the two MSCs from different networks are involved to negotiate the control of handover. Although there are several forms of GSM handover, as explained, for mobile station they all are seen as similar. There are a number of stages in undertaking a GSM handover from one cell to another or one base station to another. The GSM handover mainly depends on timing and synchronisation. The possible scenarios that might occur depend upon the level of synchronisation, as Ivan Poove writes. Old and new BTSs synchronised: Here the call will be handed over based on the details of physical channel the MS receives, later the MS confirms the action by transmitting four access bursts which are shorter than the standard bursts and thereby any effects of poor synchronisation do not cause an overlapping with other bursts. Since the synchronisation is already good in this scenario, these bursts are used only to provide fine tune adjustments. Inter-system handover: The need of handover also arises when we move from one technology to another like with the evolution of standards and the migration of GSM to other 2G technologies including 3G UMTS / WCDMA as well as HSPA and then LTE. Most of the time 2G GSM coverage will be better than others so that GSM is used as a fall back. This handover is considerably more complicated, also called as inter-rat handovers as the handover occurs between different radio access technologies. IV. SUGGESTED REMEDIAL ACTIONS In order to improve the QOS of a cellular network, some of the factors which has to be considered includes upgrading coverage rate, decreasing interference, increasing coverage continuity and enhancing cells traffic equilibrium, enhancing the need of quality of service through maintenance RF optimisation and Handover prioritisation. There are many optimisation techniques available but this paper s focus is restricted to RF optimisation and Handover prioritisation since understanding customer challenges is given more focus. RF Optimisation RF Optimization is the activity of achieving and maintaining the required quality as per the design. Every Network requires constant maintenance to improve its performance. Optimization keeps track of the network by looking into the statistics of test data collection and analysing the same. It is deeply observed for enhancing the capacity. It also helps in operation, maintenance and trouble shooting. The main objective is to improve existing network coverage, capacity, QOS and to maintain the KPIs under predefined threshold. RF optimization is applied for Non working cells /clusters or TRXs and malfunctioning radio networks. Hardware Optimisation: It includes Handover parameters, Antenna Down tilt, Antenna Relocation, Antenna Height adjustment and Frequency planning. Cell parameter optimization: It includes Neighbour list reconfiguration, Power planning, Antenna Reorientation. Time offset between synchronised old and new BTS: When there is a time offset between the old and new BTS, the time offset adjustment is provided to the mobile. The GSM handover then takes place as a standard synchronised handover. Non-synchronised handover: Here the mobile transmits 64 access bursts on the new channel which enables the base station to determine and adjust the timing of the mobile. Hence, it suitably accesses the new BTS to re-establish the connection through the new BTS with the correct timing. DRIVE TEST FOR RF OPTIMISATION [1] This test needs TEMS mobile equipment, software for drive test, a laptop, and a GPS (global positioning system). Drive tests give the 'feel' of the designed network as it is experienced in the field. The testing is applied to the network site of crisis, then the path for drive test is chosen. In drive test mobile will generate calls with a gap of few seconds. It makes continuous calls, and when the call drops it attempts for another call. The purpose of this test is to collect enough samples at a reasonable speed, in reasonable time. 39

5 During DT HO failure, RX Level and speech quality are all observed. If there are lot of dropped calls, the problem is analysed to find a solution for it and to propose changes. One of the biggest service provider, BSNL uses Ascom TEMS drive test tool kit. OMCR reports are taken in to consideration before performing the drive test. Drive Test should have route plans like primary route (street level), secondary route (street level) and miscellaneous routes. Handover remedial suggestions: There are various handover prioritization schemes. These schemes can be sorted into four different types: Reserving a number of channels exclusively for handovers Queuing handover requests Sub-rating an existing call to accommodate a handover Combination of the above classes Juan Ventura Agustina et al states in their paper that these schemes take place on the basis of PS implementation. For example, if there are a fixed number of channels (N ch ) in a GSM/GPRS cell, a fraction of channels are exclusively reserved for GPRS (N gprs ). The N gprs is referred to GPRS penetration factor. Then the rest channels (N shared ) can be used for both GSM and GPRS, where these schemes apply. Reserved channel scheme (RCS): This scheme reserves certain channels exclusively for handovers requests from N shared. The N shared channels are divided into two different groups as common channel group (N com ) and reserved channel group (N ho ). the N com channels can be used by new calls as well as handovers, whereas the N ho channels can only be used by handovers. There are two types of reservation. Pre-reservation (RCS-pre): A channel in N ho is allocated on arrival of handover request. If N ho is fully occupied by handover traffic, the handover request deals with new call channel in the N com pool. This ensures minimal handover traffic even under heavy load. Post-reservation (RCS-post): On the arrival of a new handover request, it contends with new call attempts for admission into the N com pool. If N com is full, it will be allocated in the N ho pool. In post-reserved pool, even under heavy loads extra priority is given to handovers. Reserving channels for handovers means fewer channels can be granted to new calls, so the blocking probability of new calls may increase significantly. This disadvantage can be overcome by queuing scheme in the following subsection. Queuing priority scheme (QPS): On the arrival of a new handover request, if there is no free channel in the target cell, the handover request is queued and the mobile station (MS) continues to use the old channel in the current cell until a free channel is available in the target cell. The queuing can be performed at Base Transceiver Station (BTS)/Base Station Controller (BSC) or Mobile Switch Center (MSC) in GSM system. New call in the target cell is served only when there is no handover request in the queue and a free channel is available. If any channel is released while handover requests are queued, the released channel is assigned to a handover in the queue. The next handover will be served based on queuing policies, which will also influence the performance of the scheme: FIFO priority queuing (F-QPS):The handover request is queued in FIFO discipline when it finds all channels are occupied in the target cell Measurement-based priority queuing (M-QPS): As MS spends more time in the handover area, its communication with the current BTS degrades depending on various factors viz., velocity and direction. MS continuously measures and monitors this degradation rate easily by the means of radio channel and submit to the MAHO ((Mobile Assisted Handoff) in GSM network. Later handover area will be viewed as, a region marked by different ranges of values of the power ratio. The highest priority is for the MS whose power level is closest to a receiver threshold otherwise the MS that has just issued a handover request has the least priority. The power levels are monitored continuously, and the priority of MS changes depending on its power level and queue is dynamically reordered. When a channel is released, it is granted to the MS with the highest priority. The handover can be also improved by equalizing the traffic load over the cells[8]. Traffic reason and directed retry handover make use of this principle. Traffic reason handover can be used to transfer traffic from one cell to another neighbouring when they are closed to the congestion. The traffic reason handover idea is based on the neighbouring cell having an overlapping service area. It has been proved that directed retry in the overlapping areas leads to better quality of service in the cellular system. A large overlapping area gives more capacity than a smaller overlap, but even by just having a small overlap, a significant gain is achieved. The overlap of 0.1R (where R is the radius of cell) results an overlapping area equal to 9% of the cell area gives a gain of at least 6% whereas if the overlap is equal to 0.5R means overlapping area is 75% of the cell area then the capacity gain is boost to 27%. The performance of this functionality is very dependent on the existing overlapping between cells since it is required that at 40

6 least one neighbouring cell has sufficient signal level for the mobile station to be redirected [9]. The suggestions for other performance issues could be: When Dropped Calls are caused due to either RF environments or incorrect system parameters, it needs to check the appropriate neighbour cell list, HO parameters, existing or new coverage holes interference, co-channel or adjacent channels or external interference. Even serving cells might go down causing call setup failure or covering small area. In Cell Dragging normally the call may drag a cell beyond the desired handover boundary resulting dropped calls. Ping Ponging is a situation where the serving cell keep changing and as a result of bad audio quality. Handover Boundary is a challenge occurs when the handovers do not occur at the desired HO boundary resulting in an imbalance in traffic distribution across the system. All the above three are dealt by creating an appropriate neighbour cell list, changing HO parameters such as thresholds, margin, cell barring, etc. Check cell identification of serving cell in the neighbour cell s neighbour list also check for neighbour cell BCCH, BSIC, LAC, Cell ID, Lack of dominant server and optimal antenna configuration and Poor coverage System Busy means on several call attempts site appears consistently on the traffic. Remedial suggestions are to reduce the traffic on the congested cell/site. However, proposed changes should not create any unacceptable problems such as coverage holes, dropped calls. Short term solutions are re-design the antenna configuration, add additional RTs, change BTS configuration. Long Term solution is to build a new cell site to off-load traffic. V. CONCLUSION A small effort has been made to advocate both customers as well as service providers to understand critical scenarios behind the challenges which we are facing in our day to day life in these recent days. It discusses the challenges of poor performance in terms call drops and handover failures. Call drops explore with different types, reasons for their occurrence and to some extent the suggestions to reduce the same by different optimisation techniques including RF optimisation through drive test environment. Similarly reasons for different types of handover failure are discussed with different remedial actions including various handover prioritisation schemes. Finally, it is concluded by giving suggestions for the most common performance issues in a more practical manner. REFERENCES [1] Giriraj Sharma and Ashish Kumar Bansal, "A Practical Approach to Improve GSM Network Quality by RF Optimization. Blue Eyes Intelligence Engineering & Sciences Publication Pvt. Ltd. [2]. Rohit Das1, Vikas2, Hrishikesh Narayan Tripathi3 Reducing Call Drop in Mobile Cellular Communication by using MIMO Antenna., International Journal of Advanced Research in Computer Science and software Engineering [3 ] Bilal Haider, M Zafrullah Khan, M.K.Islam, Radio Frequency optimization and QOS in operational GSM network. [4] Syed Imran Basha, Idrish Shaik: "Reducing Handover Failure Rate by RF Optimization" [5] Theodore S. Rappaport, Wireless Communications, Principles and Practice, 2nd edition,, Pearson publications. [6] Pragyan Verma a, Preeti Sharma, Sattyam Kishore Mishra,. "Dropping of call Due to Congestion in Mobile Network, Journal of Computer Applications. (JCA )ISSN: , Volume V, Issue 1, 2012 [7] ITU-T recommendation G.1000 (2001), Communication quality of Service A framework and definition. [8] Biebuma J.J.,Orakwe S.I and Igbekele, Traffic modeling for capacity analysis of gsm networks in Nigeria o.j, Continental J. Information Technology 4: 78-89, 2010 ISSN: [8] Jahangir Khan., Handover management in GSM cellular system International Journal of Computer Applications ( ) Volume 8 No.12, October 2010 [9] Nielsen, T., and Wigard, J. Performance Enhancements in a Frequency Hopping GSM Network,. Kluwer Academic Publisher, Netherlands. 41