TEAM FLY WIRELESS NETWORK DEPLOYMENTS phần 4 pps

70 Chapter 4 approximately 500 sectors, reducing to approximately 380 for a conversion ratio of 1, when we deploy CDMA on every analog cell site within the footprint. However, as we increase the traffic the CDMA coverage starts to shrink. The coverage provided by the cell sites along the border starts to shrink and we are required to deploy more cell sites within the dual-mode footprint to maintain the same coverage. As the coverage along the borders shrinks slightly, the number of cells within the guard zone is also reduced. This reduces the impact of the CDMA system on the underlying analog system while increasing the traffic capacity of the analog system. Table 1 illustrates the trend and shows the increase in the total traffic (AMPS + CDMA) with respect to the traffic in the dual-mode footprint and the size of the guard zone. As we increase the number of cell sites within the dual-mode footprint we can increase the traffic that we can carry. As the CDMA traffic increases we could opt to add an additional frequency to our CDMA system. If we go to an additional frequency we double effectively the CDMA capacity, though incurring an additional loss in the underlying analog system. From Table 1, we see that only when the conversion rate is around 0.84 or higher do we see that there is a benefit to going to another frequency. TEAMFLY Team-Fly ® Optimization of Dual Mode CDMA/AMPS Networks 71 Before the actual deployment uniform traffic amongst the cells is typically assumed. However, in a real deployment scenario, the traffic distribution will not be uniform, but will involve traffic hot spots. These hot spots will use the resources of the neighboring cell sites. We consider the baseline case of 27 CDMA cell sites within the dual-mode footprint (conversion factor = 0.73). As shown in Table 1, the CDMA traffic carried was 861 Erlangs. To simulate the hotspot areas, the traffic in the center 7 dual-mode cells was increased by 4 Erlangs/sector. This led to coverage holes, due to the excess traffic around the center. The issue facing the service provider is 5 AMPS is assumed to have nominally 18 channels per sector, at 2% blocking and CDMA to occupy 1.8 MHz of spectrum. 72 Chapter 4 whether to deploy more cell sites in the neighborhood of these hot spots, or possibly go to an additional frequency. Table 2 shows the results of the 3 options. The first option of adding another CDMA carrier in the core led to a reduction in the total traffic of 110 Erlangs. The second option of using the additional carrier everywhere reduced the overall traffic that could be carried by 530 Erlangs. This is mainly as a result of the excessive loss in analog traffic capacity of 738 Erlangs. The third option in the scenario was to increase the number of cell sites within the dual-mode footprint. For this case it just takes an additional two sites to maintain the coverage objective and results in an additional 197 Erlangs being carried. Of course as the traffic increases we can use the inherent potential of the additional carrier to allow for future growth. However, as seen in Table 2, the addition of another carrier is not always the best alternative. It is also clear that in defining the gain of CDMA over analog that the Erlang loss in traffic carrying capacity in the guard zone must also be taken into account. 2.2 Impact of CDMA-to-AMPS Handdown Procedures on Network Performance Another important network optimization issue is the flawless network operation at the boundaries of the dual-mode (mainly urban areas) and analog only (typically more rural) areas. Mobile users in the border of a CDMA coverage area—or on the edge of a “CDMA coverage hole”—must be able to get their CDMA service gracefully handed down to the AMPS system. Careful network engineering and optimization is required at the borders so that the CDMA call is handed down to an explicitly specified AMPS server well before the CDMA signal gets too weak and the call is dropped. In a similar fashion, when there is a request for a call initiation in the CDMA-AMPS boundaries, it may be necessary for the call to be immediately handed over to the overlaid AMPS service. To guarantee a flawless handdown procedure that is triggered and controlled by the network, a CDMA-AMPS handown (CAH) region should be identified at the boundary area between the CDMA and AMPS footprints. This region should be (1) contiguous, to guarantee a consistent handdown event along the CDMA-AMPS boundaries; and (2) not very wide, because this leads to a considerable loss in the traffic supported by the CDMA cells in the boundary of the dual-mode footprint. Typically, CAH strategies involve CDMA beacons, border cells, explicitly defined pilot neighbor lists Optimization of Dual Mode CDMA/AMPS Networks 73 for the sectors along the boundaries, and variable cell sizes 6 to facilitate a flawless CDMA to AMPS handover of service. We limit our studies here to the approaches based on border and beacon cells with variable cell sizes. A border cell 7 is a regular CDMA server located in the CDMA-AMPS boundary area. It is equipped with the additional functionality of first triggering a CAH event and then transferring the CDMA call to the AMPS server that is usually collocated with the border server 8 . During a border- cell-assisted CAH, a mobile user is handed down to AMPS when both the following two conditions are met: (1) all CDMA pilots in the user’s active set should be configured for this handdown type (i.e., they are border servers as well); and (2) the received CDMA pilot signal over the total interference should be below a user-specified handdown threshold T brd for all pilots in the user’s active set. A CDMA beacon is a pilot-only sector located in the dual-mode to AMPS geographical border area. It does not carry any traffic but is rather used to trigger a CAH event and maintain the necessary synchronization. During a CDMA beacon assisted handdown, a user is handed down to an AMPS server (usually collocated with the CDMA beacon) when at least one CDMA beacon with received pilot power above a user-specified threshold T beac has been detected. The sector cell size is a tuning parameter typically available on every CDMA sector in the network. In deployed networks, due to the coverage versus capacity tradeoffs, the serving areas of CDMA cells can vary dynamically in size with the changing traffic conditions. Typical base station equipment can be programmed with a distance/time related parameter referred to as cell size to limit undesirable soft-handoffs and call initiations from “weak” portables. A proper setting of the cell size helps eliminate “rogue” pilots from entering the active set of a portable, especially from base stations located far away with very tall antennas, or in areas of potential “pilot-pollution” where the portable may have to change the status of its active set much too often. Cell size is also important in facilitating intelligent CAH strategies involving CDMA beacons. A CDMA-to-AMPS handdown event that is assisted via the cell size parameter, denoted as C in our analysis, 6 This is a sector parameter which limits the distance within which the sector’s pilot will be considered as a possible server or a handoff candidate. 7 By “border cell,” we mean a sector deployed in a regular CDMA base station which is on the edge of the dual-mode network and its primary function is to seamlessly hand-down to analog gracefully. 8 This is usually the case as it reduces confusion as to which analog carrier the signal should be handed down too. 74 Chapter 4 at a CDMA beacon, necessitates that a user is handed down to an AMPS server when (a) at least one CDMA beacon with received pilot power above a user specified threshold has been detected and (b) the user’s distance from the corresponding beacon is within the range C as defined by the value of the cell size parameter set on that beacon. We have conducted numerous simulations to study the impact of the aforementioned CAH strategies on the performance of a dual-mode CDMA network. The performance criteria used were the average capacity loss and average handoff overhead per boundary cell. The average capacity loss per boundary cell represents the loss in CDMA primary traffic carried by a boundary cell with border or beacon sectors relative to the CDMA primary traffic carried by the same boundary cell without any borders or beacons. The average handoff overhead per boundary cell represents the ratio of total CDMA traffic carried by a boundary cell over the CDMA primary traffic carried by the same cell. It defines the additional number of channel elements required to support handoff (virtual) traffic and provides an indication of network efficiency. The effect of the tuning parameters T brd , and C on the system performance is described next. Additional details may be found in [9,14]. The simulation is as described previously with similar parameters except as noted below. The CDMA network considered is loaded at approximately 40% of its pole capacity. The voice coding with overhead is assumed at 14.4 kbps (rate set II). The nominal traffic channel power is set at 2.7 W with a pilot channel power of 5.0 Watts. The remaining paging and sync control channels have a total power of 2.2 Watts. Every cell site is assumed to have a noise figure of 6 dB, cable loss of 2 dB and a 110 degree antenna with a nominal gain of 11.0 dBd for both the CDMA and analog cells. One CDMA channel is used from the cellular band centred at 881.52 MHz. CDMA soft handoff parameters are set at negative 15 dB, and a of 2.5 seconds is assumed. CDMA Border-Cell-Assisted Handdown We investigate the performance tradeoffs in network coverage, capacity and handoff with respect to the number of border sectors deployed per boundary site and the handdown threshold T brd . One, two, or three border sectors are deployed per boundary cell site and T brd varies from –2 to –6 dB. Figure 6(a) shows the variation of the average capacity loss versus the handdown threshold T brd . As the handdown threshold increases, the average Optimization of Dual Mode CDMA/AMPS Networks 75 loss in capacity also increases. This is because the probability of having CDMA border pilots in the active set decreases with an increase in T brd , resulting in more CAH events and hence a loss in CDMA traffic at the boundary cells. In addition, the loss in capacity increases as the number of deployed border sectors per boundary cell increases. When one border sector is deployed, no more than 20% of the offered CDMA traffic at the boundary cell is handed down to AMPS. However, a contiguous CAH area cannot be formed around the dual-mode footprint. The CAH area becomes contiguous when two border sectors per boundary cell are deployed and the capacity loss in this case is up to 50%. Finally, most of the CDMA traffic offered at the boundary cells is handed down to analog when three border sectors per boundary cell are deployed. However, in this case, the CDMA traffic loss may be as high as 90%, causing the CDMA coverage to shrink by almost one tier of cells. Figure 6(b) shows the impact of handdown threshold T brd on the average handoff overhead at the boundary cells. As the handdown threshold increases, an increase in the handoff overhead is also observed; this increase is more apparent in the case of three border sectors per cell. For the other two cases, the impact of T brd on the handoff overhead is not significant and is similar to that observed in the case of no border sectors at the boundary cell. CDMA Beacon-Assisted Handdown Alternatively, CDMA beacons were deployed either at the boundary CDMA site or at the AMPS sites within the first tier of analog network surrounding the CDMA footprint. The impact on the loss in capacity and handoff overhead was studied by varying the handdown threshold T beac from –4 dB to –16 dB. Figure 7 (a) shows the variation of the average capacity loss with the handdown threshold T beac . As observed, a decrease in handdown threshold leads to an increase in the average capacity loss. This is because the probability of having CDMA beacon pilots in the active set increases with a decrease in T beac resulting in more CAH events and hence a loss in CDMA traffic at the boundary cells. At thresholds below –12 dB, there is no impact on the amount of traffic handed down to analog because the interference at the boundary is lower than that at the core of the CDMA footprint. The CAH strategy of placing two beacons per cell leads to the highest loss in capacity on the order of 80%, causing the CDMA coverage to shrink by almost one tier. On the other hand, placing beacons at the first tier of AMPS cells leads to the minimum 76 Chapter 4 loss in capacity. In particular, thresholds of the order of –8 dB lead to a minimum loss in capacity while maintaining a contiguous CAH area. For thresholds below –8 dB, a contiguous CAH area is maintained for all three cases. Optimization of Dual Mode CDMA/AMPS Networks 77 Figure 7 (b) shows the impact of handdown threshold T beac on the average handoff overhead at the boundary cells. Similarly with the average capacity loss, a decrease in the handdown threshold leads to an increase in the handoff overhead. Lowest handoff overheads are observed in the case when one beacon sector is deployed at the AMPS sites outside the CDMA footprint. In this case, the overhead is even lower than that observed when these are no beacon sectors at the boundary cell. The impact of T beac at values above –12 dB is more apparent for CAH strategies involving one or two beacons at the CDMA boundary cells. Cell Size - Assisted Handdown The last CAH strategy investigated involves the deployment of beacon sectors with limited cell sizes at the network boundaries. Two sub-cases are considered depending on the number of beacons (one or two respectively) deployed on every CDMA site located at the boundary tier of the CDMA network. To reduce the large search space, the handdown threshold was fixed at –15 dB. Figure 8 shows the impact of varying cell size to the average CDMA traffic loss per boundary cell and the associated handoff overhead. As the cell size C increases, the CDMA traffic carried by the outer sites decreases, or equivalently, the capacity loss increases. This lost CDMA traffic is handed down to AMPS. This is expected, since a large cell size defines a larger CAH region over which users are handed down to AMPS when they detect a strong beacon pilot. In the case of one beacon deployed per boundary site, the CDMA capacity loss increases from 15% to 57% as C is varied from 0.5 to 1.5. A similar trend is observed when two beacons are deployed per boundary site. However, the CDMA traffic carried in the latter case is significantly less when compared to that of the former. This is expected since only one sector in these sites is equipped to carry CDMA primary traffic. Another observation is that when C is lower than 1, the CAH region does not remain contiguous anymore. This would be undesirable since it does not facilitate graceful CDMA to AMPS handdown events over the entire CDMA boundary. For higher values of C, CAH contiguity is maintained although the CDMA traffic carried is significantly reduced as illustrated before. The cell size also affects the handoff overhead at the boundary sites. The two-beacon per site deployment scenario appears to be very sensitive to variations in C; even a small increase in C makes the handoff overhead high. On the other hand, variations in C appear to have a minimal impact on the handoff overhead when one beacon per site is deployed. In summary, a cell 78 Chapter 4 size on the order of the cell radius provides a contiguous CAH region while maximizing the CDMA traffic carried and keeping the handoff overhead low. Optimization of Dual Mode CDMA/AMPS Networks 79 [...]... Ganesh and V O’Byrne, “Improving System Capacity of a Dual-Mode CDMA Network, ” Proc of IEEE ICPWC, pp 42 4 42 8, 1997 [10] H Chan and C Vinodrai," The Transition to Digital Cellular", 40 th Vehicular Technology Conference, pp 191-1 94, Orlando FL, May 1990 [11] H Stellakis and R Ganesh, "CDMA to AMPS Handdown Strategies In a Dual-mode Cellular Network" , in Proceedings of Int’l Conference on Communications (ICT'98),... microcell in an existing CDMA network could be a very efficient way to improve hot-spot capacity and dead-spot coverage  84 Chapter 5 1 INTRODUCTION Microcells play an important role in expanding an existing wireless network Code Division Multiple Access (CDMA) communications systems have been deployed and commercialized all around the world [1] The traffic demand for CDMA wireless systems has grown rapidly... system is a function of the underlying analog network and its performance and any capacity, coverage or quality benefits of the overlay network will be affected by the underlying technology and how it has been deployed With the advent of 3G type services, like wireless web browsing, several techniques are being considered to increase the capacity of the network while maintaining a consistent QoS One... Communications (ICT'98), Greece, June 1998 [12] H Stellakis and A Giordano, “CDMA Radio Planning and Network Simulation”, in Proc IEEE Int Symposium On Personal, Indoor and Mobile Communications, Taiwan, 1996 [13] M Wallace and R Walton, “CDMA Radio Network Planning,” Proc of IEEE ICUPC, pp 62–67, 19 94 [ 14] M Hata, ”Empirical Formula for Propagation Loss in Land Mobile Radio Services”, IEEE Transactions... to provide capacity improvements in digital cellular networks [16] Other techniques include Tower-Mounted Amplifiers (TMA) and/or super-conducting filters [17] which can provide several dB’s improvement in reverse link coverage, which is sufficient in many cases to correct coverage holes TMA’s have not Team- Fly Optimization of Dual Mode CDMA/AMPS Networks 81 been much used in the past due to maintenance... deployment time Microcell Engineering in CDMA Networks 97 for network operators The microcell system is fully integrated with the macrocell system for alarms, operation and maintenance management The CDMA network is a dynamic system CDMA cell sites interact with each other closely and dynamically When microcells are deployed in the middle of a commercial network, the interference and soft hand-off conditions...80 Chapter 4 In summary, we investigated performance tradeoffs for various CAH procedures and identified optimum values for the associated tuning parameters When border cells are used at the network boundaries, optimum performance may be achieved when Tbrd is on the order of 4 dB Similarly, optimum performance with CDMA beacons or cell sizes... deployed and commercialized all around the world [1] The traffic demand for CDMA wireless systems has grown rapidly in recent years The emerging next generation wireless systems are hierarchical systems to serve customers’ various needs Wireless networks consist of various layers of macrocells, microcells and picocells The macrocell service area radius is usually larger than 1 kilometer The microcell... defined as the intersection of the transmission curves in Fig 2 Around the middle of the cell boundary, the forward link pilot from both cell sites should be equal Therefore, we have Team- Fly Microcell Engineering in CDMA Networks 91 Since the transmission loss from the microcell is less than that from the macrocell, the pilot transmit power from the macrocell should be set to a higher level than that... the decreases, the soft hand-off area shrinks accordingly In a commercialized CDMA network, system capacity data must be obtained from a network performance monitor A practical way to calculate capacity is based on power and interference measurement data from the forward and reverse links Microcell Engineering in CDMA Networks 93 On the forward link, the transmit power percentage of various channels . CDMA Network, ” Proc. of IEEE ICPWC, pp. 42 4 42 8, 1997. [10] H. Chan and C. Vinodrai," The Transition to Digital Cellular", 40 th Vehicular Technology Conference, pp. 191-1 94, Orlando. TMA’s have not TEAMFLY Team- Fly ® Optimization of Dual Mode CDMA/AMPS Networks 81 been. parameters except as noted below. The CDMA network considered is loaded at approximately 40 % of its pole capacity. The voice coding with overhead is assumed at 14. 4 kbps (rate set II). The nominal traffic