APPLICATION OF A VIRTUAL CONNECTION TREE OVER MULTIPLE ACCESS TECHNIQUES FOR 3G MOBILE COMMUNICATION SYSTEMS
A. Heath and R.A. Carrasco
School of Engineering and Advanced Technology, Staffordshire University
PO BOX 333, Beaconside, Stafford ST18 0DF, Staffordshire, U.K.
Emails: A.L.Heath@staffs.ac.uk, R.A.Carrasco@staffs.ac.uk 
Abstract: This paper evaluates a Virtual Connection Tree (VCT) algorithm for mobile Asynchronous Transfer Mode(ATM) handover that offers service adaptability and efficient allocation of wired resources is applied to different multiple access techniques. This creates a more distributed system where decisions about traffic conditions in different areas can be evaluated at the Base Station (BS) level and not the Master Switching Centre (MSC) level.

Keywords: 3G, VCT, ATM, Mobile Communications

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1	Introduction
In future third generation (3G) systems it is anticipated that wireless, wired and satellite communications will be integrated in one system where multimedia data will be transferred (Godara et al 1999). Data traffic is growing at a much faster rate than voice, so new and efficient mediums are required to transport this information. A medium that can be used is ATM technology, which is a wired infrastructure that provides excellent QoS. This paper presents a way in which Mobile ATM can be used to improve the QoS for many users in a network.
Extensive research has been conducted to determine the best multiple access technique for future mobile communication systems where CDMA, TDMA, PRMA, FDMA and combinations of these are employed. This paper considers the advantages of applying the VCT algorithm developed in Larrinaga et al 1997 to each of the aforementioned technologies. In the future it is thought that the MSC will be removed from the system model, as in ad-hoc networks. The use of the Call Admission Control Algorithm a more distributed system is created.
Section 2 examines how the VCT can be applied to various multiple access techniques, while section 3 presents call/carrier communication algorithm. Section 4 details the simulations and a discussion of the results and finally section 5 is used to draw conclusions and suggestions of future work.
2	Virtual Connection Tree
VCT is an ATM oriented strategy that avoids the need to involve the MSC during handoffs. The VCT consists of cellular BSs with radio transceivers connected to switching nodes thorough  and ATM wired infrastructure and mobile stations (MS), which transmit information over the shared radio channel (Larinaga et al 2000).
In order to apply the VCT to the multiple access scheme, at call set-up each MS connection is assigned a the Virtual Connection Identifier (VCI) in the VCT. Each VCI is uniquely associated to a carrier for time division duplex (TDD), or a pair of carriers for frequency division duplex FDD. The VCI-VPI combination represents the carrier and BS used at any moment by a given connection. The numbering structure is shown in Figure 1. Each BS is associated to a Virtual Path Identifier (VPI) and each mobile connection is assigned a group of VCIs, each VCI corresponding to a carrier. During a handoff the VPI changes, indicating a new BS and the VCI could change indicating a change in carrier.

Figure 1 - VCT-CRDMA Integration
The handoff procedure for MS 3 is illustrated in Figure 1.  The VPI-VCI information allows the MSC to locate any MS at any time, cells are routed correctly given this information.
By using the VCT with multiple access techniques no handoff processing is carried out at the MSC since all possible routes are pre-established. This implies a reduction in the amount of signalling and processing during a handoff, which improves the GoS. The traffic can be evaluated at the MSC since the VCNs provide the carrier frequency and BS information of every mobile without any additional signalling.
3	Carrier Communication Algorithm
In Larrinaga et al 1997 a carrier communication algorithm was presented. This involves passing in real-time, the information gathered at the MSC to all BSs in the VCT, such that a BS creates a ‘map’ of the carriers used in adjacent cells. Each BS has its table similar to that in CSMA, which contains information about its own and interfering cells. The size, S of this table, the carrier-map table, depends on the number of cells considered as interferers, Bi as well as the maximum number of carriers/codes allowed in each BS, C, and is given by:
                                               (1)
Another table is also required, the VCT active user table, in which all active users are registered relating to the MB ID number. The tables and the area they refer to are illustrated in Figure 2.

Figure 2 VCT, Carrier-map and tables
Both tables are immediately updated when changes such as new connections, handoffs and terminations occur within the VCT. The active user table is only affected by connection set-ups and terminations. The carrier-map can be updated in one of two ways; if the change is inside the cell, the BS directly updates its own table. If the change occurs outside its cell, the BS waits for the information from MSC. The set-up and termination processes are shown in Figure 3.

Figure 3 – Set-Up and Termination Process
The handoff process is illustrated in Figure 4.

Figure 4 – Handoff Process
Each change requires a different frame to be generated by the MSC, these are shown in Figure 5.
a)
b)
Figure 5– a)TDMA, FDMA, PRMA CCA, b) CDMA Frames
The carrier/code communication algorithm introduces distributed dynamic channel allocation schemes (FDMA, TDMA, PRMA) can easily be implemented since information about frequency usage is provided so the amount of interference is reduced. The system is distributed since BSs store information gathered and administer call admission control. In CDMA this is also useful as the MS can use this information to cancel out the interference from these cells.
4	Evaluation, Results and Discussion
A combination of the VCT-multiple access technique and carrier/code communication algorithm is evaluated. The bandwidth required for the carrier communication algorithm is determined. A comparison between FCA and DCA for TDMA, FDMA and PRMA and the advantages in CDMA are then evaluated. The saved amount of signalling at the MSC is then evaluated, when calls are admitted/declined at BS level. A cellular system is used to simulate these situations, using the parameters according to Table 1.
Area (M)	77.94 miles2	
No. of BS	16	
Call gen rate (avg, μ)	1, 5, 10 call/sec	
Simulation Duration	500 3600 sec	
Channels per BS	50	
Cluster sizes	3, 4 and 7	
No. channels and slots	FDMA 50; 1
TDMA 17; 3
PRMA 5; 10	
Speed	20 - 70 miles/h	
Length	30 - 240 sec.	
Directions (max)	8	
Table 1
The signalling bandwidth was measured by adding the bandwidth for the carrier/code communication algorithm, set-ups, handoffs and terminations. The bandwidths are illustrated in Figure 6, where the peak values are worst case conditions.

Figure 6 - MSC-BS Bandwidth.
The peak bandwidth varies from 5.76kbits/sec (15 ATM cells/sec) for 25 BSs at 1 call/sec generated, to 26.88kbits/sec (70 ATM cells/sec) for 49 BSs at 10 calls/sec. The bandwidth demand is small considering the rates used in ATM networks (150Mbit/sec). This occupancy may be reduced if the signalling channel is shared with other control information.
FCA and DCA are now compared in terms of failing probabilities for the VCT applied to different multiple access techniques. 
With FDMA a system consisting of 16BSs each with 50 channels is considered. For a given failing probability DCA allows a higher intensity of traffic than FCA, until about 4%, see Figure 7. Improvements with respect to FCA of 0.87, 0.63 and 0.48 Erlang/mile2 are shown for cluster sizes of 7, 3 and 4 respectively at a failing probability of 1%. DCA increases the capacity of the system, so more connections are admitted and less ongoing calls are dropped.

Figure 7- Failing Probability – FDMA
If the cell size is reduced the traffic intensity can increase compared to that with FCA.
The TDMA has 17 carriers with 3 slots for each of the 16 BSs. Again DCA is more efficient than FCA only up to a limit of 1.5% failing probability, this is not as efficient as FDMA see Figure 8. For 0.5% the improvement in traffic intensity for DCA with respect to FCA is approximately 0.43, 0.2 and 0.12 Erlang/mile2 for clusters of 7, 3 and 4 respectively at a failing probability of 0.5%.

Figure 8 - Failing Probability TDMA
This reduction in the efficiency is caused by when a channel is borrowed from an adjacent cell in TDMA three slots are locked, which can not be used by any interfering cells, whereas in FDMA only one carrier is provided.
For PRMA the system consists 16 BSs with 5 carriers with 10 slots each, FCA shows higher traffic intensity for the same failing probability with respect to DCA. DCA techniques where carriers are borrowed from one BS to another do not work well with packet switching multiple access techniques, instead a dynamic slot selection technique is required. This may improve the efficiency of TDMA systems also, this requires further investigation.
When applying the VCT to CDMA, the BER comparison for a conventional and Parallel Interference Cancellation (PIC) detectors is illustrated in Figure 9. The curve referred to as ‘1 user (no interference)’ represents a single BS with one user transmitting, and has the best performance obtainable in a given channel. The introduction of other users and cells creates interference, which reduces the BER curve. ‘3 cell cancellation’ and ‘Own Cell Cancellation’ represent the PIC detector.

Figure 9 BER/SNR Comparison Rician Channel (k = 10 dB) CDMA
3 Cell Cancellation shows an improvement from 3 Cells (no cancellation), this is due to the information about interfering codes. These improvements in the BER performance can be translated to an increase in capacity, or number of users per cell. If the number of BSs in the area decreases the probability of a call failing increases, see Figure 10. As would be expected the number of handoffs increase when the cell size is reduced, so a trade off must be made and since the BS can authorise handoff without MSC interference, decreasing the cell size is the more feasible option for increasing the capacity.

Figure 10 – Failing Probability CDMA
The processing at the MSC can be reduced using the call admission control algorithm. A  threshold value of traffic is set so that BS perform admission control. The signalling in an FDMA, FCA allocation model with different thresholds has been measured. The number of new connections in a centralised system would be infinite and this signalling may cause congestion problems. Figure 11 illustrates the signalling that would be employed in a centralised system.

Figure 11 - Signalling needed to Block New Connections (Centralised).
In a distributed system, with threshold traffic values this congestion would be avoided at the cost of a higher call failing probability, as shown in Figure 12.

Figure 12 – Threshold Failing Probability
The call dropping probability with different thresholds is greatly reduced, since some channels are saved for handoffs. Using the threshold method, once a call is accepted it has less chance of failing, but less calls are accepted, the optimum threshold is approximately 0.75.
5	Conclusion
Applying the VCT to all the different multiple access methods has proved advantageous, in the reduction signalling traffic. The peak bandwidth of the algorithm varies from 5.76kbits/sec to 26.88kbits/sec this is small considering the rates used in ATM networks (150Mbit/sec).
DCA for FDMA provided improvements of 0.87, 0.63 and 0.48 Erlang/mile2 are shown for cluster sizes of 7, 3 and 4 respectively at a failing probability of 1%. Next was TDMA with 0.43, 0.2 and 0.12 Erlang/mile2 for clusters of 7, 3 and 4 respectively at a failing probability of 0.5%. In CDMA interference was removed from signals, improving the BER curve. There is more work required for the packet switched applications and maybe IP transport protocols.  For all multiple access methods are more distributed system has been created, for FDMA the signalling saved was shown, this is similar for all the multiple access schemes.
6	References
  Godara, L.C.; Ryan, M.J.; Padovan, N. ‘Third generation mobile communication systems: Overview and modelling considerations’ Annales des Telecomms/Annals of Telecomms v 54, n 1, 1999, p 114-136
  Larrinaga, F. and Carrasco, R.A., ‘Virtual Connection Tree Concept Application over CDMA Based Cellular Systems’ IEE Coll on ATM Traffic in the Personal Mobile Communications Environment, Savoy Place, London 11 Feb. 1997.
  Larrinaga, F. and Carrasco, R.A., ‘Application of the VCT over Multiple Access Techniques using Frequency Division’, submitted to IEEE Transactions. 2000