networks & systems laboratory> research> current projects> simulation of 3G TDD-CDMA systems, exploiting asymmetry

Simulation of 3G TDD-CDMA Systems, Exploiting Asymmetry
Smart Internet Technology Research Group

Aims

- Accurately simulate the MAC layer characteristics of a TDD CDMA System

- Dynamically Exploit the Asymmetry between Downlink and Uplink bandwidth allocation.

- Minimize intercellular interference whilst maximizing throughput

Introduction

- With the advent of 3G networks there is expected to be an explosion in mobile computing, both for macro and micro device users. The main use that data users will be putting these networks to is internet and multimedia content browsing.

- More and more the applications used by mobile users will be geared towards having a higher downlink bandwidth than uplink bandwidth. This is mainly due to the information gradient between a mobile user and the internet. It is estimated that by 2005 the required downlink traffic will be approximately 4 times that of the uplink data traffic [Chaudry. P 1999]

- TDD 3G networks allow for dynamic asymmetry between uplink and downlink channels And this asymmetry can be exploited to provide optimal bandwidth to the users of the system.

Traffic Model Types

- Voice VBR with an average of 8Kbps in both the UL and DL direction

- Video VBR with an average of 96Kbps in both the UL and DL direction

- Data both VBR and CBR with an average of 96Kbps in the DL direction and 8Kbps
in the UL direction

System Model

- The system parameters used to design the simulator are those of the TDD-CDMA proposal by the UMTS.

- Primarily the object of the simulator is to determine the effectiveness of different MAC protocol schemes, including a novel one I have proposed.

- The simulator determines the performance of the system based on the ETSI UMTS Evaluation criteria.

TDD Frames

- Frames in TDD CDMA systems are conceptually different to the other major 3G technology which is WCDMA

- Frames are broken into 2 halves Uplink (Mobile to Base Station) and Downlink (Base Station to Mobile)

- The first and last timeslots of a frame are used for signalling on the downlink and uplink respectively

- This leaves a further 13 timeslots which can be used for traffic.

- There are a fixed number of codes per timeslot, depending on the spreading factor

- The number of timeslots for the uplink Tu and the number for the downlink Td can be allocated dynamically to ensure that there is optimal throughput of traffic in both directions in every frame.

- Maximising the throughput of bursty traffic is a difficult problem.

*click image to enlarge

Algorithm To Exploit Asymmetry

- Bit Error Rate in a mobile system can be decomposed into two components:

- The Transmission BER due to losses encountered through path loss and fading and other physical layer characteristics.

- The Packet Loss due to a packet not being scheduled in time.

- Different types of packets have different times to live.

- A Fair Packet Loss Sharing Algorithm can be used to schedule users calls in such a way that any packet loss due to packets not being scheduled in time is shared fairly amongst all users.

- To do this the number of packets lost for each active user needs to be monitored.

- Priority is given to packets which must be sent in the next frame or be lost because of TTL requirements. Priority is then given to users how have previously lost packets.

The Simulator

- A discrete event based Simulator. Voice and Video calls are scheduled based on a Poisson process, where as the Data length for Data calls is based on a Pareto distribution.

- Programmed using Object Oriented Methodology in Java.

- Includes a basic model of intercellular interference. In a TDD system If a timeslot in adjacent cell is allocated to a different direction it is possible to have interference.

- In the diagram the Base Station in Cell 1 is transmitting (DL) and hence interfering with the Base Station in Cell 2 which is trying to receive (UL).

Future Work: New TDD Systems

- This research maybe able to applied not only to 3G mobile technology but also to 4G technology as new systems will support more data oriented communications TDD is the most applicable option.

- Simulation to determine how well such a system would function in a heterogenous mobile environment, that is TDD cells could operate inside FDD WCDMA cells using either the DL or the UL frequency to provide shorter range data centric services as opposed to the more symmetric based long range services catered to by WCDMA.

- Harmonisation of Transport layer protocols with those in the MAC layer.

- Development of future applications for dual service TDD/FDD handsets so as to utilise the network to its full capacity and provide the user with beneficial services.

References

1. Yomo. H and Hara. S; An Up-link/Down-link Asymmetric Slot Allocation Algorithm in CDMA/TDD- Based Wireless Multimedia Communications Systems. IEEE 2001

2. Huang V and Zhuang W; QoS-Oriented Access Control for 4G Mobile Multimedia CDMA Communications. IEEE Communications Magazine 2002

3. Chaudry. P; 3GPP Proposal for IMT-2000, IEEE Communications Magazine 2002

Contact

Derek J Corbett
Professor David Everitt

 
 
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