Microsoft Word CellBreathing

3.
 
Code Assignment in CDMA 
In Wideband Code Division Multiple Access (WCDMA) based system like UMTS, 
Orthogonal Variable Spreading Factor (OVSF codes) is the physical resource. In case of 
CDMA 2000 based systems, the counterpart is Walsh codes. In WCDMA, the OVSF 
codes are used to provide different channels to different users with different data rates. 
The signal transmitted from the Node B to the UE has a symbol rate given by RC/NS, 
where RC is the chip rate and NS = 512, 256, 128, 64, … is the Spreading Factor (SF). 
Since the chip rate is fixed, the data rate is determined by the SF. In CDMA, when the 
users data rate is lowered more codes become available in the downlink at the same time 

---

interference is reduced in the uplink. Similarly, in CDMA 2000, as each transmission in 
the downlink link requires Walsh codes and the chip rate is 1.2288 Mcps. A data channel 
at 153.6 kbps has symbol rate of 307.2 ksps (4*307.2k=1.2288 Mcps). Thus for a data 
rate of 153.6 kbps, 4 Walsh codes are available. When the data rate is lowered, more 
Walsh codes become available which can be used to accommodate more users.
In our proposed algorithm, if the density of MS/UE increases the system will 
increase the spreading factor of existing user based on their traffic class and thus the 
symbol rate will decrease to keep the interference within bound. Thus the cell will not 
breathe and border users will continue to communicate. 
4.
 
Proposed Algorithm 
Our proposed algorithm is based on location information of the MSs. It is assumed that 
the MS is GPS enabled and periodically informs the BTS/Node-B about it location. The 
location of MS can also be obtained by using cell identification, signal strength, Angle of 
Arrival (AOA), uplink time (Difference) of arrival, Downlink observed time difference 
and Database correlation. Details of these methods can be found in [Cabrera 2001, 
Pavlous]. Yet, we recommend GPS because BTS in CDMA-2000 based systems are 
already using GPS for synchronization. 
The flow chart of proposed algorithm is depicted in Figure 1 and a step by step 
description of the same follows: 
•
It is assumed that MS (UE) is GPS enabled and it transmits its location 
information periodically to the BTS/Node-B. 
•
In case of multiple uses follow the same trajectory, like passengers travelling in a 
train or a bus, the BTS monitors the trajectory (railway track or highway) and 
inform the BS/RNC. 
•
The RNC based on trajectory information decide about the target Node B and 
inform the target BTS/Node-B. 
•
The target Node B checks for the availability of resources (code) for the 
approaching users at that instant. 
•
If the resources are available then multiple users are allowed to handover the 
calls/sessions and codes are assigned to them. 
•
If the resources are not available then based on the traffic classes (Streaming, 
Conversational, Interactive and Background) of users, the following decisions are 
made and resources are made available. 
o
Higher SFs (longer OVSF codes or Walsh codes) are assigned to users 
having either Interactive (like web surfing or FTP) or background (like 
email) traffic class, thereby lowering the data rate of the existing users. 
o
If still interference is not within bounds, then assign longer SF to users 
having conversational or streaming traffic class.
•
In this way, code will become available for the predicted Group handover and 
resources are thus reserved for a specific time. 
Thus in this way lower data rate will reduce interference and at the same time system 
capacity will be increased as codes will be made available for the new users for which 
the Group handover has been predicted. Consequently, Cell Breathing will be avoided. 

---

Figure 1. Flow Chart for the proposed Algorithm