The RF Interface:
In older analog FDMA system, the user occupies one frequency channel for transmit (30MHz bandwidth for AMPS) and one for receive for the duration of a phone call. These transmit and receive channels are busy until a call has been completed. During peak hours, many subscribers are unable to access the system which results in lost revenue for a network operator, and increased frustration for a user. TDMA systems improve on this capacity issue by further subdividing a given bandwidth into time slots.
CDMA and W-CDMA systems use a much broader bandwidth then either FDMA or TDMA systems. Instead of dividing users up by frequency or time, they are divided into codes, specific data streams assigned to particular users. All users transmit at the same time and multiple users share the same frequency carrier. Each mobile user is uniquely identified by specialized code and frequency.
Key Technologies used :
- Diversity
- Soft & Softer hand off
- Rake receiver
- Power Control
Diversity:
- Time diversity :
Block interleaving, error – correction .
- Frequency diversity :
The CDMA signal energy is distributed on the whole 1.23 MHZ.
- Space diversity :
- The introduction of twin receiver antennas .
- The RAKE receivers of the mobile station and the base station can combine the
Signals of different time delay .
- During a handoff ,the mobile station contacts multiple base stations and searches for the Strongest frame .
Soft and Softer handoff :
· Handoff :
W-CDMA systems have several method to hand over communications from one cell site to another , depending on the proximity to neighboring Node Bs , environmental variables .This handovers are called hard , soft , and softer .
· Hard handoff :
A hard handoff involves the transfer of a mobile from one W-CDMA cell system on one RF to another W-CDMA cell system on a different RF or perhaps to a GSM cell system . it uses break for make technique where the MS clears the connection to the current Node B before connecting to the Node B that the MS is willing to handover .Hard handovers are characterized by the potential of failure . There is no feedback mechanism and no way to recover from a faulty handover .
· Soft handover :
A soft handover occurs on the same W-CDMA channel between two different cell sites . As a mobile travels between two cells, it signals to the network that it has found a new pilot . The network tell the new cell site to allocate a traffic channel for the mobile.This will continue until one cell is too weak to use . At that point , the mobile sends a handover completion message on the reverse traffic channel and begins to exclusively use the new cell site . The soft handover procedure ensures that a mobile is always in constant communications with a cell.
· Softer handover :
A softer handover is one that occurs between two sectors of the same Node B . The signals received within the Node B are combined and sent to the network.
Rake receiver :
Unlike other digital system , W-CDMA systems have designed mechanisms to enhance signal reception when multi-path environments or other RF signal impairments may occure . The mobile phone‘s receiver has three parallel correlators or rake receivers ,and the base station has four rake receivers. These rake receivers track individual signal paths independently , adjust for time offsets , sum the resultants ,and then demodulate the resultant signal .
Power Control :
Transmitpower on all users must be tightly controlled so their signals reach the base station at the same signal level and at the absolute minimum power level necessary to ensure acceptable service quality .
· Reverse Open Loop Power Control :
- The mobile station makes a coarse initial estimation of the required transmit power , based upon the total received power .
- Problems with Reverse Open Loop Power Control :
1. Assumes same exact path loss in both directions ; cannot account for asymmetrical path loss .
2. Estimate are based on total power received ; therefore the power received from other cell sites by mobile station introduces in accuracies .
· Reverse Closed Open Power Control :
- Consists of power up ( 0 ) & power down (1 ) commands sent to the mobile stations , based upon their signal strength measured at the base station and compared to a specified threshold .
- Each command requests a 1dB increase or decrease of the mobile station transmit power.
· Reverse Outer Loop Power Control :
- Most gradual form of reverse link power control .
- Set point is varied according to the FER on the reverse Traffic Channel ( determined at the Base Station Controller ) .
· Forward Traffic Channel Power Control :
- The base station slowly decreases power to each mobile station.
- As the FER ( determined at the mobile station ) increases , the mobile station requests a Forward Traffic Channel power increases .
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