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Principles of CDMA

This presentation gives a basic overview of CDMA and PN Offset Planning

Sagar Hebbale

on 15 February 2011

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Transcript of Principles of CDMA

Principles of CDMA Need for CDMA
FDMA-Channelizes using Frequency. Each user has his own private frequency

TDMA- A user's channel is a specific frequency but it only belongs to a user for a paricular period of ime in repeaed sequence. Disadvantages of FDMA
Capacity improvement depends on
Improving SNR or SIR.
Maximum Bit rate per channel
is fixed and small.
Guard Band between two cells
results in wastage of Capacity. Disadvantages of TDMA
Disadvantage using TDMA technology
is that the users has a predefined time slot.
When moving from one cell site to other,
if all the time slots in this cell are full the
user might be disconnected. CDMA
Each userse signal is a continous unique code pattern burried within a shared signal mingled with other users code pattern.
If the users code pattern is known the presence or absence of a data can be detected. Advantages of CDMA
One of the main advantages of CDMA is that dropouts occur only when the phone is at least twice as far from the base station. Thus it is used in the rural areas where GSM cannot cover.
Another advantage is its capacity; it has a very high spectral capacity that it can accommodate more users per MHz of bandwidth. Principle of CDMA
The goal of CDMA is to break up a finite radio frequency so that multiple users can access it at the same time.
Through randomization of frequencies, CDMA divides the radio frequency spectrum to share the space for multiple users using a code to accomplish the multiplexing instead of time or frequency division.
This ability to use codes to divide up the signal, provides CDMA with a great advantage over GSM and other wireles Concepts of CDMA
Spread Spectrum

Frequency Reuse Spread Spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth.
The signal occupies a bandwidth much greater than that which is necessary to send the information.
The bandwidth is spread by means of a code which is independent of the data.
The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.

Assume two sequences x and y For instance consider two sequences x and y The properties of such a code are as follows:
Each code has equal ones and zeros.
The cross corellation between the codes is zero (or very small)
The scaled dot product of each code is unity.

Now we will illustrate how such a code can be used for Multiple access
c2.c2=1 CDMA Spreading Techniques
Walsh Codes- A 64 Chip Long Code
64 Available Walsh Codes
Mutually Orthogonal
Lasts 1/19200 secs
PN Short Code- 32K Long
Lasts 26.66 ms
Generated in a 15 bit Tapped Shift Register
PN Long Code- (2^42)-1 Chips Long
Lasts 40+ Days
Generated in a 42 Bit Tapped Shift Register

What you do, you can undo! Generating Code channels allows
us to use a universal frequency for all cells CDMA Channels
Forward Channels
1. PILOT CHANNEL- is a reference channel which the mobile station uses for acquisition, timing and as a phase reference for coherent demodulation. It is transmitted at all times by each base station on each active CDMA frequency. Each mobile station tracks this signal continuously.
2. SYNC CHANNEL carries a single, repeating message that conveys the timing and system configuration information to the mobile station in the CDMA system.
3.PAGING CHANNEL’ primary purpose is to send out pages, that is, notifications of incoming calls, to the mobile stations. The base station uses them to transmit system overhead information and mobile station- specific messages.
4.FORWARD TRAFFIC CHANNELS are code channels used to assign call (usually voice) and signaling traffic to individual users.
Reverse Channels
1.Access Channels are used by mobile stations to initiate communication with the base station or to respond to Paging Channel messages. The Access Channel is used for short signaling message exchanges such as call originations, responses to pages, and registrations.
2.Reverse Traffic Channels are used by individual users during their actual calls to
transmit traffic from a single mobile station to one or more base stations.
PN Offset Planning
Delay Spread

Pilot Search Window

PN Offset Delay Spread is a parameter that describes the time spread of the multi-path components for certain environments.
The delay spread or excess delay spread, is defined as the time window TD during which the multi-path energy is within X dB of the maximal peak. Commonly, X is about 10 dB.
A rake reciever is used to separately receive and demodulate the various multipath components of a signal. It combines these components to generate a single composite signal.

a)Delay spread directly determines the appropriate search window size for active pilots.
b)Delay spread must be incorporated in all equations that deal with ‘co-time’ pilot existence.

Sa= 2*(DS/Tc)
Sa= active Pilot Search Window size
DS= Delay Spread
Tc= Chip Duration

The MS has the capability of tracking up to six active pilots and monitoring powers of up to twelve other pilots that belong to neighbor set.
Besides that, the mobile also checks for powers of other pilots (remaining set pilots).
Due to propagation delay and multi-path, arrival times (offsets) of the pilots might be quite different from their default values.
For this reason, MS has to search for the exact PN offset of the pilot.
Both speed and accuracy of search process are limited by the parameter called search window size.
There are three types of search window size : search window for pilots in active and candidate set (Sa), for neighbor set pilots (Sn) and for remaining set pilots (Sr). PN Short Code

Co PN Offset
Adjecent PN Offset
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