When the receiver attempts to decode the signal using sender1’s code, the data is all zeros, therefore the cross-correlation is equal to zero and it is clear that sender1 did not transmit any data. From Wikipedia, the free encyclopedia. Retrieved from ” https: Since this cannot be perfectly controlled in a mobile environment, each time slot must have a guard time, which reduces the probability that users will interfere, but decreases the spectral efficiency. It was placed in the trunk of the vehicles of high-ranking officials and used a standard handset in the passenger compartment. A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. CDMA can also effectively reject narrow-band interference.

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Asynchronous CDMA offers a key advantage in the flexible allocation of resources i.

AX ZX HSUPA USB Modem | tecH Specs

A is also resistant to jamming. Since each of the 64 Walsh codes is orthogonal to all other, the signals are channelized into 64 orthogonal signals. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal-code, time-slot or frequency-channel resources. An example of orthogonal functions is shown in the adjacent picture.


This allows several users to share a band of frequencies see bandwidth. An example of 4 mutually orthogonal digital signals is shown in the figure below.


Gold codes are an example of a PN suitable for this purpose, as there is low correlation between the codes. The data signal is also binary and the time multiplication is achieved with a simple XOR function.

A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a single correlation tuned to the path delay of the strongest signal. Channel access methods Media access control.

Spread spectrum in digital communications.

Retrieved from ” https: Channel access methods Packet radio Link protocols. A PN code is a binary sequence that appears random but can mdoem reproduced in a deterministic manner by intended receivers. This is sufficient for keyboard-to-keyboard contacts between stations and for accessing local bulletin board systems and DX clusters.

If all of the users are received with the same power level, then the variance e. In the interest of brevity, the rest of this example uses codes v with only two bits. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero mdoem possible. These vectors will be assigned to individual users and are called mdem codechip codeor chipping code.

Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices. Each user in synchronous CDMA uses a code orthogonal to the others’ codes to modulate their signal. A jamming signal only has a finite amount of power available to jam the signal.


By using this site, you agree to the Terms of Use and Privacy Policy. Statistical multiplexing variable bandwidth. This is not true for asynchronous CDMA; rejection of unwanted signals is only partial.

HDLC specifies the syntax, but not the semantics, of the variable-length address field of the frame. Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal. In CDMA cellular, the base station uses a fast closed-loop power-control scheme to tightly control each mobile’s transmit power.

However, a viable widely adopted successor to AX.

Synchronous CDMA exploits mathematical properties cdmx orthogonality between vectors representing the data strings. A spreading code is a pseudo-random code that has a narrow ambiguity functionunlike other narrow pulse codes.

Retrieved from ” https: To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme where each transmitter is assigned a code.

Views Read Edit View history. HDLC protocols identify each frame by an address.