Tecnología cdma

Páginas: 7 (1532 palabras) Publicado: 7 de febrero de 2011
ESCUELA POLITÉCNICA NACIONAL

INGENIERÍA ELECTRÓNICA Y REDES DE INFORMACIÓN

COMUNICACIONES INALÁMBRICAS

ESCUELA POLITÉCNICA NACIONAL
INGENIERÍA ELECTRÓNICA Y REDES DE INFORMACIÓN
COMUNICACIONES INALÁMBRICAS

1. Define open loop, closed loop, and outer loop power control.
* Open loop power control, the mobile uses the received signal to estimate the transmission loss from themobile unit and the base station.

* Outer loop power control is used to maintain the quality of communication at the level of bearer service quality requirement, while using as low power as possible.

* Closed loop power control in the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in thedownlink.

2. What are the softer and soft handoffs?
Soft handoffs are an integral part of CDMA design. The determination of which pilots will be used in the soft handoff process has a direct impact on the quality of the call and the capacity of the system. Therefore, setting soft handoff parameters is a key element in the system design for CDMA.

In the uplink direction, soft handoffdiffers significantly from softer handoff: the code channel of the mobile station is received from both base stations, but the received data is routed to the base station controller (BSC) for combining. This is done so that the same frame reliability indicator as provided for outer loop power control is used to elect the better frame between two possible candidates within the BSC.

3.Differentiate between scrambling and spreading.

In the process called as bit scrambling the data sequence and code sequence are assumed to have the same length (one code bit for each data bit) and are used for encrypting the data bits.

To spread the data sequence, the code sequence must be much faster than the data sequence, and exhibit some random properties.
By multiplying the data sequence withthe faster code sequence, the resulting product yields a sequence with more transitions than the original data. Using suitable random-like codes, the resulting sequence will have the same rate as the code sequence.

4. What are the requirements for a DSSS?

In the DSSS system, the entire bandwidth of the RF carrier is made available to each user. The DSSS system satisfies the followingrequirements:

* The spreading signal has a bandwidth much larger than the minimum bandwidth required totransmit the desired information which, for a digital system, is baseband data.

* The spreading of the information is performed by using a spreading signal, called the code signal (see Appendix D). The code signal is independent of the data and is of a much higher chip rate than the datasignal.

* At the intended receiver, despreading is accomplished by cross-correlation of the received spread signal with a synchronized replica of the same code signal used to spread the data.

5. What are the basic differences between the DSSS and FHSS systems?

Las diferencias básicas son:

* DSSS avoids interference by configuring the spreading function in the receiver toconcentrate the desired signal but spread out and dilutes any interfering signal.

* FHSS can resist interference from spurious RF signals ten times better than DSSS.

* DSSS system is always transmitting on every frequency in the band, a nearby FHSS system may be unable to find any clear channel to hop to. In the presence of interference, FHSS usually degrades more gracefully than DSSS, butneither works well when competing at close range.

6. Which parameters are used to control the movements of the pilot in a CDMA system?

The pilot CDMA signal transmitted by a base station provides a reference for all mobile stations. It is used in the demodulation process. The pilot signal level for all base stations is much higher (about 4 to 6 dB) than the traffic channel. The pilot...
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