What's the relation between TX power and data rate in UMTS ?

power data rate relation

Hi fellows ,

I would like to know if I have certain data to be transmitted , should the Node B increase the power if the data rate increases ?

Also I read the following in a reference about the RACH trasport channel :

RACH is applicable only on the uplink, and must be able to be heard over the whole cell coverage area. To achieve this means that it is limited to low data rates. The ability to support 16kb/s RACH is a mandatory requirement for terminals, regardless of the types of services they provide.

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Why RACH should be transmitted with low rate ?

data rate in relation to power

ahmed_nasr said:

Hi fellows ,

I would like to know if I have certain data to be transmitted , should the Node B increase the power if the data rate increases ?

Click to expand...

Salamou 'alaikoum!

What I know is that the chip rate is constant = 3.84 Mchips/sec.

therfore, the datarate is variable (witch is the bitrate) ok!

And you know in CDMA, the data is spread in large bande: more it is large, more we require less power to transmit (this is demontrated accoarding to the Shanon Equation), the reason why it is : because the spreading of the data in a large band, is a form of frequency diversity witch give us a gain (in WCDMA it is called : processing gain).

Now, in WCDMA/FDD we have a band of 3.84MHz with a constant chip rate. if we spread each bit on the 3.84MHz Band, we will require less power, if we spread two bits on this band at time so for each bit we will need half of this band, so we will require more power and so on. (this is just an example)

So more the bitrate is high, more the power needed is high, and so on.

Note : Accoarding to the 3GPP, the band of the WCDMA is 5MHz, not 3.84MHz, this is because the non selectivity of the filters (the diffrence is the garde band)

Excuse me for my bad explanation (lol).

Added after 8 minutes:

And there is another point, when we talk about the power of the NodB, we are not talking here about the radiated Electromagnetic power, but the power of the Information spread and send within this power.

The power of the NodB is constant, but it is shared between channels codes users and control channels, the nodB share its power between those accoarding to the Transmission Power control on the UP link.

Wa Essalam!