Wireless Oom » Exclusive

What is TTI in umts?

wirelessoom — Wed, 17 Jun 2009 22:46:21 +0000

The MAC layer generates a new transport block every 10-ms (or multiple of blocks.)

A set of simultaneous transport blocks is the transport block set.

The transmission time interval (TTI) is the duration of each transport block set.

This can be 10, 20, 40, 80 ms.

TTI does not tell you data size. It tells you how often the transport channel data rate can be changed.

What is transport channel in umts?

wirelessoom — Wed, 17 Jun 2009 22:39:25 +0000

The transport channel is a concept applied to the interface between the physical layer and the MAC layer.

All transport channels are unidirectional.

The physical layer operates in 10-ms time slices in the connected mode (HSDPA case: 2-ms).

The data is sent using transport blocks.

Channelization and scrambling in WCDMA

wirelessoom — Wed, 17 Jun 2009 21:28:34 +0000

The spreading procedure in the UTRAN consists of two separate operations:

1. channelization

2. scrambling

Channelization uses orthogonal codes. Scrambling uses PN (Pseudo-Noise) codes.

Downlink:

channelization is for differentiating users.

scrambling is for differentiating base station.

Uplink:

channelization is for differentiating channels per UE

scrambling is for differentiating users.

———————

Orthogonality requires time synchronous properties. Thus, it cannot separate different uplink users as all UEs are unsynchronized in time.

What is the physical channel throughput in WCDMA with SF=4 and QPSK ?

wirelessoom — Wed, 17 Jun 2009 20:55:39 +0000

WCDMA has 3.84 Mcps (chip per second).

One 10-ms radio frame is divided into 15 slots.

1000 ms has 3.84 10^6 chips

10 ms has 3.84 10^4 chips

15 slots have 3.84 10^4 chips

1 slot has 38400/15 = 2560 chips

downlink spreading factor from 4 to 256

uplink spreading factor from 4 to 512

With spreading factor 4,

1 slot has 2560/4 = 640 symbols

15 slot has 640 * 15 = 9600 symbols

10 ms has 9600 symbols

1000 ms has 9600 * 100 = 960K symbols

In QPSK, 1 symbol has 2 bits.

Thus, the answer is 1.92Mbps

Note that this is only for one channel (i.e., code)! One user may have several channels simultaneously.

What is fundamental principle of spread spectrum?

wirelessoom — Wed, 17 Jun 2009 18:34:13 +0000

1. Binary XOR operation allows the original symbol reconstruction.

Do spread:

symbol sequence X (XOR) some code Y => some signal Z

Do despread:

some signal Z (XOR) some code Y => symbol sequence X

—————

2. Let’s assume there are 10 users who want to send its data X1, X2, …, X10.

Let’s assume they have their own codes Y1, Y2, …., Y10.

Let’s assume they send data at the same time.

Let Z be all summation of Xi (XOR) Yi .

I’m only interested in X1 signal. But, how can I extract X1 from all summed signal Z ?

If Yi are othogonal (i.e., Yi XOR Yj => all zero if i and j are different), then

Z (XOR) Y1 => X1

This is the fundamental concept of spread spectrum.

Why wide bandwidth is needed for high data rate?

wirelessoom — Wed, 17 Jun 2009 18:11:52 +0000

For high data rate, simply send fast.

Which one is high data rate ? 2 bits for 1 second vs. 2 bits for 6 seconds.

So, if you want a high data rate, your sending rate should be fast.

Time and frequency relationship is reverse. Short time duration means wide bandwidth.

That’s why wide bandwidth is need for faster transmission.

Why smaller spreading factor can achieve higher data rate

wirelessoom — Wed, 17 Jun 2009 18:09:53 +0000

First, think about the definition of spreading factor (SF): the number of chips for each symbol.

Second, remember that a chip duration is fixed.

Third, what is symbol? Each symbol can represent some amount of bits. e.g., QPSK, each symbol represents 2 bits.

SF: 2 SF vs. 512 SF
Time: 2*delta vs. 512*delta
Data: 1 symbol vs. 1 symbol

where delta is chip duration.

With 2 SF, you are sending symbol fast (my wife cooks very fast, so she can make lots of food)
With 512 SF, you are sending symbol slow (I cook very slow, so I can can make less amount of food than my wife)