Code Allocation in UMTS

Code Allocation:

Channelization codes, also called Spreading codes, satisfy two purpose: first, to spread the signal energy evenly over the bandwidth below the noise level ; and second, to separate the transmissions from a single source. These are based on the Orthogonal Variable Spreading Factor (OVSF) technique. Orthogonal codes are the codes that have the property that any two codes in the family, except for those that are connected to it and to the right, when multiplied together bitwise and the results of these multiplication are summed, the result is zero. That is, in mathematical way, codes in the family correlated completely with themselves and have zero crosscorrelation with any of the other codes. However, the zero crosscorrelation property of orthogonal codes works only if the codes are time aligned.

In Uplink, channelization codes are used to distinguish data and control channels from the same UE.
In Downlink, channelization codes are used to distinguish signals for different channels and users within a cell.

Scrambling codes, also called PN scrambling codes, are used to separate users and different base stations. They are used on top of spreading and it does not change the signal bandwidth. Also, scrambling codes are found to have better crosscorrelation property as compared to channelization codes even when codes are not time aligned. PN sequence or Pseudorandom number sequence is a sequence of numbers, algorithmically generated, that are distributed evenly throughout the number space but with no discernible pattern to their distribution.

In Uplink, scrambling codes are used to distinguish UE terminals.
In Downlink, scrambling codes are used to distinguish different cells.

ImportantInformation:
- Each signal is spread with the spreading code, i.e. channelization code x scrambling code
- Uplink codes are not cell specific. Channelization Code is picked by mobile and Scrambling code assigned by RNC. They can be decoded anywhere
- Downlink code is cell specific and UE must decode each of them individually

Power Control in UMTS

Power control is necessary to keep the transmitted signal power level under control so as to minimize the interference and keep the quality of signal to a desired level. The main functions are:
1. Closed-loop power control
· Outer-loop power control
- Uplink outer-loop power control
- Downlink outer-loop power control
· Inner-loop power control
- Uplink inner-loop power control
- Downlink inner-loop power control
2. Open-loop power control
· Uplink open-loop power control
· Downlink open-loop power control

Closed-loop power control is the power control mechanism used in UMTS to solve near-far problem, minimize interference and to keep the signal quality to optimum level. Closed-loop power control is used in uplink (UL) as well as downlink (DL). However, the motive in both the cases are different. In uplink, signals from different UEs reach NodeB with different power strength, thus causing the stronger signal blocking the weaker one, resulting in near-far effect. In downlink, there is no near-far effect, but the UEs near the cell-edge or in high interference region may need extra power to overcome the increased other cell interference and weak signal due to Rayleigh fading.

Closed-loop power control can be divided into outer-loop and inner-loop power control. In case of uplink, the RNC manages the outer-loop and Node B manages the inner-loop and for downlink, UE manages the outer-loop and Node B manages the inner-loop.

Inner-loop power control (also called fast closed-loop power control), operates at 1500 times per sec (1.5 kHz) [From where did this value of 1.5 kHz come from? Answer: A UMTS 10 ms frame consists of 15 TPC commands. This results in a power control frequency of 1500 Hz (15/10ms)] and relies on the feedback information from the opposite end of the link (or channel) to maintain the signal to interference (noise) ratio to a target level set by the outer-loop power control. The transmission power is increased or decreased by a certain fixed step size depending on whether the received SIR is below or above the target SIR. Precise power control can lead to optimum use of bandwidth resulting in increase cell capacity.
The UL inner-loop power control lets the UE adjust its output power in accordance with one or more TPC commands received in the downlink direction. Remember the increase and decrease in power is limited by upper and lower bounds as defined in 3GPP TS 25.101. The upper bound, i.e. UE maximum output power, is set depending on the Power class of UE. This can also be set below the maximum capability of the UE through signaling when the link is established. The lower bound, i.e. UE minimum output power defined as the mean power in one timeslot (TS), and shall be less than -50 dB.
The DL inner-loop power control is used to control the transmission power of downlink channels at Node B as per the TPC commands received from UE. However, in some situations Node B may ignore the increase/decrease these TPC commands. For example, in case of congestion (high load scenario), the Node B can ignore the TPC commands from UE.

Outer-loop power control is used to set the target quality value for inner-loop power control, i.e it adjusts the target SIR in Node B which is used during inner-loop power control. Now the question is why do we need to adjust the target SIR? Outer-loop power control tries to keep the quality of a connection to desired value. Too high quality will waste the resources.

Open-loop power control is used to set the initial power of UE (in random access) and downlink channels. The TPC commands used in inner-loop power control are relative, so it needs a starting point and this is defined by open-loop power control. Also, this is useful in setting the power level in case of common shared channels, where it is difficult to send each UE the necessary TPC commands. In case of uplink, UE and broadcasted cell/system parameters are used to set initial access power on RACH. And in case of downlink, the measurement report of UE is used to set the initial power of downlink channels.
The open loop power control tolerance is ±9dB under normal conditions and ±12dB under extreme conditions.

[References: TS 25.214, TS 25.215, Section 7.2.4.8 of TS 25.401]

Why chips are called so?

Chips are code bits used for spreading the desired signal. Chips are called so as they do not carry any useful information and to distinguish them from data (bits).

Technorati Tags: Chip, CDMA, UMTS

What are the main differences between LAPD and LAPDm in GSM?

LAPDm stands for Link Access Procedure on D channel (modified). This is a modified version of LAPD and is optimized for the GSM Air interface.
This is done in order to judiciously use the scarce radio resource. Some parts of LAPD frame are removed to save the resources. Some differences are:
1. In LAPDm, checksum is not used, as channel coding on layer 1 takes care of it(for error identification and correction).
2. In LAPDm, some of layer 2 control frames, viz. SABM and UA frames carry layer 3 information.
3. LAPDm does not support extended header formats.
4. LAPDm frames (one LAPDm frame carry a maximum of 23 octets) are shorter than LAPD frames (one LAPD frame carry a maximum of 260 octets).