Basic concepts in the WCDMA Physical Layer

From http://blog.sina.com.cn/s/blog_4d0d56900100bs3y.html

 

Community breathing:

The CDMA network is completely different from the GSM network. Because the channels and users are no longer separated, there is no difference between traditional coverage and capacity. The larger the business volume of a residential area, the smaller the residential area. In CDMA networks, increasing business volume means increasing interference. The effect of dynamic changes in residential area is called "residential breathing ".

Remote effect:

Another typical issue of CDMA networks is the so-called near-far effect. Because all users in the same community share the same frequency, it is extremely important for the entire system to transmit signals at the minimum power .. In CDMA networks, you can adjust the power to solve this problem.

In the WCDMA system, multi-path propagation is no longer a negative factor, but an ideal result. Because the receiver can combine signals with a latency of at least 1 chip (UMTS network data transmission rate of 3.84 Mbps, that is, 1 Chip = 0.26 microseconds, equivalent to 78 meters) into valid signals.

3G Switchover plan:

When the mobile station slowly moves out of the original service area and is about to enter another service area, the link between the original base station and the mobile station will be replaced by the link between the new base station and the mobile station. This is the meaning of switching.

Switching is the content of mobility management. In 3G, the RRC layer protocol is mainly responsible for this function.

Switch category:

The switchover types can be divided into soft switchover, soft switchover, and hard switchover Based on the connection establishment and release between ms and the network.

Soft SwitchWhen a mobile station starts to contact a new base station, the communication with the original base station is not immediately interrupted. Soft switchover can only be used between CDMA channels with the same frequency.

Soft switchover is definitely a same-frequency switchover. It is not necessarily a soft switchover !!!!!!!!!

Differences between soft switchover and soft switchoverThe difference is that the soft switchover occurs in the same NodeB, And the diversity signal is combined with the maximum gain ratio in NodeB. Soft switchover occurs between two NodeB nodes, and the diversity signal is merged in RNC.

Hard SwitchThis includes three scenarios: Same frequency, abnormal frequency, and inter-system switching. Note that soft switchover is performed between the same frequency, but not between the same frequency. If the target cell and the original cell share the same frequency but belong to different RNC, and there is no IUR interface between the RNC, the same frequency will be hard switched. In addition, the intra-cell codeword switch is hard switched.

During hard switching of different frequencies and hard switching of different systems, the compression mode must be enabled to measure different frequencies and different systems.

Hard switching between different systems includes switching between FDD mode and TDD mode. In r99, there are also switching between the WCDMA System and the GSM system. In r2000, it also includes switching between WCDMA and CDMA.

Switchover typical process: measurement control-> measurement report-> switch decision-> switch execution-> new measurement control.

For details about the measurement process, see the WCDMA unlimited network planning document.

WCDMAAnd GSMInter-system switching:

The WCDMA and GSM standards support switching between WCDMA and GSM. These switches are used for coverage and load balancing. In the early stage of WCDMA configuration, it is necessary to switch to the GSM system to provide continuous coverage. Switching from GSM to WCDMA can be used to reduce the load of the GSM community. When the business volume of the WCDMA network increases, it is important to perform bidirectional switching due to the load. Switching between systems is triggered by the source RNC/BSc. From the perspective of the receiving system, switching between systems is similar to switching between RNC or BSc.

Compression mode:

WCDMA adopts continuous transmission and receiving methods. If the WCDMA signal is generated without gaps, the mobile station cannot use one receiver for inter-system measurement. Therefore, both measurement between frequencies and between systems requires compression mode.

The compression mode is introduced to perform cross-frequency measurement or cross-system measurement under FDD. Because a receiver can only work on a group of sending and receiving frequencies at the same time, to measure signals of other frequencies, the receiver must stop working and switch the frequency to the target frequency for measurement. To ensure the normal transmission of the downstream signal, the original signal must be sent within the remaining sending time. This is the downlink compression mode. When the measurement frequency is close to the uplink transmission frequency, in order to ensure the measurement effect, the uplink signal transmission must be stopped at the same time, that is, the uplink compression mode.

The Inter-system switching trigger is implemented in RNC, for example, the Mobile Station runs out of WCDMA coverage;

RNC command the mobile station starts measurement between systems in compression mode;

RNC selects the target GSM cell based on the measurement of the Mobile Station;

RNC sends a switch command to the mobile platform.

Switching between GSM and WCDMA systems is initiated by the GSM BSc. Because GSM adopts non-continuous transmission and receiving modes, the WCDMA measurement values obtained from GSM do not need to be compressed.

WCDMAIntra-frequency switching

Most UMTS carriers are from 2 ~ Three FDD carriers are available, and operators can start to operate at one frequency. The second and third frequencies need to be used to deal with subsequent increases in capacity. Several frequencies can be used in two different ways. For high-capacity sites, the same site can use several frequencies, or the macro community layer and micro community layer use different frequencies. These schemes must be supported for inter-WCDMA inter-carrier frequency switching.

Same as switching between systems, switching between frequencies also requires the same compression mode measurement.

Implementation of power control:

In the WCDMA system, the power control can be divided into two categories:Open-loop control and closed-loop control.

Open-Loop ControlIs a rough estimation of the initial launch power. It estimates the path loss and interference level based on the measurement results to calculate the initial transmit power. In WCDMA, open-loop power control is used for uplink and downlink. Open Loop uses uplink interference to estimate the downlink or estimate the uplink based on the downlink, which is not closed. The inner ring has a feedback ring, which is closed.

Uplink open loop:UE calculates the uplink initial transmit power by measuring the receipt power of CPICH.

Downlink open loop:Bts3812 measures the uplink channel interference level and reports RNC. RNC determines the downlink initial transmit power of bts3812 based on the measured value.

Closed-loop power controlIt is used to quickly adjust the power of upstream and downstream links during communication, so that the quality of the link converges to the target sir. Two algorithms can be used to control the closed loop of the uplink in the 3GPP protocol. In the two algorithms, the uplink operation step is 1 or 2 dB. The adjustment of the power control step on dpcch △dpcch = △tpc * tpc_cmd. Tpc_cmd is the TPC synthesis command obtained using different algorithms. The power of dpdch is set based on the power offset between dpdch and dpcch.

 

Closed-loop power control is divided Internal control and external control

Main functions of internal ring controlIs to control the transmit power of the physical channel, so that the received Sir converges to the target sir. In the WCDMA System, the corresponding power Adjustment Command is sent by estimating the received Eb/NO (ratio of bit energy to interference power spectrum density, EB/No corresponds to sir. For example, for the 3.84 kbps speech service, the typical Eb/no value is 5.0db, and the processing gain is 10log10 (12.2 m/K) when the bitrate is 3.84mcps) = 25db. So sir = 5db-25db =-20db. Load ratio (C/I)>-20db.

Upstream internal ring control:The function is to overcome the near-far effect, shadow effect, and path loss, and partially overcome the rapid decline. Bts3812 measures the Sir value of the uplink, and compares the measured value with the preset threshold (sirtarget). If the measured value is smaller than the threshold, the TPC (transmit power control, transmit power control) command. If the value is greater than the threshold, a TPC command is issued to the UE to reduce the power. UE quickly adjusts the power according to the received TPC command, and finally converges the quality of the uplink to sirtarget. Bts3812 the frequency of upstream inner power control is 1.5 kHz. The power control step can be 0db, 0.5db, 1db, 1.5db, and 2db.

Downlink internal ring control:The role is to overcome the shadow effect, path loss, and rapid decline. Similarly, bts3812 adjusts the transmit power of the downlink dedicated link based on the TPC Command sent by UE, so that the quality of the downlink will converge to sirtarget. The frequency of the downlink internal ring is 1.5 kHz. The step length can be 0 dB, DB, 1 dB, DB, and 2 dB. The dynamic range of the downlink internal ring can reach 25 dB.

Outer Ring ControlBy dynamically adjusting the Sir target value of the internal ring, the communication quality can always meet the requirements (that is, the specified fer/bler/BER value ). The outer ring Throttling is performed in RNC. Due to the complexity of the wireless channel, the power control based on the Sir value alone does not really reflect the link quality. For example, for static users, low-speed users (Mobile speed 3 km/h), and high-speed users (Mobile speed 50 km/h), the requirements for Sir are different based on the same fer. The final communication quality is measured by fer/bler/BER. Therefore, it is necessary to dynamically adjust the Sir target value based on the actual fer/bler value.

For the WCDMA-FDD system, because the upper and lower frequency band interval is large, so the fast decline of the upper and lower rows is completely irrelevant. Therefore, open-loop Throttling is not accurate for uplink situations based on the estimation of path loss produced by downstream signals. The solution to this problem is to introduce fast closed-loop power control.

Reason for Outer Ring Control: due to the complexity of wireless channels, only the power control based on the Sir value does not really reflect the link quality, and the final communication quality is measured by fer/bler/BER, therefore, it is necessary to dynamically adjust the Sir target value based on the actual fer/bler value.

The difference between the two methods is that the open loop uses the uplink interference to estimate the downlink or estimate the uplink based on the downlink, which is not closed. The closed loop has a feedback ring, which is closed. The initial launch power of the open loop control is determined by RNC (downstream) or UE (upstream), and the closed loop control is completed by NodeB, RNC only provides the target sir value of the internal ring.

RAKE receiver

When selecting a CDMA spread spectrum code, it is required to have good self-correlation characteristics. In this way, the latency extension in the wireless channel can be considered as a re-transmission of the message number. If the delay between these multi-path signals exceeds the length of a piece of code, they will be treated as irrelevant noise by the CDMA receiver, because the multi-path signal contains available information, the CDMA receiver can improve the signal-to-noise ratio of the received signal by combining the multi-path signal. In fact, what the RAKE receiver does is to use multiple related detectors to receive various signals in the multi-path signal and merge them together. The theoretical basis is that when the propagation delay exceeds the cycle of a piece of code, the multi-path signal can be regarded as unrelated. The RAKE receiver can receive both the multi-path from the same antenna and the multi-path from different antennas,

Principle of Diversity Reception

To combat fading, you can adopt a variety of measures, such as channel codec technology, anti-fading receiving technology, or spread spectrum technology. Diversity receiving technology is considered to be an effective and economical anti-fading technology.

The signal received in the wireless channel is the synthesis of the multi-path components that reach the receiver. If the receiving end obtains several signals of different paths at the same time, the signals can be properly merged into the total receiving signal, which can greatly reduce the effect of fading. This is the basic idea of diversity. The literal meaning of diversity is to scatter several synthetic signals and centralize (merge) these signals.

Some receiving signals that are independent or basically independent from each other can be obtained by receiving methods such as different paths or different frequencies, different angles, and different polarization:

(1) spatial diversity: a number of antennas are set up at the receiving or transmitting end. Sufficient spacing is required between the positions of each antenna (generally more than 10 signal wavelengths ), to ensure that the signals transmitted or received by each antenna are basically independent of each other.

Channel Coding

Different encoding schemes have different coding gains. We usually use convolution codes, Reed-Solomon Codes, BCH codes, and Turbo codes. WCDMA uses convolution codes for speech and low-speed signaling, And Turbo codes for data.

Multi-User Detection

Multi-User Detection Technology (mud) improves system performance and increases system capacity by removing intra-cell interference. The multi-user detection technology can effectively alleviate the FAR/near effect in the direct expansion CDMA system.

For uplink multi-user detection, interference between users in the residential area can only be removed, while interference between residential areas is difficult to eliminate due to the lack of necessary information (such as the situation of users in adjacent residential areas. For downlink multi-user detection, only public channels (such as the pilot channel and Broadcast Channel) can be removed.