01principlesofthewcdmasystem-120627071558-phpapp02

WCDMAPreface
Now, the most popular term in the mobile communication field is WCDMA!
What is WCDMA? And what is its benefit?
Today, let’s go to know about WCDMA!
The third generation mobile communication system (3G) was first put forward by ITU in 1985 and named future public land mobile telecommunication system (FPLMTS) at that time. In 1996, it was renamed international mobile telecommunication -2000 (IMT-2000) . IMT-2000 means that the system works on the 2000 MHz band, supports up to 2000 kbps service rate, and will be put into commercial use in 2000. The main systems include WCDMA, CDMA2000 and TD-SCDMA.
HUAWEI TECHNOLOGIES CO., LTD.
Know the basic knowledge of the CDMA system.
Master advantages of the CDMA technology comparing with other multiple access technologies.
Know technical features of WCDMA FDD.
After studying this course, you should be able to:
CDMA Principles and Rake Receiver
Transmitted
signal
The delay for the signals reaching the receiver from different paths differs. As a result, their phases are also different. The in-phase signals are mutually strengthened, while the out-of-phase signals are mutually weakened.
For the GSM system, it is impossible to distinguish the signals from different paths. Therefore, the only way is to add the vectors. In this way, however, the received signals fluctuate greatly.
For the CDMA system, the signals from multiple paths can be distinguished through the rake receiver. Then, the in-phase signals are superimposed after phase correction. This can ensure the maximum reception efficiency and effectively resist the impact of multipath fading.
Slow fading
Fast fading
Slow fading
Slow fading, also called shadowing fading, refers to the shadowing effect caused by obstructions. In slow fading, the strength of received signals weakens but the mean value of the field strength varies slowly with the geographical changes. In slow fading, the mean value of the field strength follows logarithmic normal distribution and is related to the location or position. The fading speed depends on the movement speed of the mobile station (MS).
Fast fading
Fast fading means that the amplitude and phase of the multiplexed wavelength greatly vary with the movement of the MS. The deep fading points are spatially distributed at the interval of nearly half a wavelength. Fast fading is also called Raileigh fading because its field strength follows Raileigh distribution. The fading amplitude and phase are random.
Fast fading is further divided into the following three types:
Time-selective fading: Fast movement of a user generates a Doppler effect in the frequency domain and thus causes frequency diffusion. As a result, time-selective fading occurs.
Space-selective fading: The fading features differ in different places and on different transmission paths.
Frequency-selective fading: The fading features differ for different frequencies, thus causing delay diffusion and further frequency-selective fading.
The common methods to reduce the impact of fast fading on radio communication include space diversity, frequency diversity and time diversity.
f
f
P(f)
P(f)
Broadband
System
Suppose there is 200 KHz-wide fading on the transmission frequency point. In the narrowband GSM system, it means that the signals on this frequency point are faded because the bandwidth of one frequency point is 200 KHz in the system.
In the broadband WCDMA system, however, the useful signals are distributed on the 3.84 MHz band. Therefore, a majority of useful signals are not affected after 200 KHz-wide fading and the system can still correctly demodulate signals. This is similar to the laser holographic technology.
Therefore, the CDMA system has the enhanced capability against frequency-selective fading.
The GSM system can only provide the similar anti-fading capability by frequency hopping technology.
Static channel
Vehicle mounted channel in typical urban areas (TU30)
Vehicle mounted channel in rural areas (RA50)
Vehicle mounted channel on the highway (HT120)
TU3 indicates that the UE speed is 3 Km/h in typical urban environment (the typical pedestrian scenario).
TU30 indicates the UE speed is 30 Km/h in typical urban environment (in-car environment in typical urban areas).
RA50 indicates that the UE speed is 50 Km/h in rural environment (the in-car environment in typical rural areas).
HT120 indicates that the UE speed is 120 Km/h on the highway (the in-car environment on the highway).
The multipath feature differs in different environment. For example, the pedestrian environment in urban areas features complex multipaths and a long multipath delay. The highway environment, however, features a few multipaths and a short multipath delay. As a result, the receiver can provide different demodulation performance in different multipath scenarios.
Duplex Technology – Distinguish User’s UL and DL Signal – FDD
Frequency division duplex (FDD) : Distinguish the uplink and downlink according to the frequencies.
Adopted by the WCDMA, CDMA2000 and GSM
Advantage: It can be easily implemented.
Disadvantage: The spectrum utilization is low when the uplink and downlink services (mainly the data services) are asymmetrical.
Duplex Technology – Distinguish User’s UL and DL Signal – TDD
Time division duplex (TDD) : Distinguish the uplink and downlink according to the timeslots.
Adopted by the TD-SCDMA
Advantage: The uplink and downlink can be allocated with different numbers of timeslots when the uplink and downlink services are asymmetrical. Therefore, the spectrum utilization is high.
Disadvantage:
It cannot be easily implemented and needs precise synchronization. In the CDMA system, GPS synchronization is needed.
When it is used with the CDMA technology, it is difficult to control interference between the uplink and the downlink.
Multiple Access Technology - Distinguish Different Users
Traffic channels: different users are assigned unique code and transmitted over the same frequency band, for example, WCDMA and CDMA2000
Traffic channels: different frequency bands are allocated to different users,for example, AMPS and TACS
Traffic channels: different time slots are allocated to different users, for example, DAMPS and GSM
Frequency
Time
Power
FDMA
User
User
User
User
User
Frequency
Time
Power
TDMA
Time
Frequency
Power
CDMA
Frequency Division Multiple Access: frequency division, sometimes called channelization, means dividing the whole available spectrum into many single radio channels (transmit/receive carrier pair). Each channel can transmit one-way voice or control information. Under the control of the system, any user can be accessed to any of these channels.
For FDMA system, the frequency separation needs to be enough to avoid mutual interference between different users.
Time Division Multiple Access means that the wireless carrier of one bandwidth is divided into multiple time division channels in terms of time (or called timeslot). Each user occupies a timeslot and receives/transmits signals within this specified timeslot. Therefore, it is called time division multiple access. TDMA is a complex architecture and the simplest case is that a single channel carrier is divided into many different timeslots, each of which transmits one-way burst-oriented information. The key part in TDMA is the user part, in which each user is allocated with one timeslot (allocated when a call begins). The user communicates with a base station in a synchronous mode and counts the timeslot. When his own timeslot comes, the mobile station starts a receiving and demodulation circuit to decode the burst-oriented information sent from the base station. Likewise, when a user wants to send any information, he should first cache the information and waits for his timeslot to come. When the timeslot arrives, the information is transmitted at a burst-oriented transmission.
CDMA is a multiple access mode implemented by Spread Spectrum Modulation. Unlike FDMA and TDMA, both of which separate the user information in terms of time and frequency, CDMA can transmit the information of multiple users on a channel at the same time. That is to say, mutual interference between users is permitted. The key is that every information before transmission should be modulated by different Spread Spectrum Code-Sequence to broadband signal, then all the signals should be mixed then send. The mixed signal would be demodulated by different Spread Spectrum Code-Sequence at the different receiver. Because all the Spread Spectrum Code-Sequence is orthogonal, only the information that was be demodulated by same Spread Spectrum Code-Sequence can be reverted from the mixed signal.
In CDMA system, different users share the total power of the system. That means, the capacity of the CDMA system is restricted by the power. Therefore, power usage needs to be strictly controlled.
Soft capacity
Wideband system
Why is the WCDMA system more secure?
Why are the WCDMA handsets more environment protective?
Bit, symbol and chip
Bit (bps): the data that is obtained upon source coding and contains information.
Symbol (sps): the data obtained upon channel coding and interleaving.
Chip (cps): the data obtained upon final spreading.
The spreading rate of WCDMA is: 3.84 Mcps
Processing gain
It refers to the ratio of the final spreading rate to the bit rate (cps/bps).
In the WCDMA system, the processing gain depends on the specific service.
For the 12.2 Kbps voice service, its processing gain is as follows:
10*log10 (3.84 Mcps/12.2 Kbps) =10*log10314.75=25 db
Processing gain makes it possible to obtain the signal-to-noise ratio (Eb/Nt) required for demodulation when the carrier-to-interference ratio (Ec/Io) of the input signals is low. For example, in a typical pedestrian scenario, the Eb/Nt required to demodulate the 12.2 Kbps voice service is 7 dB (the simulation result). Since the processing gain is 25 dB, the acceptable minimum Ec/Io of the input signals is 7-25=-18 dB. That is, the signals can be submerged in the noise.
Broadband Ec/Io is also called C/I. In the GSM system, the C/I required for a high quality voice connection ranges from 9 dB to 12 dB. That is, the C/I of the useful signals need to be higher than that of the noise.
Chip rate = symbol rate spreading factor
For WCDMA, if the chip rate is 3.84 MHz and the spreading factor is 4, the symbol rate is 960 Kbps.
For CDMA2000-1x, if the chip rate is 1.2288 MHz and the spreading factor is 64, the symbol rate is 19.2 Kbps.
Symbol rate = (service rate + check code) × channel code ×repetition or punching rate
For WCDMA, if the service rate is 384 Kbps and the channel code is 1/3 Turbo, the symbol rate is 960 Kbps.
For CDMA2000-1x, if the service rate is 9.6 Kbps and the channel code is 1/3 convolutional code, the symbol rate is 19.2 Kbps.
Scrambling
Modulation
The WCDMA system adopts the adaptive multi-rate (AMR) speech coding.
A total of eight coding modes are available. The coding rate ranges from 12.2 Kbps to 4.75 Kbps.
Multiple voice rates are compatible with the coding modes used by current mainstream mobile communication systems. This facilitates the design of multi-mode terminals.
The system automatically adjusts the voice rate according to the distance between the user and the NodeB, thus reducing the number of handovers and call drop.
The system automatically decreases the voice rate of some users according to the cell load, thus saving power and containing more users.
Source coding
Channel Coding in WCDMA
Channel coding can enhance symbol correlation to recover signals in the case of interference.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Code type
Data service: Turbo code (1/3).
Source coding
Interleaving
Interleaving is used to damage symbol correlation and reduce the impact caused by fast fading and interference of the channel.
1 2 3 4 5 6 7 8 ... ... 452 453 454 ……
8
16
Ist interleaving
2nd interleaving
Self Correlation and Mutual Correlation of Code Words
Different users adopt different spreading code words, such as x1(t) , x2(t) ….
Self correlation determines multipath interference.
Mutual correlation determines multiple access interference.
Self-correlation function
0, others
Multiple access interference is caused by different code words between users. Therefore, mutual correlation of code words determine multiple access interference.
Multipath interference is caused by the correlation between the chip and the shift of the chip itself. Therefore, self correlation determines multipath interference.
Users who need to send information: UE1, UE2 and UE3
UE1 uses c1 for spreading: UE1 x c1
c1, c2 and c3 are orthogonal to each other
Information sent: UE1 x c1 + UE2 x c2 + UE3 x c3
Source coding
UE1 uses c1 for de-spreading.
(UE1 x c1 + UE2 x c2 + UE3 x c3) x c1
= UE1 x (c1 x c1) + UE2 x (c2 x c1) + UE3 x (c3 x c1)
= UE1 x 1 + UE2 x 0 + UE3 x 0
= UE1
In the same way, UE2 uses c2 for de-spreading and UE3 uses c3 for de-spreading to get their own signals.
UE1xc1 UE2xc2: 0 -2 0 -2 0 +2 0 +2
Huawei Confidential
De-spreading Principle
UE1×c1 UE2×c2 0 -2 0 -2 0 +2 0 +2
Question: How to generate those orthogonal codes like c1 and c2?
UE1 de-spreading with c1: +1 -1 +1 -1 +1 -1 +1 -1
De-spreading result: 0 +2 0 +2 0 -2 0 -2
Integral: +4 -4
Decision: +4/4 = +1 -4/4 = -1
UE2 de-spreading with c2: +1 +1 +1 +1 +1 +1 +1 +1
De-spreading result: 0 -2 0 -2 0 +2 0 +2
Integral: -4 +4
Huawei Confidential
OVSF & Walsh
OVSF codes (Walsh) are completely orthogonal and their mutual correlation is zero.
SF = 1
SF = 2
SF = 4
Cch,1,0 = (1)
Cch,2,0 = (1,1)
Cch,2,1 = (1,-1)
Cch,4,0 =(1,1,1,1)
Cch,4,1 = (1,1,-1,-1)
Cch,4,2 = (1,-1,1,-1)
Cch,4,3 = (1,-1,-1,1)
Feature: For any spreading code in use, its prefix code (father code) and lengthened (children) code cannot be used.
Why CDMA system is a self interference system?
Since all the users use orthogonal code, where does the self interference come from?
Comes from the code error during the transfer.
If there are some code error on one user’s information, it misses some orthogonal to others. Then interference comes.
UE1xc1 + UE2xc2 0 -2 0 -2 0 +2 0 +2
UE1xc1 + UE2xc2 error 2 -2 0 -2 1 +2 0 +2
C2 +1 +1 +1 +1 +1 +1 +1 +1
Result: 2 -2 0 -2 0 +2 0 +2
Integral: -2 +4
c1: +1 -1 +1 +1 +1 -1 +1 -1
Result: 2 +2 0 +2 0 -2 0 -2
Integral: +6 -4
Downlink: Different cells have different downlink scrambling codes.
Each cell is configured with a unique downlink scramble. The UE identifies a cell based on the scramble.
The OVSF code is used to distinguish different users in a cell.
Uplink: Scrambles are used to distinguish different users.
In one cell, each user is configured with a unique uplink scrambling code.
The OVSF code is used to distinguish the services of a user.
Source coding
RF transmission
Downlink: The UE distinguish the cells in the WCDMA system by identifying the scramble of each cell.
Uplink: Users are distinguishd directly based on scrambles.
OVSF codes have not good self correlation, they are not good for distinguishing the multi-paths signal.
Scrambling is using good self correlation codes to process with the signal after spreading, and the final signal will be good self correlation.
After scrambling, the chips are modulated to the higher frequency carrier then transmitted.
3.84M
3.84Mcps
3.84Mcps
Random sequence (Bernoulli sequence)
It consists of 0’s and 1’s only, with the number of 0’s equal to that of 1’s.
The probability of continuous 1 or 0 is as follows: 1/2 for one continuous 0 or 1, 1/4 for two continuous 0 or 1, 1/8 for three continuous 0 or 1….
One half of the shift sequence is the same as the original sequence and the other half is different.
m sequence - scrambling code used in the CDMA2000 system
It is generated by the shift register.
As the longest linear shift register sequence, its period is 2n-1, where “n” stands for the length of the shift register.
When the delay is 0, the self-correlation function has a maximum value. In other cases, the function value is always -1.
It meets the Bernoulli sequence.
Its disadvantages are: Only one sequence is available and different users are distinguished by different phases. So it has a high requirement for synchronization.
The CDMA2000 adopts the m sequence as the random sequence.
Gold sequence
It consists of two preferred m sequences on the Exclusive-OR basis.
Its self-correlation function has multiple values, which is worse than the m sequence.
It exceeds the m sequence in quantity.
The Gold sequence is used to distinguish the cells and users in the WCDMA system owing to its good self correlation.
Good self correlation determines the Gold sequences can be used to distinguish users, thus the multiple access function is realized.
Advantages and Disadvantages of the Gold Sequence
Advantages: The Gold sequence needs no GPS synchronization and features high system flexibility and security.
The NodeB can work in asynchronous mode.
It is easy to realize indoor coverage.
Disadvantages: The Gold sequence has greater inter-code interference than the m sequence.
Power spectrum
Allowed maximum interference level of the system
Demodulation threshold
Interference signals from other users
At the receiver, as the codes are different and are known, only the power of the intended user is de-spread.
After de-spreading (decoding) , correct data recovery requires a given value for the Eb to No ratio. Under this Eb/No ratio the noise will generate too many errors. The noise is mainly generated by the other users transmitting at the same time and at the same frequency but using different spreading codes.
Therefore, in order not to cross this maximal noise level, all the users have to share their power: In WCDMA the Time-Frequency plane is not divided among the mobile subscribers as is done in TDMA or FDMA. So the common shared resource is power.
The de-spreading process gives processing gain proportional to the bandwidth of the spreading signal. The larger the spreading factor, the larger the gain.
This means that by using a larger spreading factor, we can reduce the power (and therefore the background noise) . Thanks to this property, spread signals can operate at negative signal-to-noise ratios provided that they possess enough gain.
Example:
The narrowband signal requires an Eb/No of 12 dB to achieve a certain bit-error rate performance. What is the required Ec/No, knowing that the processing gain is 20 dB?
effectively avoids frequency-selective fading of radio channels.
Spreading code
Spreading code
Signal combination
Narrowband signal
Combiner
t
t
s(t)
s(t)
RAKE receiver help to overcome on the multi-path fading and enhance the receive performance of the system
Functions of the rake receiver:
The rake receiver receives signals from various paths through multiple correlation detectors. It can parse out the signals separately and then combine them as long as the signals from various paths arrive at the receiver at an interval of greater one chip.
The channel estimator adopts pilot symbols to estimate the channel status. The phase rotator eliminates the phase impact caused by the channel from the received symbols according to the estimated channel status. Delay estimation is to obtain signal energy distribution on different time delay positions through the matched filter, to identify the multipath positions with larger energy, and to allocate their time quantum to different receiving paths on the rake receiver.
The delay equalizer is to compensate the time difference of arrival of symbols on each path. Finally, the rake combiner adds up the symbols after channel compensation, and thus provides the multipath diversity to resist the fading.
In terms of implementation, the rake receiver supports chip-level and symbol-level processing. The correlator, local code generator and matched filter provide chip-level processing. Channel estimation, phase rotation, and combination and addition belong to symbol-level processing, and are realized through the DSP. Although the rake receiver of the UE has different implementation methods and functions from that of the NodeB, their principles are the same.
Summary - Advantages of CDMA
RAKE receiver is adopted
The time diversity effect generated by channel coherence time is efficiently used.
Frequency diversity
Low signal transmission power
Great flexibility in carrying multiple services with largely different bit rate and QoS requirement.
Different spreading factors for different services with different data rates
High spectral efficiency
Supporting soft handover and softer handover.
Fast closed loop power control: 1500Hz
Handover: soft/hard handover
Support synchronous and asynchronous NodeB operation
1,The DS-CDMA system with a bandwidth of approximately 1MHz, i.e. IS-95, is generally named as narrowband CDMA system.But WCDMA owns a chip rate of 3.84Mcps, bringing approximately 5MHz carrier bandwidth.This feature enables the system to support higher bit rate, and at the same time brings other benefits, for example, increasing of multi-path diversity.
2,In IS-95, only coherence detection is used in the downlink, but in WCDMA, coherence detection based on pilot symbol or common pilot is adopted for both the uplink and the downlink, increasing the coverage scope and the capacity of the uplink.
3,IS-95 only uses closed loop power control in the uplink, while WCDMA uses this in both of the uplink and the downlink.With the closed loop power control used in the downlink, link performance and downlink capacity is improved.
4,IS-95 system mainly aims at macro cell. Because BTS synchronization is necessary, BTS is generally placed on the roof, etc. for the sake of receiving GPS signal.In this case, a global time reference can be used. But this application is difficult to carry out in the places where it is hard to receive the GPS signals. WCDMA system supports asynchronous BTS operation, and it may not use the global reference, thus it is different with the IS-95 system requiring BTS synchronization operating mode. Thus, the application of indoor cell and microcell is much simpler. This makes that the handover of WCDMA is slightly different with that of IS-95.
Compatible with GSM-MAP core network
Comparatively steady version R99 has been released
Support open loop and closed loop transmit diversity mode
Support Common Packet Channel(CPCH) and Downlink Share Channel, adapt to Internet data access mode
Support macro diversity, selection diversity of NodeB location
Support different fast power control algorithms and open loop, out loop power control
Fully support UE locating services
Differences Between the WCDMA and GSM on the Air Interface
WCDMA
GSM
Frequency diversity
The 3.84 MHz bandwidth enables the air interface to use the rake receiver for multipath diversity.
Frequency hopping
Packet data
Not supported by the standard but can be applied.
Open loop transmit diversity: STTD and TSTD
Closed loop transmit diversity: through FBI
Macro diversity: soft handover
This course describes the WCDMA system.
This course first describes some key technologies. Then, describes the basic principles of CDMA and the WCDMA FDD mode.
After studying this course, we can have a preliminary understanding of WCDMA, thus laying a good foundation for subsequent study.
Thank You !

Documents

01principlesofthewcdmasystem-120627071558-phpapp02