Download - 2-Wcdma Ran Fundamental Issue1.0

Course code Course nameUpon completion of this course, you will be able to:
Outline the advantage of CDMA technology
Outline the fundamentals of UTRAN
Outline the key technologies of UTRAN
HUAWEI TECHNOLOGIES CO., LTD.
frequency
time
power
FDMA
frequency
time
power
TDMA
power
time
CDMA
frequency
Frequency Division Multiple Access 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. Analog cellular system is a typical example of FDMA structure.
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. This multiple access mode is adopted in both digital cellular system and GSM.
CDMA is a multiple access mode implemented by Spreading 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. The key is that every information before transmission should be modulated by different Spreading Code to broadband signal, then all the signals should be mixed and send. The mixed signal would be demodulated by different Spreading Code at the different receiver. Because all the Spreading Code is orthogonal, only the information that was be demodulated by same Spreading Code can be reverted in mixed signal.
soft capacity
A UE transmission power is interference for another UE.
In CDMA system, mutual interference between users or cells is permitted, so adjacent cells can be distributed with same frequency. That is why the spectrum efficiency is very high and the capacity is also very large in CDMA system. But it also causes self-interference, if the interference is out of control, the capacity and quality of CDMA system will be worse, so many technologies were invented to control the interference, and it is not easy.
The second feature of CDMA is security. After spreading, the narrowband signal of the user will be changed to broadband signal, is close to noise, only people who use the same spreading code can revert it. Of course, it cause the other shortcoming: more frequency band needed.
The third feature of CDMA is soft capacity. Because all of the carrier resource (the main resource is power) is “shared” by all of the users, if some user occupy more power, it will cause the capacity lower. Soft capacity will cause network planning more complex, emulation is necessary.
Correlation
Correlation is a measure of similarity of between any two arbitrary signals.
EXAMPLE:
(a)
(b)
+1
0
-1
+1
0
-1
+1
0
-1
+1
0
-1
Correlation is used to measure similarity of any two arbitrary signals. It is computed by multiplying the two signals and then summing (integrating) the result over a defined time windows. The two signals of figure (a) are identical and therefore their correlation is 1 or 100 percent. In figure (b) , however, the two signals are uncorrelated, and therefore knowing one of them does not provide any information on the other.
UE1×c1 1 1 1 1 1 1 1 1
UE2×c2 1 1 1 1 1 1 1 1
UE1×c1 UE2×c2 0 2 0 2 0 2 0 2
By spreading, each symbol is multiplied with all the chips in the orthogonal sequence assigned to the user. The resulting sequence is processed and is then transmitted over the physical channel along with other spread symbols. In this figure, 4-digit codes are used. The product of the user symbols and the spreading code is a sequence of digits that must be transmitted at 4 times the rate of the original encoded binary signal.
Dispreading
UE1×c1 UE2×c2: 0 2 0 2 0 2 0 2
UE1 Dispreading by c1: 1 1 1 1 1 1 1 1
Dispreading result: 0 2 0 2 0 2 0 2
Integral judgment: 4 (means1) 4 (means1)
UE2 Dispreading by c2: 1 1 1 1 1 1 1 1
Dispreading result: 0 2 0 2 0 2 0 2
Integral judgment: 4 (means1) 4 (means1)
The receiver dispreads the chips by using the same code used in the transmitter. Notice that under no-noise conditions, the symbols or digits are completely recovered without any error. In reality, the channel is not noise-free, but CDMA system employ Forward Error Correction techniques to combat the effects of noise and enhance the performance of the system.
When the wrong code is used for dispreading, the resulting correlation yields an average of zero. This is a clear demonstration of the advantage of the orthogonal property of the codes. Whether the wrong code is mistakenly used by the target user or other users attempting to decode the received signal, the resulting correlation is always zero because of the orthogonal property of codes.
Spreading code
Spreading code
Signal Combination
Narrowband signal
P(f)
f
Traditional radio communication systems transmit data using the minimum bandwidth required to carry it as a narrowband signal. CDMA system mix their input data with a fast spreading sequence and transmit a wideband signal. The spreading sequence is independently regenerated at the receiver and mixed with the incoming wideband signal to recover the original data. The dispreading gives substantial gain proportional to the bandwidth of the spread-spectrum signal. The gain can be used to increase system performance and range, or allow multiple coded users, or both. A digital bit stream sent over a radio link requires a definite bandwidth to be successfully transmitted and received.
Source
coding
Channel
coding
Spreading
Modulation
Source
decoding
Channel
decoding
Despreading
Demodulation
Channel coding can make the transmission more reliably.
Spreading can increase the capability of overcoming interference.
Scrambling can make transmission in security.
Through the modulation, the signals will transfer to radio signals from digital signals.
Symbol: data after channel coding and interleaving
Chip: data after spreading
Process Gain: 10log (cps/bps); for different service, the Gain is different
Process Gain is smaller, UE need more power for this service
Process Gain is smaller, the coverage of the service is smaller
For common services, the bit rate of voice call is 12.2kbps, the bit rate of video phone is 64kbps, and the highest packet service bit rate is 384kbps(R99). After the spreading, the chip rate of different service all become 3.84Mcps.
Multi-rate:
Benefit multi-mode terminal design
Adaptation: when cell load increases, the system will decrease speech rate of part of subscribers automatically so as to support more subscribers.
AMR is compatible with current mobile communication system (GSM, IS-95, PDC and so on), thus, it will make multi-mode terminal design easier.
The AMR codec offers the possibility to adapt the coding scheme to the radio channel conditions. The most robust codec mode is selected in bad propagation conditions. The codec mode providing the highest source rate is selected in good propagation conditions.
During an AMR communication, the receiver measures the radio link quality and must return to the transmitter either the quality measurements or the actual codec mode the transmitter should use during the next frame. That exchange has to be done as fast as possible in order to better follow the evolution of the channel’s quality.
Purpose:
Enhance the correlation among symbols so as to recover the signal when interference occurs.
Types
Data service: Turbo code
During the transmission, there are many interferences and fading. To guarantee reliable transmission, system should overcome these influence through the channel coding which includes convolution and interleaving.
The first is convolution that is used for anti-interference. Through the technology, many redundant bits will be inserted in original information. When error code is caused by interference, the redundant bits can be used to recover the original information.
In WCDMA network, both Convolution code and Turbo code are used. Convolution code applies to voice service while Turbo code applies to high rate data service.
Interleaving is used for continuous bit error correction
1 2 3 4 5 6 7 8 ... ... 452 453 454 ……
8
16
First interleaving
Second interleaving
In channel coding , there is another technology named interleaving. Communications over radio channel are characterized by fast fading that can cause large numbers of consecutive errors. Most coding schemes perform better on random data errors than on blocks of errors. By interleaving the data, no two adjacent bits are transmitted near to each other, and the data errors are randomized.
OVSF: Orthogonal Variable Spreading Factor, generated by Walsh matrix
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)
Orthogonal codes are easily generated by starting with a seed of 1, repeating the 1 horizontally and vertically, and then complementing the -1 diagonally. This process is to be continued with the newly generated block until the desired codes with the proper length are generated. Sequences created in this way are referred as “Walsh” code.
Spreading code uses OVSF code, for keeping the orthogonality of different subscriber physical channels. OVSF can be defined as the code tree illustrated in the following diagram.
Spreading code is defined as Cch SF, k,, where, SF is the spreading factor of the code, and k is the sequence of code, 0≤k≤SF-1. Each level definition length of code tree is SF spreading code, and the left most value of each spreading code character is corresponding to the chip which is transmitted earliest.
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Purpose of OVSF
For uplink, OVSF is used to separate different services of one connection
For downlink, OVSF is used to separate different connections
Typical service
Data rate
Downlink SF
Uplink SF
8
4
For voice service (AMR), downlink SF is 128, it means there are 128 voice services maximum can be supported in one WCDMA carrier;
For Video Phone (64k packet data) service, downlink SF is 32, it means there are 32 voice services maximum can be supported in one WCDMA carrier.
Distinguishing cells or users
Uplink
OVSF code is used for distinguishing channels of one user
In one network, usually a UE is surrounded by many base stations with cells. If UE wants to get service from system, first it should distinguish different signals from different cells. In WCDMA system, most of the cells use the same frequency, so UE can not get any information thought the frequency. Here scrambling codes are used. Different cells will be allocated different scrambling code.
Gold sequence is made by two m sequence.
Advantage: No need to use GPS as the system clock, NodeB can work in asynchronous mode, and it is also convenient for indoor coverage
Disadvantage: the interference between code is larger than m sequence
For uplink, 224 long scrambles and 224 short scrambles.
For downlink, 262143 (218 - 1) scrambles, but only 8192 scrambles( from 0 to 8191) are adopted at present.
The length of scrambling code is 38400 chips.
R99/R4: adopt QPSK
HSDPA: adopt 16QAM
DL max data rate is 14.4Mbps
QPSK: Quadrature Phase Shift Keying . Phase shift keying in which four different phase angles are used.
16QAM: 16 Quadrature Amplitude Modulation
Time
Received
signal
Transmitted
signal
A mobile communication channel is a multi-path fading channel and any transmitted signal reaches a receive end by means of multiple transmission paths, such as direct transmission, reflection, scatter, etc.
Slow fading
Fast fading
Furthermore, with the moving of a mobile station, the signal amplitude, delay and phase on various transmission paths vary with time and place. Therefore, the levels of received signals are fluctuating and unstable and these multi-path signals, if overlaid, will lead to fading. The mid-value field strength of Rayleigh fading has relatively gentle change and is called “Slow fading”. And it conforms to lognormal distribution.
Diversity technique is used to obtain uncorrelated signals for combining
Reduce the effects of fading
Improve the reliability of communication
Increase the coverage and capacity
Diversity methods
Time diversity
Frequency diversity
Space diversity
Diversity technology means that after receiving two or more input signals with mutually uncorrelated fading at the same time, the system demodulates these signals and adds them up. Thus, the system can receive more useful signals and overcome fading.
Diversity technology is an effective way to overcome overlaid fading. Because it can be selected in terms of frequency, time and space, diversity technology includes frequency diversity, time diversity and space diversity.
Time diversity: block interleaving, error-correction
Frequency diversity: frequency hopping, CDMA is also a kind of frequency diversity, the signal energy is distributed on the whole bandwidth.
Space diversity: using twin receive antennas, RAKE receivers
During a handover, the mobile station contacts multiple base stations and searches for the strongest frame, it is called macro diversity.
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
The RAKE receiver is a technique which uses several baseband correlators to individually process multipath signal components. The outputs from the different correlators are combined to achieve improved reliability and performance.
When WCDMA systems were designed for cellular systems, the inherent wide-bandwidth signals with their orthogonal Walsh functions were natural for implementing a RAKE receiver. In WCDMA system, the bandwidth is wider than the coherence bandwidth of the cellular. Thus, when the multi-path components are resolved in the receiver, the signals from each tap on the delay line are uncorrelated with each other. The receiver can then combine them using any of the combining schemes. The WCDMA system then uses the multi-path characteristics of the channel to its advantage to improve the operation of the system.
Correlator
Code
Generators
Q
For the digitized signals input to the baseband, dispreading and integration of subscriber data symbols is completed via the correlator and local Code Generator, specifics are as follows: Channel Estimator uses the pilot signal to estimate the channel status; Phase Rotator deletes the phase affection caused by the channel from the received signal according to the estimated channel status. The function of Delay Equalizer is to obtain the signal energy distribution at different delay positions via the matching filter, identify the multipaths with large energy, and allocate their time values to different receive paths of the RAKE receiver. The delay equalizer is to compensate the difference of symbol arriving time for each path. At last, the RAKE combiner adds the symbols after channel compensation to provide multipath diversity to withstand fading. From the aspect of realization, the processing of RAKE receiver can be based on either chip level or symbol level. The correlator, local code generator and matching filter belong to the chip level processing, and this is generally realized via ASIC device; Channel estimation, phase spinning and combination belong to symbol level processing, and this is realized via DSP. Though the realization methods and functions of the RAKE receiver between UE and BTS are different, the principles are complete the same.
Without power control
With power control
The rate of power control can be up to1500 times per second, which is faster than that of fading, thus, it can overcome shadow fading and fast fading effectively
Power control also decrease interference of system, and increase system capacity and quality.
For UE, Power control can save power, and expand conversational time