ثمات لجوالات الجيل الثالث

[BLACKDREAM 5]

اربع ثمات روعه لعيون احلى اعضاء

 

 

و ها هو رابط التحميل اتفضلو

 

إضغط هنا

تم تعديل May 4, 2007 بواسطه BLACKDREAM

[AMR DAWAM 7]

السلام عليكم

جزاك الله خيرا اخى BLACKDREAM

وجعله الله فى ميزان حسناتك

وفعلا ثمات اكثر من رائعة

بس اعتقد ان بيهم مشكلة ما ......

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[BLACKDREAM 5]

في ايه حبيبي عمر

 

خير انشالله

 

انا حجبلك غيرهم من عيوني والله انت تامر بس يا عسل

[AMR DAWAM 7]

ربنا يخليك حبيبى يامن على كلامك الجميل

بس هما للاسف بيهم مشكلة فى التنفيذ والعمل

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[jo_magdi 0]

لاول مرة عرض معنة 3g ؟

ياعني سرعة 906kbpsالي5mbpsياعني السرعة كبيرة جدا

data increase

[jo_magdi 0]

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Code Division Multiple Access (CDMA)

 

Introduction to CDMA

 

CDMA is the third multiple access technique among the two more classical schemes FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access). To recall, FDMA separates the medium by putting them on different frequency bands, while TDMA separates them by allocating different time-slots on the same frequency. In contrast, CDMA is a spread-spectrum (SS) approach to user multiplexing. Users in a CDMA cellular environment simultaneously share the same frequency band and can be separated at the receiver end with the knowledge of their unique code.

 

 

Figure 1: Multiple access schemes

 

Besides of the multiple access schemes explained above there is also SDMA (Space Division Multiple Access), which uses the spatial dimension. Since most users can be separated by their different positions this can be exploited by antenna array processing.

In general combinations of different multiple access schemes are applied e.g. GSM uses both TDMA and FDMA and even SDMA with sectorized cells as well as antenna arrays.

In this text we will focus on the CDMA scheme. There are several ways of employing the code in spread-spectrum transceivers, the direct-sequence (DS) approach is most common. With this technique, the carrier frequency is mixed (or more correctly modulated by) a periodic spreading sequence. There are several possible spreading sequences. They can be complex valued or just real valued, quite long or very short and there are codes with various statistical properties to choose from.

One of the most common is pseudo-random binary sequence also often denoted as PN-code (pseudo noise). This is a unique digital data sequence that repeats itself periodically, say, every 1023 bits (or even (242-1) bits in the case of long codes). A symbol of the spreading code (here PN-code) is called a chip.

 

 

Figure 2: Realisation of spreading in time and frequency domain

 

The spreading process is displayed on the right hand side of figure (2). The data stream, with a symbol duration Ts, is ''multiplied'' with a spreading sequence with Tc denoting the chip duration. The spreading factor is defined as

SF= Ts/Tc

which is corresponding with the number of chips within a symbol duration. The period of the spreading sequence used is greater or equals the spreading factor. This multiplication with a sequence of higher frequency also yields a resulting signal of higher rate, therefore the spectrum of the original signal is spread by the factor SF. This signal is broadcast and can be seen on the right hand side of the upper figure. One of the main advantages of this scheme is its robustness against narrow band noise or interference. For instance this becomes quite handy in military transmissions or e.g. if a microwave oven is nearby transmitting interference in the same frequency band, which is the case for the Universal Mobile Telecommunication System (UMTS) and its 2 GHz band. The receiver multiplies the incoming signal with the (conjugate complex) spreading sequence again. After an integrate and dump (I&D-) circuit, a decision can be made. This multiplication with the (conjugate complex) spreading sequence results in a spreading of narrow band noise. The I&D-operation works as a lowpass and suppresses a mayor portion of the narrow band noise. Unfortunately this does not effect additive white gaussian noise (AWGN), in respect to the same Eb/N0. Therefore the BER within an AWGN environment of a DS-CDMA system in a single user case is identical to the one of the classical scheme. The transmission scheme for a QPSK based system is depicted in figure (3). This kind of receiver is also known as correlation receiver. Instead of using a plain real valued spreading sequence a complex valued random spreading sequence with the values [1+j,1-j,-1+j,-1-j] is used with j denotes the square root of -1 and n(k) denotes the additive noise.

 

 

Figure 3: DS-CDMA scheme with correlation receiver

 

Alas wireless systems have to deal with a transmission channel that has several unwanted properties (Many guys in mobile communications would need a new job, if the channel would only consist of AWGN):

• Additive White Gaussian Noise (AWGN)

• Frequency selectivity due to multipath propagation

• Time variance of the channel due to Doppler spread also called fast fading

• Multiple Access Interference (MAI) in CDMA-Systems

In the following figure (4) a CDMA-system is depicted within a multipath environment. The transmitter on the right hand side (from the right side to the left side) spreads the (QPSK-) modulated data. The resulting signal is transmitted over a frequency selective fading channel. If we only consider one branch of the receiver called Rake finger the receiver is mostly identical with the correlation receiver displayed above. The main difference is the correction of the phase shift introduced by the channel with the help of an estimation of the channel coefficient h0(i). In general we have multipath propagation and the best thing to do is to exploit it and therefore to enhance the performance of the receiver. In order to achieve this goal, we try to synchronise all fingers by a chipswise tap-delay line. If we have L paths we can apply up to L correlation receivers. The path corresponding to the channel coefficient h0(i) is the first one and hL-1(i) the last one to arrive at the receiver. After channel correction with all channel coefficients we can combine the correlator outputs. There are several possibilities to correct the phase offset of every path introduced by the channel. The most used way is by multiplying the conjugate complex estimate of the considered channel coefficient. This method is called Maximal Ratio Combining (MRC), and is emphasised by the blue background colour in figure (4). The Rake receiver becomes a matched filter receiver which is known to get the best SNR for white noise at the receiver input. The Rake receiver can be seen as an adaption of the mached filter g(t) to spread spectrum communication.

g(t) = h*(T0-t)

and g(t) denotes the matched filter at the receiver to the channel h(t). The negative time index can be archieved by setting the estimates in reverse order, as can be seen in lower figure and T0. The constant T0 keeps the filter causal and for this specific figure below it can be set to T0=(L-1)Tc.

 

 

Figure 4: DS-CDMA scheme with Rake receiver within a multipath environment

 

Besides of this coherent detection scheme, incoherent schemes are known. Further information on incoherent ones can be found in our publication database.

One property of CDMA is its ability to be used by multiple user at the same time and frequency. Therefore the choice for a class of codes according to their statistical properties is quite important. Another factor to consider in this choice is if our communication system is used for uplink (mobile to basestation) or downlink (basestation to mobile) direction.Do we want to do a joint detection or just one user at the time (a mobile does not need to detect all users but a basestation has to, therefore a joint detection is a bad idea for a cellular due to its much higher computation costs). Or do we just have some sort of broadcast system with one code only, actually this is not a CDMA scheme any more but still a Direct Sequence Spread Spectrum system (DS-SS) e.g. IEEE 802.11b. What kind of channel do we have to expect? All these kinds of considerations and a few more have to be taken into account for the choice of the best possible spreading code because there does not exist something like perfect codes for every application. The correlation properties of the codes have to be examined for the planned transmission scheme. Unfortunately under practical conditions there is one main point to state: The more users we get the worse the performance for every user becomes. Every user is disturbing every other user. This effect is called Multiple Access Interference (MAI) or Multiple User Interference (MUI). In order to get the degradation of the system performance with raising number of users as small as possible is a research topic in the community at the moment. In general there are two classes of systems:

• The linear systems try to suppress the MUI with the help of some sort of filtering.

• The interference cancellation schemes are non linear systems and try to get rid of the interference by substracting disturbing signals before data for the considered user is decided. More informations about these techniques can be found here.

 

Summary

For several reasons, spread spectrum communications is regarded a promising technique for mobile radio networks. It is already used in UMTS. It is widely accepted that spectrum communications results in efficient spectrum use. Some benefits of CDMA and, in particular, advantages over TDMA are:

• CDMA is less prone to deep multipath fading caused by transmissions arriving at the receiver that have followed different propagation paths. In fact, one approach in common use with CDMA systems, the Rake receiver, takes advantage of multipath, normally a major source of interference and signal degradation in other systems. In a Rake receiver, the signals of several correlation receivers belonging to the strongest multipath components are combined to provide an enhanced signal with better voice quality.

• CDMA systems can operate with much lower transmit powers leading to smaller handsets and smaller batteries and longer life.

• CDMA techniques can reduce interference between cells in cellular networks and improve "hand-over" by summing and correlating transmissions from adjacent cells. Therefore, rather than having to use a "hard hand-off" between cells in a "break-before-make" fashion, CDMA system can manage a "soft hand-off" in a "make-before-break" manner.

• CDMA systems have the ability to co-exist with conventional narrow-band transmissions.

• CDMA can simplify cell planning by removing the need to specify rigid frequency allocations to individual cells.

• CDMA is able to increase capacity more easily than TDMA by exploiting variable data rates and/or voice activity detection.

It is fair to say that this short note does not attempt to fully address the pros and cons of CDMA versus TDMA, and in fact a balanced discussion would also need to discuss the benefits of TDMA over CDMA. Spread spectrum is still a relatively new technology in a commercial environment with little practical experience behind it compared to e.g. GSM (which is TDMA-based). In effect, many of its benefits are theoretical with few real field trials in urban environments to allow evaluation and to ascertain how it would co-exist with other services. All this aside, CDMA will be used in next generation systems and its advantages and problems will be shown soon in a commercial^M environment.

Current work on CDMA in our research group

The program of our research projects address basic transmission-technical problems of 3G mobile communication systems whereby the underlying transmission technique is based on direct-sequence CDMA (DS-CDMA). In the field of coding and modulation the Rake receiver in combination with SDMA-Schemes (Space-Time Codes, closed Loop) and enhancement of the system using MUI-suppresion techniques will be investigated in particular in the course of this project.

The following topics have been subject to our CDMA research work:

• New Algorithm for the UMTS UTRA FDD Downlink

• Space time processing for UMTS

• Pilot based channel estimation for UMTS

• Interference (MUI) surpression schemes for DS-CDMA

• Interference cancellation and Multiuser detection for DS-CDMA schemes

• Optimization of a novel CDMA Concept using Hybrid Modulation.

• Optimization of noncoherent Rake receivers for the transmission of CDMA-signals in cellular networks.

 

For further information please refer to our papers on the above topics and CDMA in general. Additionally, see also slides.ps (161 kB) or slides.pdf (1,2 MB) in german language.

 

 

 

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