2-CDMA Reception and System Performance

PowerPoint Presentation

CDMA Reception and System Performance

By/ Eng. Abdelmonem Abdelghany ElbawabAMAGSMB

2012CDMA

Multi-User CDMA Coding and DetectionInterference and Processing GainRAKE ReceptionPower ControlQuadrature Spreading and Modulation

ContentsMulti-User CDMA Coding and Detection

Multi-User CDMA Coding and Detection

Multi-User Downlink Overview









Multi-User CDMA Coding and DetectionMulti-User Uplink Overview

Interference and Processing GainInterference and Processing GainTraditional Systems

Performance is measured by signal-to-noise ratio (S/N). The desired user's signal versus noise.

CDMA Systems

Performance is measured by signal-to-interference ratio (S/I). The desired user's signal versus interference from other users signals.Interference and Processing Gain

Interference and Processing Gain

Interference and Processing GainProcessing gainIs the ratio of the spread (or RF) bandwidth to the unspread (or baseband) bandwidth. It is usually expressed in decibels (dB).

G (processing gain) = Fc/Fb = Tb/Tc

Fb =1/Tb (the bit rate of the input signal)Fc =1/Tc (the chip rate of the spreading code)

Consequently, the greater the process gain, the larger the allowable interference.

Interference and Processing GainProcessing gain

where M = number of users

Assume that IS-95 uses a chip rate of 1.2288 Mbps and an input data rate of 9.6 kbps ( there is another rate for data). The processing gain (G) would be 128 = 21.07 dB

Interference and Processing GainHow many users can one CDMA carrier accommodate? The key determinate of capacity is voice quality, but voice quality is subjective

ScoreQuality Scale5Excellent ( Speech Perfectly Understandable)4Good (speech easily understandable, some noise )3Fair ( speech understandable with a slight effort, occasional repetition needed )2Poor ( speech understandable only with considerable effort, frequent repetitions needed)1Unsatisfactory ( speech not understandable)The mean opinion score ( MOS). Usually select voice quality of MOS >= 4Interference and Processing GainHow many users can one CDMA carrier accommodate? Adding users should end when S/I reaches 6 dB. This corresponds to acceptable speech quality. It also corresponds to a theoretical maximum of 32 interferes or a total of 33 users.

No exact calculation of the number of users on a carrier is possible because it requires such unattainable information as the environment, speed, and direction of each mobile unit. Empirical evidence suggests that 15 to 20 users can be accommodated with today's technology.

Traditional cellular systems at 850 MHz such as AMPS (Advanced Mobile Phone System) separate users by a technique called FDMA (Frequency Division Multiple Access) that assigns each user a separate RF channel. AMPS provides 395 channels for each of two competitive service providers.

GSM have 124 carriers and each can hold 8 users. So, the total users number is 992.Interference and Processing Gain

The AMPS system uses the technique of sectorizing cells to reduce interference with other sectors using the same frequencies.

AMPS uses sectorization to improve S/I (speech quality), not capacity.Interference and Processing Gain

With an IS-95 CDMA system, there are fewer RF carriers available than with AMPS.Two IS-95 carriers use the same amount of spectrum as 100 AMPS channels or can provide up to 8 RF carriers in the AMPS spectrum allocated for 395 AMPS channels. Currently, an IS-95 system can support about 14 or 15 users per RF carrier. Therefore, in a CDMA system with two carriers, we can have 28 to 30 users in the same spectrum that can handle 100 AMPS users.

So, how can it be that IS-95 can be used to increase the capacity of a system over AMPS? The secret is in the way the frequencies are reused.Interference and Processing GainAssume that the same seven-cell area is sectorized into 21 sectors. In an IS-95 CDMA system, the same RF carrier frequency is allowed to be used in every cell and every sector and thus is said to have a frequency reuse factor of "1". That is because with CDMA, the spreading PN codes are used to recover each user's signal, instead of separate RF channels as with AMPS.

Using today's technology that can support approximately 15 users per IS-95 CDMA carrier, the seven-cell area has the potential to accommodate 2,520 active users (multiply 21 sectors by 8 carriers by 15 users).

Keep in mind that CDMA also uses the PCS spectrum. Therefore, CDMA systems have the potential to support many thousands of active users in each seven-cell area.Interference and Processing Gain

RAKE ReceptionMultipaths and Delay Spread

The received signal is actually the sum of several signals, each traveling over a separate path and arriving at a different time.

TM, the length of time between reception of the first and last path of the signal, is referred to as delay spread. Typically, delay spread is between 2 and 5 sRAKE Reception

Effects of Multipath PropagationIntersymbol Interference

RAKE Reception

Effects of Multipath PropagationRayleigh Fading

Severe local variations in signal strength as these multipath signals bounce back and forth due to the buildings and houses, they form many standing-wave pairs in space, those standing-wave pairs are summed together and become an irregular wave-fading structure. This type of variation is called Rayleigh Fading ( because it follows Rayleigh distribution).

When a mobile unit is standing still, its receiver only receives a signal strength at that spot, so a constant signal is observed. When the mobile unit is moving, the fading structure of the wave in the space is received.

The recorded fading becomes fast as the mobile move faster.RAKE Reception

Effects of Multipath PropagationDoppler ShiftThe third effect of multipath propagation is caused by the movement of the mobile station. This effect is known as doppler shift and causes each receive signal to be shifted in frequency as a function of the direction and speed of the mobile. Shifts as much as +100 and +200 Hz can take place in cellular systems at 900 MHz and 1800 MHz respectively. RAKE ReceptionMultipath reception has traditionally posed a significant engineering challenge. In older wireless technologies, delay spread adds noise to the system that cannot be totally eliminated. Fading is a common result.

With CDMA it is quite a different story. The digital coding inherent to CDMA processing enables CDMA receivers to reject most multipaths in much the same way you've seen that they can reject noise (other users).RAKE ReceptionNon-RAKE CDMA Receptionassuming that the receiver is synchronized to the time delay and phase associated with path a.RAKE Reception

Non-RAKE CDMA ReceptionDelay > 1 Tc Delay < 1 Tc

RAKE Reception

Non-RAKE CDMA ReceptionDelay Difference and Differential Distance Between PathsSpeed = Distance * Frequencytime delay = td = distance(m) / [3x108 m/s]

Thus a CDMA system with a chip time of 1s can suppress multipath signals that have a differential distance of 300 m or more.RAKE ReceptionTime Delay (s)Differential Distance (m)Feet0.3331003281.30098410.3000984033.310,00032,800RAKE Receiver Operation

RAKE Reception

RAKE Receiver Operation

RAKE Reception


Once the multipaths are aligned (synchronized), it becomes possible to combine them resulting in a net processing gain.The resulting signal has a higher S/N than any one of the individual multipaths that was received.

RAKE ReceptionRAKE Receiver Operation

RAKE Reception


RAKE Reception

Power ControlNear/Far ProblemServe the maximum number of subscribers and extends the dynamic range of the system. Use optimum power levelsMobile battery & Healthy.Decrease interference to other base stations

Power ControlPower Control

Remember at least a 6 dB signal-to-interference (S/I) ratio is required to properly recover the signal. The S/I ratio is equivalent to the signal energy in each bit (Eb) over the noise from other users (N0). Therefore, in IS-95 systems, the base stations and mobiles measure Eb/N0 to determine how well the signals are received.In order to correct for the near/far problem, CDMA systems use a technique called "Power Control." This technique controls the transmitter power by monitoring both the received signals (Eb) and noise (N0). The goal of power control is to achieve optimum power levels in order to serve the maximum number of subscribers.

Power control is used on both downlink and uplink signal paths. However, power control is done independently in uplink and downlink directions. Each direction uses a different power control procedure.Power Controllinear decrease (mean path loss) with increasing distance (assuming dB and a log scale for distance).

A slow variation (shadowing) about the linear decrease.

A rapid variation (Rayleigh fading) superimposed on the other two.Power Control

Propagation Path Loss:40 dB/decade ( mobile moves 10 Km)Increase with frequency and distance. Power Control

Shadowing:Due to terrain contour between the base station and the mobile unit. Power Control

Rayleigh Fading:

Power Control

Power Control on the Up Link:

1- Autonomous Power Control2- Directed Power Control3- Access Channel Power ControlPower control1- Autonomous Power ControlThis approach to controlling the transmitted power of the mobile unit comes from realizing that an area that causes the base station's signal to be weak at the mobile also causes the mobile's signal to be weak at the base station.

The downlink and uplink signals are at different frequencies, fluctuations do not exactly match. But since the frequencies are very close, the differences between uplink and downlink are negligible.

One way to dynamically control the power level received at the base station is for the mobile unit to vary the power of its broadcast inversely with the power level of the signal it receives. Thus, if the mobile detects that the received power from the transmitter has decreased by a factor of 2 or 3 dB, it will increase its transmit power by that same factor.

Power control1- Autonomous Power ControlPower control

IS-95 refers to this power control technique as "Open Loop" or "Reverse Open Loop" power control1- Autonomous Power ControlAutonomous control measures the received signal strength of all base station pilot signals at the mobile unit. A strong pilot signal received at the mobile means that less transmitter power is required to send uplink information to the base station.Autonomous control is useful in countering large, slow changes in path loss.

Power control2- Directed Power ControlWith directed power control, there are 16 power control bits transmitted in every 20 ms frame. Therefore, Eb/N0 is measured every 1.25 ms.The base station measures Eb/N0 and sends power control bits over the downlink to the mobile to instruct the mobile to either increase or decrease its transmit powerDirected control is much faster than autonomous control and is useful in countering rapid changes in path loss such as shadowing and slow fading.

Power control

2- Directed Power ControlDirect control uses two types of closed loops:Closed outer loop is computed once per frame.Closed inner loop computes changes in transmit power 16 times per frame.

Power control2- Directed Power ControlThe closed outer loop computes a target value of Eb/N0 once per frame. This target value is called the setpoint. The goal of the mobile is to adjust its transmit power to the level indicated by the setpoint. Notice in the example, there are two frames and each frame indicates a calculated setpoint. Power control

2- Directed Power ControlThe closed inner loop receives power control bits from the base station telling the mobile to increase or decrease transmit power in attempt to bring the measured Eb/N0 closer to the target value.

If the measured Eb/N0 is greater than the current setpoint, the base station tells the mobile to decrease power by 1 dB.

If the measured Eb/N0 is less than the current setpoint, the base station tells the mobile to increase power by 1 dB.Power control

2- Directed Power ControlThe closed outer loop reduces the setpoint a small amount (labeled as "a" in the diagram) each frame until a frame error occurs. It takes about 35 frames to lower the setpoint by 1 dB, assuming no other changes occur.Once a frame error occurs, the setpoint is raised a large amount (labeled as "100a" in the diagram) that equals about a 3 dB increase.

Power control

2- Directed Power Control

Power control

3- Access channel Power ControlThe access channel is used to send call requests and messaging from the mobile to the base station prior to establishing a voice or data connection. IS-95 specifies that the access channel must use a power control technique to optimizing transmit power.When the mobile needs to send an access channel message, it uses an access probe sequence consisting of 16 probes. Each access probe consists of an access channel preamble and an access channel message capsule. Power control

3- Access channel Power ControlThe process of sending one message on the access channel and receiving (or failing to receive) an acknowledgement (ACK) for that message is called an access attempt. Within an access attempt, access probes are grouped into access probe sequences of 16 probes each.

The mobile begins by sending the first access probe at a specified power level relative to the autonomous power level. It then waits a specific amount of time for an ACK. If no ACK is received, the mobile waits a random amount of time then sends the next probe in the sequence. Each subsequent probe is transmitted at a higher level than the previous one until the base station responds with an ACK.

Once the mobile receives an ACK, it remembers what power level works.

Power control

Power Control on the DownlinkDownlink power control attempts to use the minimum power needed to meet a Frame Error Rate (FER) threshold for each mobile independently. Note that the base station can increase the S/N for one mobile by increasing the amplitude of its modulating signal before combining the signals for all mobiles.The mobile unit measures the downlink FER and compares it with a set threshold. If the FER has exceeded the threshold, the mobile sends error reports back to the base station, which may increase the modulation (index) for that mobile.The central strategy for downlink power control can be described as follows. When a frame is received without error, there is the possibility that it could have been transmitted successfully at a lower power level. So the following frame is transmitted at a slightly lower power level. Thus each successive frame is transmitted a successively lower power level until, finally, a frame error occurs. The base station then increases its power significantly for the next frame, and the "trickle down" process resumes. A service provider can set values for U and D as required on a per base station basis.Power control

Quadrature Spreading and Modulation


Quadrature Spreading and ModulationBPSK

One Bit a time

QPSK

2 Bits a timeQuadrature Spreading and Modulation


Quadrature Spreading and ModulationNotice that each leg of the quadrature modulator uses a different PN code to spread the incoming signal.

The PN-I(t) and PN-Q(t) spreading codes used on the I- and Q-channels have a low cross-correlation with each other and are neither the same code nor are they orthogonal to each other.

Quadrature Spreading and ModulationQuadrature Despreading and RAKE Receiving

/ .

Documents

2-CDMA Reception and System Performance