02-03-2011, 09:29 AM
[attachment=9314]
CDMA Mobile Communication & IS-95
Most of the slides are stolen from Prof. Abhay Karandikar’s lecture
Spread Spectrum Priniciples
Does not attempt to allocate disjoint frequency or time slot resources
Instead, this approach allocates all resources to simultaneous users, controlling the power transmitted by each user to the minimum required to maintain a given SNR
Each user employs a noise-like wideband signal occupying the entire frequency allocation
Each user contributes to the background noise affecting all users, but to the least extent possible.
Spread Spectrum Priniciples
This restriction on interference limits capacity, but because time and bandwidth resource allocations are unrestricted, the resulting capacity is significantly higher than the conventional system
Spread Spectrum Priniciples
Suppose each user use a wideband Gaussian noise carrier
Suppose each user’s transmission is controlled so that all signals received at the BS are of equal power
Let Ps be the power of each user, and the background noise be negligible.
Then the total interference power, I, presented to each user’s demodulator is
I = (K-1) Ps (1) where K is the number of users
Spread Spectrum Priniciples
Let’s say demodulator of each user operates at bit-energy-to-noise-density level of Eb/N0.
So the noise density received by each user’s demodulator is N0 = I/W (2), where W Hz is the bandwidth of the wideband noise carriers
The received energy per bit is the received signal power divided by the data rate R (bits/s), i.e., Eb = Ps/R (3)
Spread Spectrum Priniciples
Combining (1), (2) and (3) we get
K – 1 = I/Ps = (W/R) / (Eb/N0) (4)
If W >> R then the capacity of the system can be large
i.e., transmission bandwidth should be much larger than the message bandwidth
If Eb/N0 is small, then also the capacity can be large. (since Eb/N0 α SNR, this means SNR should be as small as possible)
Code Division Multiple Access - CDMA
Multiple users occupying the same band by having different codes is known as CDMA - Code Division Multiple Access system
Let
W - spread bandwidth in Hz
R = 1/Tb = Date Rate
S - received power of the desired signal in W
J - received power for undesired signals like multiple access users, multipath, jammers etc in W
Eb - received energy per bit for the desired signal in W
N0 - equivalent noise spectral density in W/Hz
In conventional systems W/R 1 which means, for satisfactory operation J/S < 1
Example Let R = 9600; W = 1.2288 MHz
(Eb/N0)min = 6 dB (values taken from IS-95)
Jamming margin (JM) = 10log10(1.2288*106/9.6*103) - 6
= 15.1 dB 32
This antijam margin or JM arises from Processing Gain (PG) = W/R = 128
If (Eb/N0)min is further decreased or PG is increased, JM can be further increased
CDMA (contd…)
JM can be used to accommodate multiple users in the same band
If (Eb/N0)min and PG is fixed, number of users is maximized if perfect power control is employed.
Capacity of a CDMA system is proportional to PG.
Spreading Codes
A noise-like and random signal has to be generated at the transmitter.
The same signal must be generated at the receiver in synchronization.
We limit the complexity by specifying only one bit per sample, i.e., a binary sequence.
Desirable Randomness Properties
Relative frequencies of “0” and “1” should be ½ (Balance property)
Run lengths of zeros and ones should be (Run property):
Half of all run lengths should be unity
One - quarter should be of length two
One - eighth should be of length three
A fraction 1/2n of all run lengths should be of length n for all finite n
Desirable Randomness Properties (contd…)
If the random sequence is shifted by any nonzero
number of elements, the resulting sequence
should have an equal number of agreements and
disagreements with the original sequence
(Autocorrelation property)
PN Sequences
A deterministically generated sequence that nearly satisfies these properties is referred to as a Pseudorandom Sequence (PN)
Periodic binary sequences can be conveniently generated using linear feedback shift registers (LFSR)
If the number of stages in the LFSR is r, P 2r - 1 where P is the period of the sequence
PN Sequences (contd…)
However, if the feedback connections satisfy a specific property, P = 2r - 1. Then the sequence is called a Maximal Length Shift Register (MLSR) or a PN sequence.
Thus if r=15, P=32767.
MLSR satisfies the randomness properties stated before
Randomness Properties of PN Sequences
Balance property - Of the 2r - 1 terms, 2r-1 are one and 2r-1–1 are zero. Thus the unbalance is 1/P. For r=50; 1/P10-15
Run length property - Relative frequency of run length n (zero or ones) is 1/ 2n for n r-1 and 1/(2r - 1) for n = r
One run length each of r-1 zeros and r ones occurs. There are no run lengths for n > r
Autocorrelation property - The number of disagreements exceeds the number of agreements by unity. Thus again the discrepancy is 1/p
PN Sequences Specified in IS-95
A “long” PN sequence (r =42) is used to scramble the user data with a different code shift for each user
The 42-degree characteristic polynomial is given by:
x42+x41+x40+x39+x37+x36+x35+x32+x26+x25+x24+x23+x21+x20+x17+x16+x15+x11+x9+x7+1
The period of the long code is 242 - 1 4.4*102 chips and lasts over 41 days
PN Sequences Specified in IS-95 (contd…)
A short PN sequence (r = 15) is specific to a base station and its period is (215−1)Tc = 27ms.
Two “short” PN sequences (r=15) are used to spread the quadrature components of the forward and reverse page link waveforms
Power Control in CDMA
CDMA goal is to maximize the number of simultaneous users
Capacity is maximized by maintaining the signal to interference ratio at the minimum acceptable
Power transmitted by mobile station must be therefore controlled
Transmit power enough to achieve target BER: no less no more
Two factors important for power control
Propagation loss due to propagation loss, power variations up to 80 dB
a high dynamic range of power control required
Channel Fading
average rate of fade is one fade per second per mile hour of mobile speed
power attenuated by more than 30 dB
power control must track the fade
Power Control in IS-95A
At 900 MHz and 120 km/hr mobile speed Doppler shift =100Hz
In IS 95-A closed loop power control is operated at 800 Hz update rate
Power control bits are inserted (‘punctured’) into the interleaved and encoded traffic data stream
Power control step size is +/- 1 dB
Power control bit errors do not affect performance much
Rake Receiver
Mobile station receives multiple attenuated and delayed replicas of the original signal (multipath diversity channels).
Two multipath signals are resolvable only if their relative delay exceeds the chip period Tc
Amplitudes and phases of multipath components are found by correlating the received waveform with multiple delayed versions of the signal (delay = nTc).
Searcher performs the above task for up to 3 different multipath signals.
3 parallel demodulators (RAKE fingers) isolate the multipath components and the RAKE receiver combines them.
Handoff in CDMA System
In GSM hard handoff occurs at the cell boundary
Soft Handoff
Mobile commences Communication with a new BS without interrupting communication with old BS
same frequency assignment between old and new BS
provides different site selection diversity
Softer Handoff
Handoff between sectors in a cell
CDMA to CDMA hard handoff
Mobile transmits between two base stations with different frequency assignment
Soft Handoff- A unique feature of CDMA Mobile
Advantages
Contact with new base station is made before the call is switched
Diversity combining is used between multiple cell sites
Diversity combining is the process of combining information from multiple transmitted packets to increase the effective SNR of received packets
additional resistance to fading
If the new cell is loaded to capacity, handoff can still be performed for a small increase in BER
Neither the mobile nor the base station is required to change frequency
CDMA Mobile Communication & IS-95
Most of the slides are stolen from Prof. Abhay Karandikar’s lecture
Spread Spectrum Priniciples
Does not attempt to allocate disjoint frequency or time slot resources
Instead, this approach allocates all resources to simultaneous users, controlling the power transmitted by each user to the minimum required to maintain a given SNR
Each user employs a noise-like wideband signal occupying the entire frequency allocation
Each user contributes to the background noise affecting all users, but to the least extent possible.
Spread Spectrum Priniciples
This restriction on interference limits capacity, but because time and bandwidth resource allocations are unrestricted, the resulting capacity is significantly higher than the conventional system
Spread Spectrum Priniciples
Suppose each user use a wideband Gaussian noise carrier
Suppose each user’s transmission is controlled so that all signals received at the BS are of equal power
Let Ps be the power of each user, and the background noise be negligible.
Then the total interference power, I, presented to each user’s demodulator is
I = (K-1) Ps (1) where K is the number of users
Spread Spectrum Priniciples
Let’s say demodulator of each user operates at bit-energy-to-noise-density level of Eb/N0.
So the noise density received by each user’s demodulator is N0 = I/W (2), where W Hz is the bandwidth of the wideband noise carriers
The received energy per bit is the received signal power divided by the data rate R (bits/s), i.e., Eb = Ps/R (3)
Spread Spectrum Priniciples
Combining (1), (2) and (3) we get
K – 1 = I/Ps = (W/R) / (Eb/N0) (4)
If W >> R then the capacity of the system can be large
i.e., transmission bandwidth should be much larger than the message bandwidth
If Eb/N0 is small, then also the capacity can be large. (since Eb/N0 α SNR, this means SNR should be as small as possible)
Code Division Multiple Access - CDMA
Multiple users occupying the same band by having different codes is known as CDMA - Code Division Multiple Access system
Let
W - spread bandwidth in Hz
R = 1/Tb = Date Rate
S - received power of the desired signal in W
J - received power for undesired signals like multiple access users, multipath, jammers etc in W
Eb - received energy per bit for the desired signal in W
N0 - equivalent noise spectral density in W/Hz
In conventional systems W/R 1 which means, for satisfactory operation J/S < 1
Example Let R = 9600; W = 1.2288 MHz
(Eb/N0)min = 6 dB (values taken from IS-95)
Jamming margin (JM) = 10log10(1.2288*106/9.6*103) - 6
= 15.1 dB 32
This antijam margin or JM arises from Processing Gain (PG) = W/R = 128
If (Eb/N0)min is further decreased or PG is increased, JM can be further increased
CDMA (contd…)
JM can be used to accommodate multiple users in the same band
If (Eb/N0)min and PG is fixed, number of users is maximized if perfect power control is employed.
Capacity of a CDMA system is proportional to PG.
Spreading Codes
A noise-like and random signal has to be generated at the transmitter.
The same signal must be generated at the receiver in synchronization.
We limit the complexity by specifying only one bit per sample, i.e., a binary sequence.
Desirable Randomness Properties
Relative frequencies of “0” and “1” should be ½ (Balance property)
Run lengths of zeros and ones should be (Run property):
Half of all run lengths should be unity
One - quarter should be of length two
One - eighth should be of length three
A fraction 1/2n of all run lengths should be of length n for all finite n
Desirable Randomness Properties (contd…)
If the random sequence is shifted by any nonzero
number of elements, the resulting sequence
should have an equal number of agreements and
disagreements with the original sequence
(Autocorrelation property)
PN Sequences
A deterministically generated sequence that nearly satisfies these properties is referred to as a Pseudorandom Sequence (PN)
Periodic binary sequences can be conveniently generated using linear feedback shift registers (LFSR)
If the number of stages in the LFSR is r, P 2r - 1 where P is the period of the sequence
PN Sequences (contd…)
However, if the feedback connections satisfy a specific property, P = 2r - 1. Then the sequence is called a Maximal Length Shift Register (MLSR) or a PN sequence.
Thus if r=15, P=32767.
MLSR satisfies the randomness properties stated before
Randomness Properties of PN Sequences
Balance property - Of the 2r - 1 terms, 2r-1 are one and 2r-1–1 are zero. Thus the unbalance is 1/P. For r=50; 1/P10-15
Run length property - Relative frequency of run length n (zero or ones) is 1/ 2n for n r-1 and 1/(2r - 1) for n = r
One run length each of r-1 zeros and r ones occurs. There are no run lengths for n > r
Autocorrelation property - The number of disagreements exceeds the number of agreements by unity. Thus again the discrepancy is 1/p
PN Sequences Specified in IS-95
A “long” PN sequence (r =42) is used to scramble the user data with a different code shift for each user
The 42-degree characteristic polynomial is given by:
x42+x41+x40+x39+x37+x36+x35+x32+x26+x25+x24+x23+x21+x20+x17+x16+x15+x11+x9+x7+1
The period of the long code is 242 - 1 4.4*102 chips and lasts over 41 days
PN Sequences Specified in IS-95 (contd…)
A short PN sequence (r = 15) is specific to a base station and its period is (215−1)Tc = 27ms.
Two “short” PN sequences (r=15) are used to spread the quadrature components of the forward and reverse page link waveforms
Power Control in CDMA
CDMA goal is to maximize the number of simultaneous users
Capacity is maximized by maintaining the signal to interference ratio at the minimum acceptable
Power transmitted by mobile station must be therefore controlled
Transmit power enough to achieve target BER: no less no more
Two factors important for power control
Propagation loss due to propagation loss, power variations up to 80 dB
a high dynamic range of power control required
Channel Fading
average rate of fade is one fade per second per mile hour of mobile speed
power attenuated by more than 30 dB
power control must track the fade
Power Control in IS-95A
At 900 MHz and 120 km/hr mobile speed Doppler shift =100Hz
In IS 95-A closed loop power control is operated at 800 Hz update rate
Power control bits are inserted (‘punctured’) into the interleaved and encoded traffic data stream
Power control step size is +/- 1 dB
Power control bit errors do not affect performance much
Rake Receiver
Mobile station receives multiple attenuated and delayed replicas of the original signal (multipath diversity channels).
Two multipath signals are resolvable only if their relative delay exceeds the chip period Tc
Amplitudes and phases of multipath components are found by correlating the received waveform with multiple delayed versions of the signal (delay = nTc).
Searcher performs the above task for up to 3 different multipath signals.
3 parallel demodulators (RAKE fingers) isolate the multipath components and the RAKE receiver combines them.
Handoff in CDMA System
In GSM hard handoff occurs at the cell boundary
Soft Handoff
Mobile commences Communication with a new BS without interrupting communication with old BS
same frequency assignment between old and new BS
provides different site selection diversity
Softer Handoff
Handoff between sectors in a cell
CDMA to CDMA hard handoff
Mobile transmits between two base stations with different frequency assignment
Soft Handoff- A unique feature of CDMA Mobile
Advantages
Contact with new base station is made before the call is switched
Diversity combining is used between multiple cell sites
Diversity combining is the process of combining information from multiple transmitted packets to increase the effective SNR of received packets
additional resistance to fading
If the new cell is loaded to capacity, handoff can still be performed for a small increase in BER
Neither the mobile nor the base station is required to change frequency
