Outline. 2.4.1 Introduction 2.4.2 Pseudo-Noise Sequences 2.4.3 Direct-sequence Spread Spectrum 2.4.4 Frequency-Hop Spread Spectrum 2.4.5 Time-Hop Spread Spectrum 2.4.6 Acquisition and Tracking 2.4.7 Performance in a Jamming Environment 2.4.8 CDMA and Smart Antenna 2.4.9 Summary
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2.4.2 Pseudo-Noise Sequences
2.4.3 Direct-sequence Spread Spectrum
2.4.4 Frequency-Hop Spread Spectrum
2.4.5 Time-Hop Spread Spectrum
2.4.6 Acquisition and Tracking
2.4.7 Performance in a Jamming Environment
2.4.8 CDMA and Smart Antenna
CDMA (code division multiple access) is a wireless technology that allows multiple radio subscribers to share the same frequ-ency band at the same time by assigning each user a unique code.
Smart antennas are a technology for wireless systems that use a fixed set of antenna elements in an array. The signals from these antenna elements are combined to form a movable beam pattern that can be steered to a desired direction that tracks mobile units as they move.
Both smart antenna technology
and CDMA promise to
revolutionize the field of
CDMA offers some advantage over TDMA and FDMA.
A U.S. digital cellular system based on CDMA which promises increased capacity has been standardized as Interim Standard 95 (IS-95) by the U.S. Telecommunications Industry Association (TIA).
IS-95 allows each user within a cell to use the same radio channel, and users in adjacent cells also use the same radio channel, since this is a direct sequence spread spectrum CDMA system.
CDMA completely eliminates the need for frequency planning within a market.
CDMA IS-95 uses a different modulation and spreading technique for the forward and reverse links.
On the forward link, the base station simultaneouslytransmits the user data for all mobiles in the cell by using a different spreading sequence for each mobile.
A pilot code is also transmitted simultaneously and at a higher power level, thereby allowing all mobiles to use coherent carrier detection while estimating the channel conditions.
On the reverse link, all mobiles respond in an asynchronous fashion and have ideally a constant signal level due to power control applied by the base station.
An essential element of the reverse link is tight control of each subscriber’s transmitter power, to avoid the ‘near-far’ problem that arise from varying received powers of the users.
A combination of open-loop and fast, closed-loop power control is used to adjust the transmit power of each in-cell subscriber so that the base station receives each user with the same received power.
The commands for the closed-loop power control are sent at a rate of 800 b/s, and these bits are stolen from the speech frames.
Without fast power control, the rapid power changes due to fading would degrade the performance of all users in the system.
At both the base station and the subscriber, RAKE receivers are used to resolve and combine multipath components, thereby reducing the degree of fading.
The RAKE receiver exploits the multipath time delays in a channel and combines the delayed replicas of the transmitted signal in order to improve link quality. In IS-95, a three finger RAKE is used at the base station.
The IS-95 architecture also provides base station diversity during ‘soft’ handoffs, whereby a mobile making the transition between cells maintains links with both base stations during the transition.
The mobile receiver combines the signals from the two base stations in the same manner as it would combine signals associated with different multipath components.
An M-branch (M-finger) RAKE receiver implementation is shown in Figure.
Each correlator detects a time shifted version of the original CDMA transmission, and each finger of the RAKE correlates to a portion of the signal which is delayed by at least one chip in time from the other finger.
The long PN sequence is uniquely assigned to each user is a periodic long code with period chips. It is specified by the following characteristic polynomial
i = 0 1 2 3 5 6 7 10 16 17 18 19 21 22 25 26 27 31 33 35 42
However, the output of the linear feedback shift register (LFSR) is not used directly for spreading. Instead, the long code comprises an altered version of this sequence which is produced using a Long Code Mask.
The reverse traffic channel is spread by the long code PN sequence which operates at a rate of 1.2288 Mcps. This corresponds to repeating approximately 41.4 days.
Consider a single cell CDMA system in which K mobile units simultaneously transmit to a single base station.
In IS-95-based CDMA system, the uplink bit error rate is a complex function of different error control coding methods, orthogonal 64-ary modulation, multiple spreading codes, power control, and other factors.
A critical parameter to measure link performance is the Carrier-to-Interference-and-Noise-Ratio (CINR) available for each subscriber.
The CINR for each subscriber is measured after despreading.
On the reverse channel, the CINR for a particular subscriber is measured at the input to the Walsh Chip Matched Filter at the base station.
We define the CINR after despreading, as the ratio of the desired signal to the sum of interference and noise.
where is the power of the desired signal at the input to the despreader at the base station, and is the power from every other user for . The spreading factor N is defined as
The CINR Eq. reflects the fact that spreading reduces the impact of multiple access interference (MAI), . The noise variance,
, represents the noise contribution to the decision variable after dispreading.
Multiplying the numerator and denominator of above Eq. by the bit duration, , we have
where the term is the energy per bit for the desired subscriber signal.
After dispreading, the noise bandwidth is approximately . If the thermal noise has a power spectral density of , then we can write the CINR as
The term represents the power spectral density of the total MAI after dispreading.
Rather than using the spreading factor to compute the CINR in complex systems such as IS-95, it is more appropriate to computer the CINR using the processing gain.
For the IS-95 uplink, the chip rate is 1.2288 Mcpc. For Rate Set 1, the maximum symbol rate out of the convolutional encoder is 28.8 Ksps and uses a 1/3 rate convolutional encoder on the reverse link, the processing gain is
or PG = 21.1 dB.
It is also important to account for the fact that IS-95-based CDMA system take advantage of voice inactivity.
Because the vocoder reduces its output rate when the speaker is silent, the subscriber unit does not transmit continuously, but is gated on and off with a duty cycles low as 1/8 during silent periods. This is captured in the voice activity factor, ν.
Typically, the voice activity factor reduces the average MAI level seen by the base station receiver by 50%-60% relative to the case where all subscribers are transmitting continuously.
We can modify the above CINR Eq. to take into account the improvement due to the voice activity factor:
For the reverse link in a single cell CDMA system, consider the case in which each portable unit has an omni-directional antenna
And the base station uses a directive antenna with the gain of the horizontal portion of the pattern is and the gain of the overall pattern is .
We assume that K users in the single cell CDMA system are uniformly distributed throughout a two-dimensional cell as shown in Figure in next page.
If perfect power control is applied so that the power incident at the base station antenna from each user is the same, said . The average total interference power received at the central base station may be expressed as 
The cell geometry shows a central base station surrounded by uniformly distributed subscribers (The horizontal pattern is assumed to be a perfect sectorization with zero sidelobes).
Assume that the system is set to operate with , so that the system is not thermal noise-limited, then the mean CINR is found
Thus, up to 97 subscribers can be supported on the reverse link in single cell.