Cdma Basic Concepts 4m2b5r

CDMA Basic Concepts

Definition „

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CDMA is a digital wireless air interface and networking standard based on the principle of spread-spectrum techniques, which allow multiple s to access the system simultaneously on the same carrier frequency. CDMA is a method in which s occupy the same time and frequency allocations, and are channelized by unique assigned codes. The signals are separated at the receiver by using a correlator that accepts only signal energy from the desired channel. Undesired signals contribute only to noise.

Spread Spectrum Techniques „

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In spread spectrum, the signal occupies a bandwidth in excess of the minimum necessary to send the information The band spread is accomplished by means of a code that is independent of the data, and a synchronized reception with the code at the receiver is used for despreading and subsequent data recovery. Using Spread Spectrum, Processing gain = W/R can be achieved, where W is the final bandwidth of the spread message and R is the band-width of the baseband message.

Advantages of CDMA „ „ „ „ „ „ „

Increased Capacity Simplified frequency reuse Multipathing Soft Hand-off Variable rate speech coding Security Increases Mobile Unit Battery Life

Capacity • • •

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No of s that can be simultaneously ed Dynamic in nature Reverse link capacity Depends on ¾ Distance from cell site ¾ Cell loading ¾ Traffic distribution within and outside the cell ¾ Voice activity Forward Link Capacity Depends Upon ¾ Cell site Tx. Power ¾ Need to maintain a minimum S/N at each mobile

CDMA Power Control In CDMA, the base station communicates to the mobile station, instructing the mobile to adjust its power up or down. The mobile station transmits only enough power to maintain a link. The primary design goal of a CDMA system is for all s to be received by the base station at the same power level, and to make that power level as low as possible while still maintaining a high quality call. Any more power than needed adds unnecessarily to the overall noise level on the CDMA channel, and cuts down capacity.

Soft Handoff With traditional hard handoffs, which are used in all other types of cellular systems, the mobile drops a channel before picking up the next channel. When a call is in a soft handoff condition, a mobile is monitored by two or more cell sites and the transcoder circuitry compares the quality of the frames from the two receive cell sites on a frame-by-frame basis. The system can take advantage of the moment-by-moment changes in signal strength at each of the two cells to pick out the best signal. Soft handoffs also contribute to high call quality by providing a "make before break" connection. This eliminates the short disruption of speech one hears with non-CDMA technologies when the RF connection breaks from one cell to establish the call at the destination cell during a handoff.

Vocoder The vocoder used by the IS-95 system is variable rate, which means that the output rate of the vocoder is adjusted according to a ’s actual speech pattern. For example, if the is not speaking during part of the conversation, the output rate of the vocoder is lowered to prevent power from being transmitted unnecessarily. The effect of this variable-rate vocoding is the reduction of overall transmitted power and hence interference. Speech statistics shows that a in a conversation typically speaks between 40% and 50% of the time. By employing variable-rate vocoding, the system reduces the total interference power by this voice activity factor.

Spatial Diversity Spatial Diversity refers to the use of two receive antennas separated by some physical distance. The principle of spatial diversity recognizes that when a mobile is moving about, it creates a pattern of signal peaks and nulls. When one of these nulls falls on one antenna it will cause the received signal strength to drop. However, if a second antenna is placed some physical distance away, it will be outside of the signal null area and thus receive the signal at an acceptable signal level.

Spectrum Requirement „ „ „

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CDMA Carrier Bandwidth = 1.23 MHz Guard Band =0.27 MHz on either side CDMA Channel Band: - Forward Link 869 – 894 MHz - Reverse Link 824 – 849 MHz Frequency Calculations: Transmitter Channel No. Frequency Mobile 1<=N<=866 .03N + 825 MHz Cell Site 1<=N<=866 .03N + 870 MHz

CDMA Carrier for Cellular band (850 MHz)

CDMA Reverse & Forward Link Physical Layer

Interleaving is a technique for randomizing the bits in a message stream so that burst errors introduced by the channel can be converted to random errors. The output from the vocoder and input into the convolutional encoder is 9.6 Kbps (at full rate for Rate Set 1) The Rate 1/2 convolutional encoder doubles the baseband rate to 19.2 Kbps for error correction. After the baseband signal has been channel coded for error control, the signal is further transformed in order to allow multiple access by different s. Multiple access refers to the sharing of a common resource in order to allow simultaneous communications by multiple s, and this common resource is the RF spectrum. In CDMA, each ’s narrowband signal is spread over a wider band-width. This wider bandwidth is greater than the minimum bandwidth required to transmit the information. Each ’s narrowband signal is spread by a different wideband code. All the spread wideband signals (of different s) are added together to form a composite signal, and the composite signal is transmitted over the air in the same frequency band. The receiver is able to distinguish among the different s by using a copy of the original code. The receiver shifts the desired out of the composite signal by correlating the composite signal with the original code. All other s with codes that do not match the code of the desired are rejected. The IS-95 CDMA system has asymmetric links (i.e., the forward and the reverse links have different link structures). The differences range from the modulation scheme to error control methods. In addition, each link uses different codes to channelize individual s. The forward link uses Walsh codes, while the reverse link uses pseudorandom noice (PN) codes for channelization. The IS-95 CDMA system uses QPSK for both forward and reverse links. The reverse link, in particular, uses a variant of QPSK called offset quadrature phase-shift keying (OQPSK). OQPSK differs from the conventional QPSK in that prior to carrier multiplication, a delay of a half-bit interval (with respect to the I path) is placed in the Q path (see Figure 3.22). This is done to avoid a 180-degree phase transition that occurs in conventional QPSK systems.

CDMA Forward Link Physical Layer 1.2288Mbps

I SHORT CODE

WALSH COVER

Vocoded Speech Data

20 msec blocks

Convolutional Encoder 9.6 Kbps

Mbps

Interleaver

19.2 Kbps

1.2288

19.2 Kbps

FIR

I

FIR

Q

19.2 Kbps

LONG CODE 1.2288 Mbps

WALSH CODE GENERATOR

Q SHORT CODE 1.2288Mbps

CDMA Reverse Link Physical Layer 1.2288Mbps 64-ARY MODULATOR

I SHORT CODE

1 OF 64 WALSH CODES

WALSH CODE 63

Vocoded Speech Data

Convolutional Encoder

Mbps

Interleaver WALSH

20 msec blocks

9.6 Kbps

1.2288

28.8 Kbps

28.8 Kbps

FIR

I

FIR

Q

307.2 Kbps

CODE 0

T/2

1/2 CHIP DELAY

Q SHORT CODE 1.2288Mbps

LONG CODE 1.2288Mbps

Encoding The CDMA transmission process uses several codes to ensure end-to-end communication. „ „

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Long Code The Long Code is used to encrypt the content of the transmitted signal, and thus ensures privacy. Pilot Channel PN Code The Pilot Channel PN code is used by the mobile to identify the antenna and the base station with which the mobile is communicating. Walsh Code The Walsh code is used to uniquely define individual communication channels. Walsh codes are “orthogonal” mathematical codes. As such, if two Walsh codes are correlated, the result is intelligible only if these two codes are the same. As a result, a Walsh-encoded signal appears as random noise to a PCS CDMA capable mobile terminal, unless that terminal uses the same code as the one used to encode the incoming signal.

Forward CDMA Channel

Reverse CDMA Channel

CDMA Call Processing