The CDMA receiver system in an IS-98-A standard

Transcription

1 The CDMA receiver system in an IS-98-A standard Reciprocal mixing, crossmodulation, spurious response and other issues should be considered carefully in the design of a CDMA receiver. The paper offers a derivation of essential specifications, blockand front-end level, for the receiver section of a CDMA handset. By Walid W. Ali-Ahmad Senior Member of Technical Staff Wireless Communications Group Maxim Integrated Products The harsh environment in which mobile phones operate imposes tough conditions on the handset design. These conditions mandate high performance for the frontend and the DSP back-end, especially for decoding speech information in a received when the carrier-to-interference ratio (C/I) is less than 0dB. The article examines minimum standard levels for the receiver section of a CDMA handset, as described in the TIA/EIA IS-98-A standard. Understanding these standards allow derivation of the minimum measurable specifications that affect system- and blocklevel performance for the front-end of a CDMA cellular radio. BW CDMA Informationmodulated carrier As a primary advantage, spreadspectrum systems have excellent immunity to interference, while hiding their transmissions in the background noise. In CDMA systems, based on direct-sequence spread spectrum (DS- SS) technique, the desired information carrier is modulated by a digital code comprising of a pseudorandom noise (PN) sequence. The PN sequences, however, are not random; they are deterministic and periodic. The PN code s are independent of the data, and have a data rate much higher than that of the desired information. Consequently, the bandwidth of this digital code is much larger than the minimum required for transmitting the baseband data in a digital system. Speed CW carrier interferer BW Informationmodulated carrier R INFO System noise CW Mcps Digital filtering Filtered output R INFO CW Interferencerelated noise CW carrier interference Carrier PN source CDMA correlator Figure 1: The correlator in a CDMA receiver despreads the received. 2 Electronics Engineer July 2000

2 N + N 0 TX Duplexer LNA Rx PA What s Online Technical challenges and solutions for narrowband CDMA ART_ HTM Self-synchronizing a CDMA cellular network ART_ HTM SAW filters in CDMA cellular networks ART_ HTM Figure 2 sources N o and N TX. This act of modulating the information carrier with a digital code, in which carrier bandwidth can be as large as the code bandwidth, spreads the carrier bandwidth by an amount that depends on the modulation employed, such as biphase, quadriphase or minimum shift keying. The receiver then despreads this by cross-correlating it with a synchronized replica of the original PN code. Figure 2: CDMA receiver sensitivity is affected by interference from the white Gaussian noise Basic architecture On the downlink side, from base station to handset, the information to each user is encoded with a different digital coding, PN sequence. Because the PN sequences are orthogonal to each other, they exhibit minimum cross-correlation and therefore minimum interference. However, they occupy the same transmission bandwidths when the spread s of different users are multiplexed on top of each other in the base station transmitter section. This type of multiple access is called codedivision multiple access (CDMA). As one can see from the block diagram of the correlator (despreader) in a baseband CDMA system (Figure 1), it develops processing gain through spreading and despreading of the main information-carrier. After being downconverted to baseband, the CDMA is fed to the input of the CDMA correlator along with other interfering s. When a correlator PN sequence matches the PN sequence embedded in the CDMA, the desired information collapses to its original bandwidth before spreading. On the other hand, input s unmatched to this sequence (receiver noise, CW jamming s or other CDMA s not in exact code synchronism) are spread by the correlator PN sequence to a bandwidth equal to that of the PN coding sequence. Offset Offset The digital filter, which follows the despreader and has a bandwidth equal to the information bandwidth, completely selects the desired information carrier while letting through only a portion of the interference s spread spectrum. Eb/Nt is defined as the ratio of average energy per information bit to the effective noise power density at the correlator output. Noise power density consists of thermal noise and interference from other jamming sources. With spread spectrum systems, the interference is transformed to noise in the despreader. The term jamming margin expresses IO N Interference Figure 3: First interference caused by single-tone s result from reciprocal mixing. 3 Electronics Engineer July 2000

3 leakage Figure 4 4: The second interference component caused by single-tone s result from cross modulation. the ability of a CDMA system to perform in the presence of jamming interference. It is instructive to consider the reaction of a DS-SS system to interference from a single-frequency CW. One might assume that a CDMA system is most affected by interference from other CDMA systems, but that is true only if the interfering has a high correlation with the desired. An interference of wider bandwidth at the input causes a wider bandwidth at the despreader output. The resulting lower-power density delivers less power to the following digital filter, with the result that the interfering has less effect on the system performance. We can, therefore, predict that the most effective source of interference to a CDMA system is a narrowband, such as single-tone CW interference, because the power density in the correlator output due to a CW carrier is higher than that due to wideband s. LNA leakage Cross-modulation interface These code channels consist of a pilot channel, no more than one sync channel, as many as seven paging channels and as many as 63 traffic channels. These code channels share the same base station pilot PN code offset, but they are distinguished from one another at the mobile station receiver by a set of 64 binary orthogonal PN codes based on Walsh functions. Because one traffic channel transports the s for a single user, mainly encoded speech information, the remainder of the discussion concentrates on deriving receiver performance for the forward traffic channel. The full-speech information rate at the output of the handset vocoder is 9.6Kbps. To allow error detection and correction at the receiver, the speech information to be transmitted is encoded convolutionally, interleaved to combat fast fading, and then scrambled for privacy. Next, the encoded, interleaved and scrambled symbols of a single user s traffic channel are BPSK modulated by an assigned orthogonal Walsh code and then QPSK modulated by a pair of base station PN codes. The final data rate for a single-user traffic channel equals the chipping rate (1.2288Mcps). At the output of a base station s sector transmitter, the traffic channels along with pilot, sync and paging channels are all multiplexed on top of each other and assigned the same radio-channel frequency. Hence, the power in each user s traffic channel represents a fraction of the total power for the forward CDMA channel. After bandlimiting by a digital filter, the 3dB bandwidth of a CDMA carrier or forward channel is 1.23MHz. For a forward CDMA channel incident on the handset through a single path, we can define the received spectral power of forward channel, measured as 1.23MHz at the handset antenna connector. The channel received at the mobile station antenna is filtered, amplified, downconverted and demodulated in the receiver s analog front end. After clock recovery through the pilot channel and frame synchronization through the sync channel, the forward Forward CDMA channel While defining the receiver performance of a CDMA handset, you must consider the forward CDMA channel, from base station to mobile station. The forward channel contains one or more code channels, which are transmitted on the same CDMA frequency and share a common pilot PN code phase or offset related to the base station or cell sector assignment. f f -f /2 Half- f 2-by-2 product Figure 5: A single-tone halfway between the CDMA and the frequency produces the 1 / 2- spurious problem. 4 Electronics Engineer July 2000

4 IM 3 interferer f 2 f 1 f 2f 2 -f 1 -f f 2 -f f 1 -f 2f 1 -f 2 -f Figure 6 6: The ability of a CDMA receiver to operate in the presence of two interfering CW tones is indicated by its intermodulation spurious response attenuation. traffic channel assigned to the user or mobile station is decoded in the receiver s modem section. Sensitivity and dynamic range In the IS-98-A standard, receiver sensitivity for a CDMA cellular phone (defined at the antenna connector) equals the minimum received forward CDMA channel power for which the receiver s frame-error rate (FER) does not exceed 0.5 percent. In CDMA systems, the frame is a basic timing interval 20ms long that consists of information on the traffic channel (voice or data), the access channel and the paging channel. Because the link between base station and handset is established on a frame-by-frame basis, the performance of a CDMA mobile phone is evaluated in terms of frame error rate (FER). The IS-98-A standard specifies the minimum sensitivity and dynamic range for a CDMA mobile station receiver. When designing a CDMA mobile phone for high performance, the receiver s sensitivity and dynamic range must meet these minimum standards with a high margin. Regarding receiver sensitivity, two sources of interference are purely white Gaussian noise: the receiver s input-referred thermal noise power spectral density (No) and the transmitter s thermal noise power spectral density (NTX) in the receivefrequency band (Figure 2). No is determined by the receiver s noise figure (NF), and NTX is determined by the amount of transmitter-output thermal noise leaking through the duplexer to the mobile station s receiver input. Because information in a CDMA system is bi-phase modulated, the resulting modulated envelope is non-constant. A typical peak-toaverage ratio for a forward CDMA channel is 10dB. For proper received detection and demodulation, therefore, the receiver should remain linear within the allowed range for received power. In a typical CDMA receiver, the mixer input 1dB compression point is the main limiter of system linearity for high-level received s in the range -30dBm to -20dBm. To resolve this problem in practical mixer designs, the front-end LNA should have a linear gain control with more than 15dB of range or a low-gain mode with a gain-reduction step between 15dB and 20dB. Single-tone desensitization As defined in the IS-98-A standard, single-tone desensitization is a measure of the handset s ability to receive a CDMA at its assigned channel frequency, and in the presence of a single tone spaced at a given frequency offset from the CDMA s center frequency. This single-tone interferer is usually a narrowband AMPS transmitted from a nearby analog cellular base station (AMPS s have a 30kHz bandwidth, compared to the 1,230kHz bandwidth of CDMA s). Inside the front-end of the handset s receiver, the single-tone generates two interfering components whose levels add to it when referred to the receiver s input. Figure 3 shows that the first interference component caused by the single-tone (IRMXG) results from reciprocal mixing phenomena, in which the received of the forward CDMA channel suffers from interference due to phase noise (FN) in the receiver s UHF voltage control oscillator (VCO). This interference is defined at a frequency offset equal to the separation between the wanted and the single-tone frequency, which first mixes with the single-tone in the receiver s front-end mixer and then gets downconverted to. The second interference component caused by a single-tone (IXMOD) results from cross-modulation phenomena (Figure 4), in which third-order nonlinearities in the receiver s front-end cause crossmodulation of the single-tone with the transmission power. Cross modulation occurs mainly in the front-end of the low-noise amplifier (LNA), assuming the transmission leakage is filtered out by the band-pass filter that follows the LNA. Cross-modulation in the LNA generates in-band 5 Electronics Engineer July 2000

5 interference to the received forward CDMA channel. The simulation of cross-modulation phenomena in CDMA systems has produced an equation for estimating the cross-modulation product in CDMA-based cellular systems. The power amplifier s output power is +28dBm when the mobile station is receiving CDMA s close to the sensitivity level. Isolation for a typical RX-TX duplexer is -58dB. Spurious response Choosing a low for the receiver can result in the 1 /2- spurious problem, especially for the PCS band where the bandwidth of the receiver is 60MHz wide. This problem occurs when the frequency of a single-tone, lying inside the receiver s bandwidth, is halfway between the desired CDMA and the UHF local oscillator (Figure 5). The single-tone will be downconverted to in the [2-by-2] mixer spurious product, which acts as an inband interferer to the wanted at the output. As defined in the IS-98-A standard, a receiver s intermodulation-response attenuation is a measure of its ability to receive a CDMA on the assigned channel frequency, in the presence of two interfering CW tones. These tones are separated from the assigned channel frequency and from each other as the third-order intermodulation product of the two interfering CW tones. That occurs in the receiver s odd-order nonlinearities, and produces an interfering that is in-band to the desired CDMA (Figure 6). The IS-98-A standard calls for three test cases of different two-tone levels and desired CDMA level, for which the receiver s frame-error rate (FER) should not exceed 1 percent. It is valid to assume in these test cases that the receiver s third-order nonlinearity is the dominant source of third-order intermodulation products. Essential to the design of highperformance CDMA mobile-station receivers is a thorough derivation of the important system- and block-level specifications based on the IS-98-A CDMA standard. Reciprocal mixing, cross-modulation, 1 /2- spurious response and other -system issues related to CDMA receivers should be considered carefully in the design of any CDMA receiver system. You may your comments to Walid Ali-Ahm-ad at walid_ali-ahmad@ mximhq.com, or fax Electronics Engineer July 2000