Performance characteristics of the IS-95 standard for CDMA spread spectrum mobile communication systems

Transcription

1 Retrospective Theses and Dissertations 1996 Performance characteristics of the IS-95 standard for CDMA spread spectrum mobile communication systems Vijayalakshmi R. Raveendran Iowa State University Follow this and additional works at: Part of the Digital Communications and Networking Commons, Signal Processing Commons, and the Systems and Communications Commons Recommended Citation Raveendran, Vijayalakshmi R., "Performance characteristics of the IS-95 standard for CDMA spread spectrum mobile communication systems" (1996). Retrospective Theses and Dissertations This Thesis is brought to you for free and open access by Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact

2 Performance characteristics of the IS-95 standard for CDMA spread spectrum mobile communication systems by Vijayalakshmi R. Raveendran A Thesis Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Department: Electrical and Computer Engineering Major: Electrical Engineering Signatures have been redacted for privacy niversity 1996 Copyright Vijayalakshmi R. Raveendran, All rights reserved

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4 III TABLE OF CONTENTS CHAPTER Overview INTRODUCTION Motivation and Scope of Research. CHAPTER 2. LITERATURE REVIEW. 2.1 Introduction Previous Work on System Simulation 2.3 Previous Work on Channel Impulse Response Measurements 2.4 Previous Work on RAKE Receivers 2.5 Relevance to Research CHAPTER 3. OVERVIEW OF THE IS-95 STANDARD Introduction Forward CDIVIA Channel Structure Receiver at Mobile Station. 3.3 Reverse CDMA Channel Structure 3.4 Power Control. 3.5 Voice Privacy. 3.6 Call Processing 3.7 Handoff Procedures

5 IV CHAPTER 4. COMMUNICATION SYSTElVl MODEL Introduction Forward CDMA Channel Structure Code Channel Data Generator Convolutional Encoder Symbol Repetition Block Interleaver Walsh Code Generator Long Code Generator PN Sequence Generators Baseband Filter QPSK Modulator I Data Scrambler and Frame Quality Indicators 4.3 Reverse CDMA Channel Structure Code Channel Data Generators Convolutional Encoder Symbol Repetition Block Interleavers Walsh Code, Long Code, PN Sequence Generators and Baseband Filter OQPSK Modulator Data Burst RandomizeI' and Frame Quality Indicators 4.4 Receiver Structure Integrator and Sampler

6 v RF Demodulators RAKE Receiver.. CHAPTER 5. MULTIPATH CHANNEL MODEL 5.1 Introduction The Multipath Channel Propagation Path Loss Long-Term Fading Short-Term Fading Simulated Multipath Channel Model CHAPTER 6. SIMULATION RESULTS 6.1 Introduction BER Estimation Monte Carlo Simulation Importance Sampling Extreme Value Theory 6.2 SNR Estimation Transmitter Characteristics 6.4 Receiver Characteristics Multipath Channel Characteristics CHAPTER 7. CONCLUSIONS BIBLIOGRAPHY APPENDIX

7 VI LIST OF TABLES Table 2.1 Radio Channel Allocation... 6 Table 3.1 Sync Channel Modulation Parameters Table 3.2 Paging Channel Modulation Parameters Table 3.3 Forward Traffic Channel Modulation Parameters Table 3.4 Forward CDMA Channel I and Q Mapping Table 3.5 Reverse Traffic Channel Modulation Parameters Table 3.6 Access Channel Modulation Parameters Table 4.1 Block Interleaver Parameters Table 6.1 Observed Channel Characteristics... 71

8 Vll LIST OF FIGURES Figure 2.1 Typical Power Delay Profiles for various propagation channel environments Figure 3.1 Figure 3.2 Forward CDMA Channel Structure Forward COMA Channel Signal Constellation and Phase Transition Figure 3.3 Example of a forward CDMA channel transmitted by a base station Figure 3.4 Figure 3.5 Reverse COMA Channel Structure Example of a reverse CDMA channel received by a base station. 18 Figure 4.1 Figure 4.2 Figure 4.3 Figure 4.4 Figure 4.5 Figure 4.6 Figure 4.7 Figure 4.8 Forward COMA Channel Structure Random Data Generator Convolutional Encoder, K = 9, Rate = 1/ Walsh Function Waveform Generator Long Code Generator I Channel PN Sequence Generator Q Channel PN Sequence Generator Forward COMA Channel QPSK Modulator

9 Vlll Figure 4.9 Reverse CDMA Channel Structure Figure 4.10 Convolutional Encoder K = 9, rate = 1/ Figure 4.11 OQPSK Modulator Figure 4.12 Receiver Structure Figure 4.13 Integrator and Sampler Figure 4.14 Typical QPSK Demodulator Figure 4.15 RAKE Receiver Structure Figure 5.1 Propagation path loss for different environments Figure 5.2 Figure 5.3 Long Term Fading Experimental record of received signal envelope in an urban area Figure 5.4 Figure 5.5 Sample Power Delay Profile Multipath Channel Model Figure 6.1 Schematic representation of implementation of Monte Carlo estimation procedure Figure 6.2 Figure 6.3 Figure 6.4 Figure 6.5 SNR Estimation Sub-optimal multipath diversity receiver architecture Simulation test setup Comparison of BER Vs SNR for optimum and four-way combining RAKE diversity receiver

10 IX LIST OF ABBREVIATIONS Access Channel: A Reverse CDMA Channel used by mobile stations for communicating to the base station. The Access Channel is used for short signaling message exchanges such as call originations, responses to pages, and registrations. The Access Channel is a slotted random access channel. Active Set: The set of pilots associated with the CDMA Channels containing Forward Traffic Channels assigned to a particular mobile station. Analog Voice Channel: An analog channel on which a voice conversation occurs and on which brief digital messages may be sent from a base station to a mobile station or from a mobile station to a base station. AWGN: Additive White Gaussian Noise. Base Station: A station in the Domestic Public Cellular Radio Telecommunications Service, other than a mobile station, used for communicating with mobile stations. Depending upon the context, the term base station may refer to a cell, a sector within a cell, an MSC, or other part of the cellular system. BERSIM: Bit Error Rate Simulator. bps: Bits per second. CDMA: See Code Division Multiple Access. CDMA Channel: The set of channels transmitted between the base station and

11 x the mobile stations within a given CDMA frequency assignment. See also Forward CDMA Channel and Reverse CDMA Channel. CDMA Channel Number: An ll-bit number corresponding to the center of the CDMA frequency assignment. CDMA Frequency Assignment: A 1.23 MHz segment of spectrum centered on one of the 30 khz channels of the existing analog system. Cellular Communication System: As defined by the Federal Communication Commission (FCC): A high capacity land mobile system in which assigned spectrum is divided into discrete channels which are assigned in groups to geographic cells covering a cellular geographic service area. The discrete channels are capable of being reused in different cells within the service area. Code Channel: A sub channel of a Forward CDMA Channel. A Forward CDMA Channel contains 64 code channels. Code channel zero is assigned to the Pilot Channel. Code channels 1 through 7 may be assigned to the either Paging Channels or the Traffic Channel. Code channel 32 may be assigned to either a Sync Channel or a Traffic Channel. The remaining code channels may be assigned to Traffic Channels. Code Symbol: The output of an error-correcting encoder. Information bits are input to the encoder and code symbols are output from the encoder. See Convolutional Code. Convolutional Code: A type of error-correcting code. A code symbol can be considered as the convolution of the input data sequence with the impulse response of a generator function. CRC: See Cyclic Redundancy Code.

12 Xl Cyclic Redundancy Code (CRC): A class of linear error detecting codes which generate parity check bits by finding the remainder of a polynomial division. Data Burst Randomizer: The function that determines which power control groups within a frame are transmitted on the Reverse Traffic Channel when the data rate is lower than 9600 bps. The data burst randomizer determines, for each mobile station, the pseudorandom position of the transmitted power control groups in the frame while guaranteeing that every modulation symbol is transmitted exactly once. Direct Sequence Spread Spectrum: See Spread Spectrum Signals. dbc: The ratio (in db) of the sideband power of a signal, measured in a given bandwidth at a given frequency offset from the center frequency of the same signal, to the total inband power of the signal. For CDMA, the total inband power of the signal is measured in a 1.23 MHz bandwidth around the center frequency of the CD MA signal. dbm: A measure of power expressed in terms of its ratio (in db) to one milliwatt. dbm/hz: A measure of power spectral density. dbm/hz is the power in one Hertz of bandwidth, where power is expressed in units of dbm. dbw: A measure of power expressed in terms of its ratio (in db) to one Watt. Deinterleaving: The process of unpermuting the symbols that were permuted by the interleaver. Deinterleaving is performed on received symbols prior to decoding. DSSS: See Spread Spectrum Signals. Eb: The energy of an information bit. Effective Radiated Power (ERP): The transmitted power multiplied by the antenna gain referenced to a half-wave dipole.

13 Xll Electronic Serial Number (ESN): A 32-bit number assigned by the mobile station manufacturer, uniquely identifying the mobile station equipment. Encoder Tail Bits: A fixed sequence of bits added to the end of a block of data to reset the convolutional encoder to a known state. ERP: See Effective Radiated Power. ESN: See Electronic Serial Number. FHSS: See Spread Spectrum Signals. Forward CDMA Channel: A CDMA Channel from a base station to mobile stations. The Forward CDMA Channel contains one or more code channels that are transmitted on a CDMA frequency assignment using a particular pilot PN offset. The code channels are associated with the Pilot Channel, Sync Channel, Paging Channels, and Traffic Channels. The Forward CDMA Channel always carries a Pilot Channel and may carry up to one Sync Channel, up to seven Paging Channels, and up to 63 Traffic Channels, as long as the total number of channels, including the Pilot Channels, is no greater than 64. Forward Traffic Channel: A code channel used to transport user and signaling traffic from the base station to the mobile station. Frame: A basic timing interval in the system. For the Access Channel, Paging Channel, and Traffic Channel, a frame is 20 ms long. For the Sync Channel, a frame is ms long. Frame Category: A classification of a received Traffic Channel frame based upon transmission data rate, the frame contents (primary traffic, secondary traffic, or signaling traffic), and whether there are detected errors in the frame. Frame Offset: A time skewing of Traffic Channel frames from System Time in

14 Xlll integer multiples of 1.25 ms. The maximum frame offset is ms. Frequency Hopped Spread Spectrum: See Spread Spectrum Signals. Global Positioning System (GPS): A US government satellite system that provides location and time information to users. See Navstar CPS Space Segment / Navigation User Interfaces rcd-cps-200 for specifications. GPS: See Global Positioning System. Handoff: The act of transferring communication with a mobile station from one base station to another. Hard Handoff: A handoff characterized by a temporary disconnection of the Traffic Channel. Hard Handoffs occur when the mobile station is transferred between disjoint Active Sets, the CDMA frequency assignment changes, the frame offset changes, or the mobile station is directed from a CDMA Traffic Channel to an analog voice channel. See also Soft Handoff : Interim Standard - 95, endorsed by the telecommunications Industry Association (TIA) as the "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System. Idle Handoff: The act of transferring reception of the Paging Channel from one base station to another, when the mobile station is in the Mobile Station Idle State. Interleaving: The process of permuting a sequence of symbols. khz: Kilohertz (10 3 Hertz). ksps: Kilo-symbols per second (10 3 symbols per second). Layering: A method of organization for communication protocols. A layer is defined in terms of its communication protocol to a peer layer in another entity and

15 XIV the services it offers to the next higher layer in its own entity. Layer 1: See Physical Layer. Layer 2: Layer 2 provides for the correct transmission and reception of signaling messages, including partial duplicate detection. See also layering and Layer 3. Layer 3: Layer 3 provides the control of the cellular telephone system. Signaling messages originate and terminate at layer 3. See also Layering and Layer 2. Long Code: A PN sequence with period that is used for scrambling on the Forward CDMA Channel and spreading on the Reverse CDMA Channel. The long code uniquely identifies a mobile station on both the Reverse Traffic Channel and the Forward Traffic Channel. The long code provides limited privacy. The long code also separates multiple Access Channels on the same CDMA channel. See also Public Long Code Mask, and Private Long Code. Long Code Mask: A 42-bit binary number that creates the unique identity of the long code. See also Public Long Code and Private Long Code Mask. LSB: Least significant bit. Maximal Length Sequence (m-sequence): A binary sequence of period 2 n -1, n a positive integer, with no internal periodicities. A maximal length sequence can be generated by a tapped n-bit shift register with linear feedback. Mcps: Megachips per second (10 6 chips per second). Message: A data structure that conveys control information or application information. A message consists of a length field (MSGLENGTH), a message body (the part conveying the information), and a CRC. MHz: Megahertz (10 6 Hertz). MIN: See Mobile Station Identification Number.

16 xv Mobile Station: A station in the Domestic Public Cellular Radio Telecommunications Service intended to be used while in motion or during halts at unspecified points. Mobile stations include portable units (e.g., hand-held personal units) and units installed in vehicles. Mobile Station Identification Number (MIN): The 34-bit number that is a digital representation of the 10-digit directory telephone number assigned to a mobile station. Mobile Switching Center (MSC): A configuration of equipment that provides cellular radiotelephone service. Also called the Mobile Telephone Switching Office (MTSO). Modulation Symbol: The output of the data modulator before spreading. On the Reverse Traffic Channel, 64-ary orthogonal modulation is used and six code symbols are associated with one modulation symbol. On the Forward Traffic Channel, each code symbol (when the data rate is 9600 bps) or each repeated code symbol (when that data rate is less than 9600 bps) is one modulation symbol. ms: Millisecond. Multiplex Option: The ability of the multiplex sublayer and lower layers to be tailored to provide special capabilities. A multiplex option defines such characteristics as the frame format and the rate decision rules. See also Multiplex Sublayer. Multiplex Sublayer: One of the conceptual layers of the system that multiplexes and demultiplexes primary traffic, secondary traffic, and signalling traffic. ns: Nanosecond. Null Traffic Channel Data: One or more frames of 16 'l's followed by eight 'O's sent at the 1200 bps rate. Null Traffic Channel data is sent when no service

17 XVI option is active and no signaling message is being sent. Null Traffic Channel data serves to maintain the connectivity between the mobile station and the base station. Paging: The act of seeking a mobile station when a call has been placed to that mobile station. Paging Channel (Analog): See Analog Paging Channel. Paging Channel (CDMA): A code channel in a Forward CDMA Channel used for transmission of control information and pages from a base station to a mobile station. Physical Layer: The part of the communication protocol between the mobile station and the base station that is responsible for the transmission and reception of data. The physical layer in the transmitting station is presented a frame by the multiplex sublayer and transforms it into an over-the-air waveform. The physical layer in the receiving station transforms the waveform back into a frame and presents it to the multiplex sublayer above it. Pilot Channel: An unmodulated, direct-sequence spread spectrum signal transmitted continuously by each CDMA base station. The Pilot Channel allows a mobile station to acquire the timing of the Forward CDMA Channel, provides a phase reference for coherent demodulation, and provides a means for signal strength comparisons between base stations for determining when to handoff. Pilot PN Sequence: A pair of modified maximal length PN sequences with period 2 15 used to spread the Forward CDMA Channel and the Reverse CDMA Channel. Different base stations are identified by different pilot PN sequence offsets. Pilot PN Sequence Offset Index: The PN offset in units of 64 PN chips of a pilot, relative to the zero offset pilot PN sequence.

18 XVll Pilot Strength: The ratio of receive pilot energy to overall received energy. See also Eel10. PN Chip: One bit in the PN sequence. PN Sequence: Pseudonoise sequence. A periodic binary sequence. Power Control Bit: A bit sent in every 1.25 ms interval on the Forward Traffic Channel to signal the mobile station to increase or decrease its transmit power. Power Control Group: A 1.25 ms interval on the Forward Traffic Channel and the Reverse Traffic Channel. See also Power Control Bit. PPM: Parts per million. Primary CDMA Channel: A CDMA Channel at a preassigned frequency assignment used by the mobile station for initial acquisition. see also Secondary CD MA Channel. Primary Paging Channel (CDMA): The default code channel (code channel 1) assigned for paging on a CDMA Channel. Primary Traffic: The main traffic stream carried between the mobile station and the base station, supporting the active primary service option, on the Traffic Channel. See also Secondary Traffic, Signaling Traffic, and Service Option. Private Long Code: The long code characterized by the private long code mask. See also Long Code. Private Long Code Mask: The long code mask used to form the private long code. See also Public Long Code Mask and Long Code. Public Long Code: The long code characterized by the public long code mask. Public Long Code Mask: The long code mask used to form the public long code. The mask contains the ESN of the mobile station. See also Private Long Code

19 XVlll Mask and Long code. Reverse CDMA Channel: The CDMA Channel from the mobile station to the base station. From the base station's perspective, the Reverse CDMA Channel is the sum of all mobile station transmissions on a CDMA frequency assignment. Reverse 'Iraffic Channel: A Reverse CDMA Channel used to transport user and signaling traffic from a single mobile station to one or more base stations. Secondary CDMA Channel: A CDMA Channel at a preassigned frequency assignment used by the mobile station for initial acquisition. See also Primary CDMA Channel. Secondary Traffic: An additional traffic stream that can be carried between the mobile station and the base station on the Traffic Channel. See also Primary Traffic and Signaling Traffic. SIRCIM: Simulation of Indoor Radio Channel Impulse Response Models. Soft Randoff: A handoff occurring while the mobile station is in the Mobile Station Control on the Traffic Channel State. This handoff is characterized by commencing communications with a new base station on the same CDMA frequency assigned before terminating communications with the old base station. See also Hard Handoff. Spread Spectrum Signals: Signals used for transmission of digital information that are distinguished by the characteristic that their bandwidth W is much greater than the information rate R in bits per second [6]. When the PN (pseudonoise) sequence employed to spread the information bits is used in conjunction with PSK modulation to shift the phase of the PSK signal pseudo-randomly, the resulting modulated signal is called direct-sequence spread spectrum (DSSS) signal. When the

20 xix PN sequence is used in conjunction with FSK to select the frequency of the transmitted signal pseudo-randomly, the resulting signal is called frequency-hopped spread spectrum (FHSS) signal. sps: Symbols per second. Symbol: See Code Symbol and Modulation Symbol. Sync Channel: Code channel 32 in the Forward CDMA Channel which transports the synchronization message to the mobile station. Sync Channel Superframe: An 80 ms interval consisting of three Sync Channel frames (each ms in length). System: A system is a cellular telephone service that covers a geographic area such as a city, metropolitan region, county, or group of counties. See also Network. System Time: The time reference used by the system. System time is synchronous to UTC time (except for leap seconds) and uses the same time origin as GPS time. All base stations use the same System Time (within a small error). Mobile stations use the same System Time, offset by the propagation delay from the base station to the mobile station. Time Reference: A reference established by the mobile station that is synchronous with the earliest arriving multipath component used for demodulation. Traffic Channel: A communication path between a mobile station and a base station used for user and signaling traffic. The term Traffic Channel implies a Forward Traffic Channel and Reverse Traffic Channel pair. See also Forward Traffic Channel and Reverse Traffic Channel. Voice Channel: See Analog Voice Channel. Voice Privacy: The process by which user voice transmitted over a CDMA

21 xx Traffic Channel is afforded a modest degree of protection against eavesdropping over the air. Walsh Chip: The shortest identifiable component of a Walsh function. There are 2N Walsh chips in one Walsh function where N is the order of the Walsh function. On the Forward CDMA Channel, one Walsh chip equals 1/ MHz, or ns. On the Reverse CDMA Channel, one Walsh chip equals 4/ MHz, or ILS. Walsh Function: One of 2N time orthogonal binary functions (note that the functions are orthogonal after mapping '0' to '1' and '1' to '-1').

22 XX1 ACKNOWLEDGMENTS I am grateful to my major professor, Dr. John F. Doherty for his valuable guidance and encouragement through the course of my research. I would like to thank the members of my committee, Dr. Steve F. Russell, Department of Electrical and Computer Engineering and Dr. Stephen B. Vardeman, Department of Statistics for critically evaluating this work. This research was funded in part by the U.S. Army Research Office under grant no. DAAH I would like to thank them for their support.

23 1 CHAPTER 1. INTRODUCTION 1.1 Overview Mobile communication systems provide access to the capabilities of the global network at any time, irrespective of the location or mobility of the user. A cellular mobile communication system overcomes the limitations of its conventional counterpart such as Hmited service capability, poor service performance and inefficient frequency spectrum utilization. The IS-95 "Mobile-Station Base-Station Compatibility Standard For Dual-Mode Wideband Spread Spectrum Cellular System" endorsed by the US Telecommunications Industry Association j Electronic Industry Association (TIAjEIA) based on CDMA technology describes one such system underway in North America [1]. The dual-mode operation of facilitates the coexistence of analog and digital cellular systems. This is necessary since the North American cellular system has no additional spectrum allocated for the digital system. CDMA direct sequence spread spectrum technique, incorporated into this system, enables the accommodation of a large number of users in one radio channel depending on the voice activity level [2]. This feature also provides immunity to jamming signals and enables resolution of multipath components in a time-dispersive radio propagation channel [3]. Chapter 3 provides a description of the standard.

24 2 A simulation-based approach to performance evaluation is adopted here. A communication system based on the IS-95 standard is simulated using SIMULINK in MATLAB. Power delay profiles in various mobile radio propagation channels are used to obtain measurement-based channel models for urban, suburban, indoor and open area environments. Statistics of the path loss characteristics are then used to estimate the number of taps and tap gains. These are then used in an optimum combining RAKE receiver structure for signal detection. Performance comparison of the simulated system with regard to bit error rate (BER) for four-way and optimum combining RAKE receivers is presented. Various multipath propagation channels are characterized based on the number of discrete paths and average delay spreads. The need for simulation is described as follows. The performance of a communication system can be evaluated using formula-based calculations, waveform level simulation or through hardware prototyping and measurements [4]. Formula-based techniques are based on the analysis of simplified models of the system using analytical methods. Hence the degree of accuracy of system model and performance evaluation decreases with the complexity of communication systems. Hardware prototyping is an accurate and credible method, but with a loss of flexibility. It is also time consuming and expensive. A simulation-based approach, on the other hand, can model systems with any level of detail and the design space can be explored more efficiently than the above methods. Moreover, mathematical and empirical models can be combined and measured characteristics incorporated into the system model in a better way. Simulation waveforms obtained can be used for rapid prototyping using a Real-time Workshop.

25 3 1.2 Motivation and Scope of Research As the demand for digital wireless communication systems grows, the accurate prediction of average and instantaneous BER in multi path channels becomes increasingly important. These predictions enable the determination of acceptable modulation methods, coding techniques and receiver implementations in the operating environments. However, such predictions become extremely difficult when there are numerous system and channel parameters involved (e.g. SNR, data rate, impulse noise and mobile speed). Moreover, the time-varying nature of mobile-radio channels complicates the optimization of these parameters using analytical techniques. Hence the performance of a complex mobile communication system based on IS-95 is evaluated using a simulated system model. Multipath fading is the major cause of communication impairments In a Illicrowave radio link [5J. It is mainly caused by multipath reflections of a transmitted wave by local scatterers such as houses, buildings and forests surrounding the mobile station. When a direct wave is present in the fading signal along with the reflected waves, the channel is described as a Rician fading environment, otherwise it is called a Rayleigh fading environment. Various measurement based channel models with multi path fading are developed and presented to the system. Also, optimum combining RAKE receiver structures are developed from the characteristics of various propagation channels obtained from [5-11], and the overall performance of the system is presented.

26 4 CHAPTER 2. LITERATURE REVIEW 2.1 Introduction Computer-based modeling and simulation is a relatively new dimension in the performance evaluation of communication systems. Though it has developed only over the past two decades, a variety of modeling and simulation techniques have been developed since. The current generation software simulation packages (BOSS [12], SPW [13],COS SAP [14]) offer interactive, graphical and user-friendly frameworks for simulation [4]. 2.2 Previous Work on System Simulation The simulation methodology implemented in the bir error rate simulator (BERSIM) allows subjective evaluation of link quality between a source and a sink via real-time bit-by-bit error simulation for mobile radio systems. In this package, communication system parameters like modulation scheme, data rate, SNR and receiver speed may be specified permitting performance comparisons. Indoor and outdoor multipath fading channels, AWGN and co channel interference effects are also simulated here [15]. BERSIM uses the measurement-based statistical indoor channel model in Simulation of Indoor Radio Channel Impulse Response Models (SIRCIM) described in [16].

27 5 Open Erwironmert 0 0 In In "0 c: "0 c:.~ -10 :.~ -10 ~ &. : '.' ~ a. ~ -20.lll ":-.... """.~ -20 SutllJilan Environment 12 ".; Q: Q: '" Time Delay in us Time Delay in us Urban Environmert ndoor Erwironmert 0 0 In In "0 "0 c: c: ~ ~ a. ". ~ -20 ~ -20.lll /.lll '" Q: '. Q: Time Delay in us Time Delay in us '" '" :'/" Figure 2.1: Typical Power Delay Profiles for various propagation channel environments. 2.3 Previous Work on Channel Impulse Response Measurements The performance of a communication system is lower-bounded under the worstcase operating conditions. A reasonable sample of worst case multipath profiles and typical wideband path loss characteristics (for urban, suburban, open area and indoor environments, see Figure 2.1) were obtained from the measurements presented in the following papers. The measurements were restricted to those of RF signals in the range of 800 MHz to 910 MHz. This is because the Federal Communication Commission (FCC) has allocated a 50-MHz bandwidth in the MHz frequency range for high-capacity mobile radio telephone use. This bandwidth is equally divided between transmit and receive bands with mobile transmit channels in the MHz range and mobile receive channels in the MHz range [17]. Systems based

28 6 Transmitter Mobile Base Table 2.1: Channel No. N 1 ~ N ~ ~ N ~ ~ N ~ < N < 1023 Radio Channel Allocation Center Frequency (MHz) 0.03N (N ) N (N ) Range on the IS-95 standard conform to these regulations. The distribution of channels between the mobile and base station are given in Table 2.l. Statistical descriptions of the time delays and Doppler shifts associated with multipath propagation in a suburban mobile radio environment obtained from bandpass impulse response measurements are presented in [6]. These provided average power delay profiles made up of over 200 individual profiles for 910 MHz radio signals. Distributions of delay spread for 910 MHz Gaussian wide-sense stationary uncorrelated scattering (GWSSUS) channels at different locations in New York City are presented in [5]. The regions are representative of heavily built-up areas of many large cities in US. Over 10 percent of the areas covered here exhibited more than 2.5 J-LS for rms delay spread. Experimental propagation loss measurements taken in the Ottawa region at 910 MHz are presented in [7]. In addition, a comparative study of these profiles with statistical models described by Hata-Okumura, Egli, Edwards and Durkin, Blomquist and La dell, Allsebrook and Parsons under applicable conditions are also presented. Time delay spread and signal level measurements of 850 MHz radio waves in indoor propagation channels are presented in [8]. Average delay spreads of 420 ns were obtained in residential environments. Measurements and cumulative probability distributions of power delay profiles

29 7 for suburban, dense suburban, urban and dense urban radio propagation environments at different locations in the city of Leeds, UK are presented in [9]. Also, the number of discrete paths and their corresponding time delays are deduced from the distributions. Microcellular radio measurements at 900 MHz undertaken at University of Liverpool are presented in [10]. These were representative of power delay profiles of urban propagation channels. Typical and worst case rms delay spreads, excess delay spreads and mean channel path loss at 900 MHz in four European cities are presented in [11]. Path loss changes as a function of the distance between base-station and mobile-station are also determined here using a power law propagation model. Data obtained from measurements described above are used in developing mobile propagation channel models as described in Section Previous Work on RAKE Receivers The RAKE tropospheric scatter technique is described in [18]. Propagation measurements were carried out using this technique with an RF carrier of 900 MHz, pseudo randomly phase-shift modulated at 100 ns intervals. This provided a multi path resolution of 100 ns. Scattering functions thus obtained were one of the first set of presentations for tropospheric-scatter transmission paths. Noncoherent combining of the outputs of the taps of the RAKE receiver m a CDMA DSSS system with binary DPSK modulation is presented in [19]. This study showed that RAKE receivers are more appropriate for DS systems with smaller number of chips (50) per data symbol while a correlation receiver is preferred in

30 8 systems with large number of chips (400) per data symbol. 2.5 Relevance to Research Comprehensive software packages described in [4] are expensive in terms of resources and infrastructure required to install them. Signal Processing Workstation (SPW) requires a Sun workstation. BERS1M AND S1RC1M system simulation softwares, described in [15] and [16] can be used to simulate modulation, filtering, propagation and detection aspects of communication systems in general. In order to study the performance characteristics of an 1S-95 based system, a specific system that met all the required specifications (e.g. chip rate, encoder structure, PN sequence length etc. ), needed to be simulated. Propagation channel impulse responses for radio signals in the UHF range ( MHz) are described in Section 2.3. Power delay profiles which represent the probability density function (pdf) of the average power of the received signal with respect to time for the four multipath channels were obtained from these data. Through appropriate analysis of their statistics, delay spread, average delay, number of discrete paths and excess delay are obtained. This type of analysis is called direct data reduction. The parameters and the bounds on their variability provide a good means of assessing the performance of the system. The number of taps and the tap gains to be provided in the RAKE receiver are obtained from the number of discrete paths, their time delays and the amplitude distributions of the multipath channel. The RAKE receiver makes a binary decision based on a real-valued decision statistic which is obtained by the non coherent combination of the individual tap outputs. A comparison of the BER versus signal-

31 9 to-noise (SNR) characteristics for four-way and optimum combining RAKE receivers for a suburban environment is presented.

32 10 CHAPTER 3. OVERVIEW OF THE IS-95 STANDARD 3.1 Introduction This chapter describes the IS-95 standard for digital cellular systems. Forward (base station to subscriber) and reverse (subscriber to base station) CDMA channel structures and signals, power control, message encryption and privacy, call processing and handoff procedures are the topics discussed here. 3.2 Forward CDMA Channel Structure The forward CDMA channel consists of four code channels: the pilot channel (always required), the sync channel, paging channels (1 to 7) and traffic channels (55 to 63)(see Figure 3.1) [l].a pilot channel is transmitted at all times by the base station on each active forward CDMA channel. It is an unmodulated spread spectrum signal used for synchronization by a mobile station operating within the coverage area of the base station. The sync channel is a modulated spread spectrum signal used by mobile stations to acquire initial time synchronization. The paging channel is also a modulated spread spectrum signal used to transmit system overhead information by the base station and specific messages by the mobile station. The forward traffic channel is used for the transmission of user and signaling information to a specific mobile station during a call.

33 11 Sync Channel Data 1200 bps Pilot Channel (All O's) i WO ~ Mcps.. ~ T To Quaw:ature Spreadmg W Mcps To Quadrature Spreading Pag ing Channel Data 1200 bps Forward Traffic Channel Data 9600 bps 4800 bps 2400 bps 1200 bps To Quadrature Spreading I-Channel PN Sequence Quadrature Spreading Baseband I rv-.-1 T}------Q--I~,--_Fi_I_Iter_-,.. \.6J t Q(t) Q-Channel PN Sequence sin(2pifct) ~ set) Figure 3.1: Forward CDMA Channel Structure Data rates at the input are: Pilot channel (all O's) at 19.2 kbps; Sync channel at 1.2 kbps; Paging channel (fixed data rate) at 9.6, 4.8 or 2-4 kbps; Traffic channel (variable data rate) at 9.6, 4_8, 2.4 or 1.2 kbps. The sync, paging and traffic channel data are then convolutionally encoded using rate 1/2, constraint length 9 code with generator functions 753 (octal) and 561 (octal). This generates two code symbols for each input data bit. The sync channel encoded symbols are repeated twice to achieve a modulation symbol rate of 4.8 ksps. While for paging and traffic channels, the number of repetitions is such that the modulation symbol rate is 19.2 ksps. The modulation parameters are listed in Tables 3.1, 3.2, 3.3.

34 12 Table 3.1: Parameter PN Chip Rate Code Rate Code Repetition Modulation Symbol Rate PN Chips/Modulation Symbol PN Chips/Bit Sync Channel Modulation Parameters Data Rate (bps) / Units Mcps bits/code sym mod sym/code sym sps PN chips/mod sym PN chips/bit Table 3.2: Parameter PN Chip Rate Code Rate Code Repetition Modulation Symbol Rate PN Chips/Modulation Symbol PN Chips/Bit Paging Channel Modulation Parameters Data Rate (bps) /2 1/ Units Mcps bits/code sym mod sym/code sym sps PN chips/mod sym PN chips/bit After symbol repetition, block interleaving (of span 20ms equivalent to 384 modulation symbols) is performed to avoid burst errors while data is being transmitted through a multipath fading environment. Data scrambling of the interleaved symbols is done using the first of every 64 bits of a long-code (of length 242_1) at the PN chip rate for paging and traffic channels. A long code mask that modulates the long code in traffic channels is used for voice privacy (see Section 3.5). Each code channel transmitted on the forward CDMA channel is spread with a Walsh function at a fixed chip rate of Mcps to provide orthogonal channelization among all code channels. One of sixty-four time orthogonal Walsh functions is used. A code channel spread using Walsh function n is assigned to code channel number n (n = 0 to 63). Code channel number zero (64 O's) is always assigned to

35 13 Table 3.3: Parameter PN Chip Rate Code Rate Code Repetition Modulation Symbol Rate PN Chips per Modulation Symbol PN Chips/Bit Forward Traffic Channel Modulation Parameters Data Rate (bps) /2 1/2 1/2 1/ Units Mcps bits/code sym mod sym/ code sym sps PN chips/mod sym PN chips/bit Table 3.4: I o 1 1 o I and Q Mapping Q Phase o 7r/4 o 37r /4 1-37r/4 1-7r/4 the pilot channel. If the sync channel is present, it is assigned code channel number 32. Paging channels, if present, are assigned code channel numbers 1 through 7 and the rest are assigned to the forward traffic channels. After orthogonal spreading, each of these code channels are spread by a quadrature pair of maximal-length PN sequences (length 2 15 ) at a fi..'{ed chip rate of Mcps. The spread polynomials of the I and Q channel PN sequences are PI (x) X 15 + X 13 + x 9 + x 8 + x 7 + x (3.1) PQ{x) _ X 15 + X12 + xli + x 10 + x 6 + x 5 + X4 + x (3.2) The binary (O's and 1 's) I and Q at the output of quadrature spreading are baseband filtered and mapped into phase according to Table 3.4. The resulting signal constellation and phase transitions are shown in Figure 3.2.

36 14 Q-channel (1,0) (0,0) (I,Q) *" I-channel (1,1) (0,1) Figure 3.2: Forward CDMA Channel Signal Constellation and Phase Transition PN sequence time offsets are used in code channels for synchronization by a mobile station. The pilot PN sequence time offset is used to identify a forward CDMA channel. Time offsets may be reused within a CDMA cellular system. The I and Q channel PN sequences for the sync, paging and forward traffic channels use the same offset as the pilot channel for a given base station. The base station transmits the forward CDMA channel signal at MHz with a channel spacing of 30 khz. The corresponding dual-mode mobile station transmit channel is at MHz. This is termed channel number 1. The maximum effective radiated power (ERP) and antenna height above average terrain (HAAT) is coordinated locally on an ongoing basis. A typical forward CDMA channel transmitted by a base station is shown in Figure 3.3.

37 15 Figure 3.3: Example of a forward CDMA channel transmitted by a base station Receiver at Mobile Station The mobile station demodulation process involves complimentary operations to the base station modulation process. The mobile station also performs tracking and demodulation of multipath components of the forward CDMA channel in addition to scanning and estimation of the signal strength at each pilot PN sequence offset. This is used during the idle or initialization stage and to determine when and from which base station handoff needs to be requested. 3.3 Reverse CDMA Channel Structure The reverse CDMA channel is composed of access channels and reverse traffic channels. The mobile station does not establish a system time as at the base station. Hence the reverse channel signal does not use coherent detection. The modulation characteristics for the forward and reverse channels are different. The reverse channel

38 16 is 64-ary orthogonal modulated at data rates of 9.6, 4.8, 2.4 or 1.2 kbps as shown in Figure 3.4 at point A. The actual burst transmission rate is fixed at code symbols per second. This results in a fixed Walsh chip rate of thousand chips per second. Each Walsh chip is spread by four PN chips. The rate of the spreading PN sequence is fixed at Mcps (million chips per second). The reverse traffic channel and access channel modulation parameters are listed in Tables 3.4 and 3.5 respectively. The reverse traffic channel is used for the transmission of user and signaling information to the base station during a call. The access channel is used by the mobile station to initiate communication with the base station and to respond to Reverse Traffic cb:mne1 bits ~ ( , 40 or kbps Bits/framc) 4.0 kbps L..--'--_... 2.Okbps 0.8 kbps Frame Data Rate ~,00 s(2pifc1l...:1-t~~1 B~::d ~~ L...-_---l Baseband ~---J '--_Fil_ter----'Il Q(I) sin (2pifCI)T <P~ s(l) Code Code Access Channel bib (172.80,40 or16 Bits/framc) 4A kbps --_"'1 mock Interleaver OOS(2PifCt ~ 1...:1-t~~1 B=band 1(t). Filter '------' ~~I B~::d Q-Ch:IIIDel PN Sequence H---J ' 'Sin(2PifcI)t Q(I) ~ S(I) Figure 3.4 Reverse CDMA Channel Structure

39 17 Table 3.5: Reverse Traffi~ Channel Modulation Earameters Data Rate (bps) Parameter Units PN Chip Rate Mcps Code Rate 1/3 1/3 1/3 1/3 bits/code sym Transmit Duty Cycle % Code Symbol Rate sps Modulation code sym/mod sym Modulation Symbol Rate sps Walsh Chip Rate kcps Modulation Symbol PN chips/mod sym Duration J.1,S PN Chips/Code Sym PN chips/code sym PN Chips/Mod. Sym PN chips/mod. sym PN Chips/Walsh Chip PN chips/walsh chip Table 3.6: Parameter PN Chip Rate Code Rate Code Symbol Repetition Transmit Duty Cycle Code Symbol Rate Modulation Modulation Symbol Rate Walsh Chip Rate Modulation Symbol Duration PN Chips/Code Symbol PN Chips/Modulation Symbol PN Chips/Walsh Chip Access Channel Modlliation Parameters Data Rate (bps) / Units Mcps bits/code sym symbols / cod sym % sps code sym/mod sym sps kcps J.1,S PN chips/code sym PN chips/mod. sym PN chips/walsh chip

40 18 Reverse CDMA channel (1.23 MHz channel received by the base station) ====... f AddressedbylongcodePN Figure 3.5 Example of a reverse CDMA channel received by a base station. paging channel messages. The mobile station transmits information on the reverse traffic channel at variable data rates of 9.6, 4.8, 2.4 or 1.2 kbps and on the access channel at a fixed data rate of 4.8 kbps. These are then convolution ally encoded by rate 1/3, constraint length 9 codes with code generators 557, 663 and 711 (octal). The code symbols are then block-interleaved with a span of 20 ms (576 code symbols) and modulated by a 64-ary orthogonal modulator using 64 Walsh functions. The reverse traffic channel and the access channel are then direct sequence spread by the long code (with period = chips) satisfying the linear recursion specified by the polynomial (3.3) The subscriber address is contained in the long code mask which modulates the long code before spreading. Furthermore, the waveform is spread by a pair of

41 19 PN codes (identical to the ones used in the forward traffic channel), common to all subscribers and to the access channel in an OQPSK arrangement. The final waveform is then filtered to generate a spectrum with MHz double-sided 3 db bandwidth. The mobile station transmits the reverse CDMA channel signal at MHz with a channel spacing of 30 khz. The corresponding dual-mode base station transmit channel is at MHz. This is termed channel number 1. The maximum effective radiated power (ERP) with respect to a half wave dipole for any class mobile transmitter is 8 dbw (6.3 Watts). 3.4 Power Control CDMA power control is essential in a cellular CDMA system for the reverse link transmission in order to reduce near-far interference. The mobile station receives a signal that has undergone log-normal and Rayleigh fading from the forward link. Also, the Rayleigh fading on the forward channel and the reverse channel are not the same since CDMA uses duplex channels. Hence, the desired average transmit power needs to be sent back to the base station on the reverse link. At the base station (or cell site), the available information on the instantaneous value versus the expected value of frame error rate (FER) (1 frame = 192 bits) of the received signal is examined to determine whether to command a particular mobile to increase or decrease its transmit power. This mechanism is called CDMA closed-loop power control. The base station reverse traffic channel receiver estimates the received signal strength of the particular mobile station it is assigned to over a 1.25 ms period (1/16 of 20 ms). The estimate is used to determine the value of the power control bit ('0'

42 20 or 'I') corresponding to an indication of increase or decrease in the mean output power to the mobile station) to be transmitted on the corresponding forward traffic channel Each power control bit replaces two consecutive modulation symbols of the forward traffic channel after data scrambling. The change in mean output power for every power control bit is 1 db nominal within ±0.5 db of the nominal change. 3.5 Voice Privacy Voice privacy is provided in the CDMA system by means of the private longcode mask used for PN spreading. Voice privacy control is provided on the traffic channels only. All calls are initiated using the public long-code mask. To initiate a transition to the private long-code mask, the base station or the mobile station sends a long-code transition request order on the traffic channel 3.6 Call Processing The mobile station call processing consists of the following states: Initialization state: The mobile station selects the base station corresponding to the highest signal strength in its coverage area. It then acquires the pilot channel of the CDMA system within 20 ms and obtains the system configuration and timing information. The mobile station synchronizes its timing to that of the CDMA system. Idle state: The mobile station performs paging channel monitoring procedures. Unless otherwise specified, it transmits an acknowledgement in response to any message received from the base station addressed to the mobile station. It also maintains active registration timers like the power-up, power-down, timer-based, distance and zone-based registrations.