Design of spread spectrum multicode CDMA transport architecture for multimedia services

Design of Spread Spectrum Multicode CDMA

Transport Architecture for Multimedia Services

Po-Rong Chang, Member, IEEE, and Chin-Feng Lin

Abstract—In this paper, we investigate a new application of

the well-known spread spectrum code division multiple access (SS-CDMA) techniques to multimedia services related to the development of the next-generation wireless mobile networks interconnecting with a wireline ATM-based broadband network. Such services allow users to share novel multimedia applications without any geographical restrictions. However, since the mobile radio channel has a fixed limited bandwidth, the traditional SS-CDMA system may not be sufficient to accommodate the variable bit rate (VBR) multimedia services requested by multiple mobile users simultaneously. Moreover, the traffic load at the base station can change dynamically due to the time-varying throughput requirement of these requested multimedia services. To tackle this difficulty, a multicode CDMA (MC-CDMA) tech-nique is proposed to provide multirate multimedia services by varying the number of spreading codes assigned to each user in order to meet its throughput requirement. In MC-CDMA, a spreading code can be used to transmit information at a basic bit rate. Users (video or data) who need higher transmission rates can use multiple codes in parallel. Meanwhile, the maximum available number of codes in the MC-CDMA system is still limited. Hence, a cost-effective dynamic code allocation scheme has then been proposed to dynamically assign appropriate number of codes to each user for achieving the maximum resource utilization for multiuser multimedia services via the mobile radio channel. Finally, a number of real multimedia titles generated from the well-known MacroMind Director are conducted to evaluate the performance of the proposed wireless multimedia CDMA system.

Index Terms—Multicode code division multiple access (CDMA),

multimedia, object-composition Petri-net (OCPN).

I. INTRODUCTION

T

system currently being developed is intended to integrate all the different services of second-generation systems and cover a much wider range of broadband services (audio, data, video, multimedia) consistent and compatible with the tech-nology developments taking place within the broadband ISDN wireline networks [1], [2]. The concept of wireless mobile communication networks suggests itself as a replacement for wired multimedia communication networks in order to avoid expensive installation and relocation and to provide portability and mobility to various pieces of equipment. Furthermore, the use of radio for mobile multimedia communication in

Manuscript received October 2, 1998; revised April 17, 1999. This work was supported in part by the National Science Council, Taiwan, R.O.C under Con-tract NSC-87–2218–E-009–046.

The authors are with the Department of Communication Engineering, National Chiao-Tung University, Hsin-Chu, Taiwan, R.O.C. (e-mail: pr-chang@cc.nctu.edu.tw).

Publisher Item Identifier S 0733-8716(00)00191-8.

cars, remote hospitals, military services, homes, and office automation systems is an attractive proposition. It would free the users from cords or optical fibers tying them to particular locations within the building, thus offering true mobility which convenient and sometimes even necessary. The development of multimedia terminals will support the ever-growing demand for mixed data, audio, and video applications and will connect the portable pen pad and lap-top devices to backbone information resources and computational facilities. The possibility of multimedia services will allow services such as dial-up video conference, video-on-demand (VOD) services, and portable PC-based applications incorporating video/audio/data transfer to any location. Moreover, a number of different mobile users can simultaneously request multimedia data from one or more multimedia servers on the network. Each multimedia server is capable of catering to multiple data requests from multiple users, simultaneously. Presentation of preorchestrated multimedia information requires synchronous playback of time-dependent multimedia data according to some prespeci-fied temporal relations. At the time of creation of multimedia information, a user needs a model to specify temporal con-straints among various data objects which must be observed at the time of playback. Usually, the temporal relationships of multimedia information may be characterized by a timeline diagram which is the commonly used tool in commercial multimedia authoring products. Fig. 1 depicts an example of a timeline diagram and its associated multimedia title generated by the most commonly used product called MacroMind Di-rector. Although the timeline diagram is a useful description tool, it has a lot of redundancies in characterizing the temporal relationships and is not suitable for further analysis and system evaluation, however. To tackle this difficulty and to obtain a more compact multimedia representation, a well-known model called object-composition Petri-net (OCPN) [3], [4] is able to describe the temporal relationships of the various components of a multimedia document and represents them in the form of a graph. Since preorchestrated multimedia information has highly time-varying bandwidth, the fixed bandlimited constant bit rate (CBR) wireless channel may not be appropriate for the variable bit rate (VBR) multimedia services. Therefore, it is desirable to design a dynamic mechanism to manage and allocate bandwidth according to the changing levels of concurrencies of multimedia data streams. Woo et al.[5] have introduced a dynamic RF channel capacity allocation to deal with the OCPN-based multimedia data stream. In this paper, an alternative method has been proposed to provide a cost-effective resource allocation scheme for the OCPN-based multimedia services by employing the well-known antimultipath spread 0733–8716/00$10.00 © 2000 IEEE

Fig. 1. Timeline diagram of a multimedia title generated from MacroMind Director.

spectrum code division multiple access (SS-CDMA) tech-niques.

In terms of the multiplexing/multiple access techniques, CDMA has gained significant attention in recent years as a competing technology for wireless mobile networks [6]. Unfor-tunately, a major weakness of the conventional CDMA is that for a given system bandwidth, spectrum-spreading limits the peak user data rate to a relatively low value. For example, even with 50 MHz bandwidth and a spreading gain of 512, user bit rate is limited to just 100 kb/s. Meanwhile, multimedia services should be supported with a high transmission bit rate within a wider allowable bandwidth. This problem could be ameliorated via multiple code CDMA transmission per user [7]–[9], [15]. In other words, higher data rates are achieved by allocating more than one code to a single user in order to create more than one virtual channel for the user. The code number assigned to each user is proportional to the dynamic throughput requirement of multimedia services requested by the user. However, since the maximum available code number is limited, total code number requirements may exceed the available number. Therefore, a dynamic code allocation mechanism has been proposed to assign an appropriate code number to each user in order to achieve the maximum resource utilization for the OCPN-based multiuser multimedia services. Furthermore, in order to avoid the self-interference that a user employing multiple codes may incur, the multiple codes to/from one user should be made orthogonal. This particular spread-coding scheme is called the concatenated orthogonal/PN spreading code [7]–[9], which is capable of subdividing a high rate stream belonging to a user into several parallel lower basic bit rate streams.

II. OBJECTCOMPOSITIONPETRI-NET(OCPN) MODELS FOR MULTIMEDIAINFORMATION

A form of timed Petri nets called the OCPN has been used in [3] and [4] to specify synchronization and the relationship

between multimedia objects. The OCPN has been demonstrated to capture all possible temporal relationships between objects required for multimedia presentation. Formally, the OCPN is defined as a bipartite directed graph , specified by the

tuple where:

set of transitions (bars); set of places (circles); set of directed arcs;

mapping from the set of places to the real numbers (time durations);

mapping from the set of place to a set of object types, corresponding to video, audio, or textural objects. mapping from the set of place to the integer, which represents the number of tokens in a specified place. Associated with the definition of the Petri-net is a number of firing rules governing the semantics of the model. A transition fires immediately when each of its input places contains an un-locked token. Upon firing, the transition removes a token from each of its input places and adds a token to each of its output places. After receiving a token, a place remains in the active state for the time interval specified by the duration . During the interval, the token is locked. When the place becomes inac-tive, or upon expiration of the duration , the token becomes unlocked. For multimedia information modeling, a place in an OCPN represents the playout process of multimedia object . Attributes associated with the object include its type, size, throughput requirements, and the duration of its presentation. Moreover, a transition in an OCPN represents a synchroniza-tion point as it marks the playout start time of new concurrent objects.

Fig. 2. Example of OCPN representation for a multimedia title.

Fig. 2 illustrates an example of OCPN. In this figure, represents the initial place in the OCPN which indicates the start of the multimedia presentation, and denotes the final place which indicates the completion of presentation. Places and correspond to the playout processes of some video data; places

and correspond to the audio and text data, respectively; and plays out an image data when it has a token.

Since intermedia synchronization is ultimately desired, to synchronize multiple data streams of an OCPN, it requires concurrent streams to be played out at identical synchronization times . Such a playout time instant is called the playout deadline. Little and Ghafoor [3] have proposed a serialize-net algorithm to determine those playout deadlines based on the transitions and the playout durations . Hence, each object in the OCPN is associated with the playout deadline and the playout duration .

Moreover, the OCPN, being only a specification model for presentation of multimedia information, does not contain the communication and synchronization requirements over a net-work. Various multimedia data types have different performance requirements for network transmission and playback at the des-tination. However, the rates of communication and presentation of an isochronous object, such as video or audio, need to be equal in order to provide continuity in playback. In order to achieve this purpose, isochronous objects can be divided into smaller units of information to be used for maintaining syn-chronization. The smallest unit is referred as a synchronization interval unit (SIU) [4]. As an example, the synchronization in-terval for a video object can be 1/30th of a second, which corre-sponds to the playback duration of a single video frame. Hence, a video frame represents an SIU. For audio data, its SIU can be audio sample. As a result, a complete multimedia object is transmitted as a stream of SIU’s.

III. TRANSMISSION OFOCPN-BASED MULTIMEDIA INFORMATIONSTREAMS VIAMULTICODECDMA MOBILE

COMMUNICATIONNETWORKS

In the future, the mobile communication network will allow high mobility users to access multimedia information stored at various network sites. These multimedia servers are connected over a B-ISDN wireline network. A base station serves an in-terface between the wireline and the mobile networks. Fig. 3 shows this concept. A server retrieves the requested multimedia

data from its databases and communicates it on multiple vir-tual channels (MVCS) to the base station and has the connec-tion to the requesting mobile user via mobile multipath fading channels. Each connection between the multimedia server and the base station consists of multiple virtual channels, each of which is used to transmit an object of multimedia type. The buffers at the base station provide temporary storage for some fraction of the multimedia data objects, in order to compensate for the rate difference between the wireline and wireless net-work. These buffers also smooth the jitter delay that occurs in-side the wireline network. In the multiple access system for mul-timedia transmission of interest, the system is assumed to multi-plex a set of multimedia objects by applying CDMA techniques to the mobile networks. Each CDMA spreading code is able to create a virtual channel for multimedia transmission via the mo-bile channel. Since the momo-bile radio channel has a fixed lim-ited channel bandwidth, the traditional CDMA system may not be sufficient to accommodate all the VBR multimedia objects. Moreover, the traffic load at the base station can change dynam-ically due to various factors, such as the throughput requirement of each object, the changing level of concurrency of objects in OCPN’s, and the number of users concurrently served by the base station. To overcome this difficulty, a multicode CDMA technique is applied to the multimedia transmission in order to increase its transmission bit rate via the bandlimited mobile channels. In the multicode CDMA system, when a user needs times the basic source rate, it converts its associated concur-rent multimedia objects using a multiplexer into basic rate streams, encodes each with a different code, modulates them with a different Walsh modulator, and superimposes them be-fore upconverting for mobile transmission. In other words, each code in the multicode CDMA carries a basic rate .

codes in parallel will provide a single user times the basic rate capability. Note that each user admitted into the system has a primary PN code assigned to it. These PN codes are not orthogonal between users. To avoid the self-interference that a user employing multiple codes may incur, the multiple codes to/from one user should be made orthogonal. If is the primary PN code of user with a basic transmission bit rate , and the th object of the user requires a throughput of , new concatenated codes, ’s, can be derived

from by , , where ,

, and , denotes the smallest

integer greater than , and is the level of concurrent objects. It should be mentioned that the total number of orthogonal

Fig. 3. Overall schematics of multimedia transmissions (downlink) via multicode CDMA mobile multipath fading channels interconnecting with wireline networks for two users, MVCS represents the multiple virtual channels, andc and d denote the PN and orthogonal Walsh–Hadamard (WH) codes, respectively.

codes assigned to user depends on both the throughput re-quirement of each object and the level of concurrent objects in a specific time interval. Due to the orthogonality requirement, the maximum number of orthogonal codes per user is the ratio of the channel chip rate and the Walsh modulator output rate. is termed as the spreading-sequence length. Hence, . The previous coding strategy is called the subcode concatenated scheme that orthogonal sequences are concatenated with a PN sequence to increase the randomness of the orthogonal sequence. The binary orthogonal sequences used in this paper were the well-known Walsh–Hadamard (WH) codes which have zero cross correlation at zero time delay. They are used when synchronization of transmission can be maintained. Unfortunately, the multipath fading in a cellular radio environment introduces nonzero time delays that destroy the orthogonality between WH codes. Fong et al. [7] showed that a sufficiently long PN sequence is concatenated with WH codes to randomize and eliminate their unsatisfactory and inhomogeneous behavior at nonzero time delays. The long PN sequence may be chosen as either an -sequence or Gold sequence.

A. Multicode CDMA Transmitter Model with Concatenated Orthogonal/PN Spreading Code Scheme

For multimedia OCPN’s transmitted over a mobile channel, each of them is divided into parallel data streams (virtual channels) for user at time , where a specific WH code is assigned to each virtual channel. The value of is dynamically proportional to both the throughput requirement of user and the level of concurrent objects belonging to user at time . For simplicity, is assumed to be a constant

in the th time interval , i.e.,

for . Hence, the total number of WH codes assigned to all the users in the new time interval is where denotes the th intersection interval of the time intervals for all the OCPN’s, i.e., . Note that WH codes assigned to different users may be iden-tical. It should be mentioned that the earlier arguments are valid when each user has a continuous multimedia presentation. However, this assumption may not be true since some users may not have multimedia objects during a specific time interval, i.e., . To tackle this difficulty, the value of for this particular interval is set to zero. Therefore, the earlier argument becomes valid again. For simplicity, we assume that each of the OCPN’s has its corresponding multimedia objects during . The transmitting binary phase shift keying (BPSK) signal of the th data stream (virtual channel) belonging to the th user during the th time interval is expressed as

(1) where

transmission power of the base station;

random phase angle, uniformly distributed between 0 and 2 , introduced by the modulator;

data signal which consists of a sequence of rectan-gular pulses of duration , i.e.,

where . The concatenated spreading code can be expressed as

(3)

where is the concatenated spreading sequence

which is equal to the product of a PN sequence used by the th user and a WH code sequence assigned to its th virtual channel, i.e.,

(4) In (2) and (3), is the unit pulse function of duration , defined by

else. (5)

The duration of each data bit is , while the duration of each chip in the spreading code is . The number of chips per bit is , where is an integer. The period of the WH code sequence is equal to the processing gain . The long PN sequence has a period that is much greater than . Moreover, and are chosen to be relatively prime so that every possible chip of the PN sequence can occur at the beginning of some data bit.

As a result, the total signal transmitted to users is

(6)

B. Mobile Radio Channel Model

The Rayleigh multipath fading model is the general accepted channel model for macrocellular mobile communications [10]. In this paper, we adopted the Rayleigh fading model for perfor-mance analysis in our multicode CDMA system. The channel impulse response for the th virtual channel of the th user is given by

(7) where

th Rayleigh distributed random path gain; th random path phase, uniformly distributed be-tween zero and 2 ;

th uniformly distributed random delay ranging from zero to one data bit period, ;

the unit impulse function;

the number of resolvable multipaths for the th virtual channel of user .

In addition, it should be mentioned that these channel parame-ters vary with the transmitter–receiver distance. It may be shown

that and for

, since all the parallel virtual channels introduced by the same user are transmitted over the same propagation environ-ment between the transmitter and receiver and then would have identical channel characteristics, where denotes the variance of a random variable.

C. Receiver Model

The received signal at the input to the matched filter in the mobile receiver is given by

(8)

where

complex envelope of ;

denotes the real part of complex number;

white Gaussian noise with two-sided power spectral

density .

For simplified analysis, the first virtual channel of the first user is chosen as the reference for calculating the probability of error of its data symbol in the th sampling time interval . The receiver is able to coherently re-cover the carrier phase and locking to the th path as a reference path between the transmitter of reference and its corresponding receiver. All other paths constitute interference. That is, we assume without loss of generality that and . The envelope of the matched-filter output at the th sampling time instant is denoted by and can be expressed as

(9)

where

(10)

intramultiuser interference (self-interference) indi-cating the interference introduced by the other vir-tual channels of reference user;

intramultipath interference;

denotes the intermultiuser interference.

(11)

(12)

(13)

In (11), (12), and (13), , , and

are the well-known continuous periodic cross correlation and partial cross correlation/autocorrelations of the regenerated code and a delayed version of the interfering codes [7], [8], respectively. They are defined as follows:

(14)

(15)

(16) where is the initial phase of the PN sequence used by the th user.

Fong et al. [7], [8] have shown that is

al-ways equal to zero when and are the

concate-nated spreading codes. This implies that the orthogonality of the concatenated spreading codes eliminates the intramultiuser interference (self-interference).

D. Determination of Maximum Number (Capacity) of WH codes Based on the Quality Requirement of Multimedia Service

Usually, the bit error rate (BER) can be regarded as the per-formance index used to evaluate the quality of multimedia ob-ject, such as audio, video, and text. For the simplified derivation of the BER, the Gaussian assumption is to take all the self-in-terference, intramultipath inself-in-terference, and intermultiuser inter-ference terms as Gaussian noise. To calculate the overall vari-ance of the interference terms, one should evaluate a term like as shown in (17), shown at the bottom of the page.

Pursley [11] showed that is a constant which has the value . Since the interference terms in (9) are all mutual condi-tionally independent, the overall variance becomes

(18)

where and . It is shown in

Section III-B that and for

and . Therefore, the expressions of and

are, respectively, given by

(19)

(20) Moreover, since all the signals including the desired signal and the interfering signals caused by the other users relative to the reference user (user 1) are transmitted to the first mobile receiver from the same base station (downlink) and have the identical propagation environment between the base station and the receiver for user 1, it can be shown that and . From the previous discussion, the variance of the total interfering signals then becomes

(21)

where represents the received signal energy per bit via the th path (reference path), , and

. Similarly, the received signal power is found to

be . Hence, the average value of half

the signal-to-noise plus interference power ratio becomes

(22)

In addition, assume that . Then, becomes

(23) Note that is always greater than one. For simplicity, assume that the path gains for all the virtual channels in the same user are independently identically distributed (i.i.d.), i.e., , . Thus, the value of is identical to , i.e., , and the expression of (23) for large reduces to

(24) Moreover, for the application of multimedia presentation ser-vices, the interference term can be reduced by a multimedia pre-sentation activity factor of [6] given by

(25) where and are the activity factor and the presentation oc-currence frequency for object , respectively. Thus, with mon-itoring multimedia presentation activity, is increased relative to (24)and becomes

(26) Proakis [10] showed that the BER for both the nondiversity coherent receiver and a receiver with maximal ratio combining (MRC) of order can be expressed as a form in terms of

for nondiversity receiver

for MRC of order . (27)

In (27), for MRC, the quantity represents the av-erage signal-to-noise ratio (SNR) per combined path, and

.

Assume that BER denotes the BER requirement for sup-porting the acceptable quality of multimedia presentation

ser-vice. The value of BER (in dB) can be obtained by calcu-lating the weighted sum of BER (in dB), BER (in dB), and BER (in dB) and is given by

BER BER BER

BER (28)

where BER and are the BER requirement and

weighting factor for object , respectively, where

BER (dB). Note that the logarithmic

scale is able to make BER , BER , and BER be in the same order of magnitude. Thus, the maximum number of WH codes assigned to all the multimedia users during the time interval is determined by

BER (29)

where denotes the inverse function of which is a monotonically decreasing function of BER . Equation (29) shows that is increasing when BER is increasing or the degree of multimedia presentation quality is decreasing. However, it should be noted that of (29) is used to sup-port the presentation quality of OCPN belonging to the refer-ence user (user 1). In order to maintain the presentation quality of BER for all the OCPN’s, of (29) should be se-lected as follows:

BER

(30) where

maximum number of WH codes assigned to user ; exp exponential function with a base of ten.

In other words, , ,

, and .

denotes the presentation activity factor of the th OCPN. and represent the number of multipaths and the re-ceived signal energy per bit from the base station to the th mo-bile receiver, respectively. Therefore, the quality of all the OCPN’s may satisfy the BER requirement. In the near future, we will propose a new power assignment technique to select an appropriate level of transmitting power for each user in order to maximize the value of (30). For this case, the transmitting power for user of (1) may be different and can be selected to achieve the maximum capacity.

IV. DYNAMICSPREADING CODEASSIGNMENT FOR OCPN-BASEDMULTIMEDIASERVICES

The base station is responsible for assigning the appropriate number of WH codes for every multimedia object in all the con-current OCPN’s in each time interval. It is therefore essential

that information about OCPN’s, i.e., the playout deadline , the playout duration , and the throughput requirement of ob-ject in the OCPN’s must be provided to the base station at the time of connection establishment prior to assigning WH codes (or virtual channels) to those objects. When the total number re-quirement of WH codes assigned to all the OCPN’s is greater than of (30), and the total number of WH codes assigned to each OCPN is also less than , the transmission rate of mul-timedia objects over the mobile radio channel can be matched with the playout rate at the mobile terminal, and the presenta-tion quality of each OCPN is guaranteed with a BER value less than BER of (28). However, the maximum number of WH codes may not be sufficient to accommodate all the OCPN’s for a certain time interval. In this case, reassignment of WH codes is needed at the occurrence of transitions in OCPN’s. The possible way to code reassignment is based on either dropping some SIU’s in order to decrease total number requirement of WH codes assigned to all the OCPN’s or increasing the value of BER in order to increase to value of . Both strategies result in the degradation in presentation quality. Woo et al. [4] have shown that the tolerable loss of SIU’s can be expressed in terms of reliability requirement . For example, , , . In other words, the presentation of ob-ject with loss of % information is tolerable.

We can define the loss of information as the “dropping ratio” of object . It represents the degradation in the transmis-sion of object as a result of that the code number require-ment for all the users exceeds the maximum number of avail-able WH codes. As an example, for a video object whose code number requirement is less than a given , the number of video frames has been dropped with a ratio of % in order to re-duce the code number requirement. However, the quality of each survived video frame is still maintained with a constant perfor-mance of BER during the presentation period. In terms of SIU’s of object , the ratio is given as

number of SIU's dropped in

Total number of SIU's in (31)

Woo et al. [4] indicated that expressing the ratio in terms of SIU’s provides a better control mechanism for distributing the total droppage of object uniformly over all its SIU’s.

A. Spreading Code Assignment for Concurrent Multimedia Objects in A Fixed Time Interval

A fair code assignment policy requires that the degradation should be evenly spread across all the objects that are being transmitted concurrently if transmission needs to be degraded. The degradation in transmission occurs when either code number requirement for all the OCPN’s (users) or code number requirement for each OCPN (user) exceeds its maximum avail-able code number in time interval . The previous argument implies that all the objects have roughly equal dropping ratios. Moreover, all the dropping ratio should be selected to minimize the total degradation loss of all the OCPN’s. Under the earlier discussions, nonlinear programming (NLP) [12] is proposed to find the dropping ratio for objects in an interval to conform the limited available number of WH

codes, where is a set of all the concurrent objects in an in-terval . Prior to describing the procedure for code assignment, notations used in the procedure are defined as follows: denotes the throughput requirement of object , represents the basic transmission bit rate for the multicode CDMA system, is the set of all the concurrent objects belonging to the th user in an interval , and and are the code number requirements in an interval for all the users and the th user, respectively. They are defined as:

and .

A procedure for code assignment in an interval is described as follows.

Step 1) Satisfaction for all the code number requirements.

a) If and ,

then perform the following procedure. Other-wise, go to Step 2.

b) Code assignment:

number of WH codes assigned to object

(32) c) Terminate the procedure, and then output . Step 2) Violation for the code number requirement per user. a) If and there exists a user, i.e., user

such that , then perform the fol-lowing procedure. Otherwise, go to Step 3. b) Drop some SIU’s of objects in such that

. Solve the NLP:

NLP Minimize (33)

subect to i)

(34)

ii) Search range:

(35) where and are the costs used to mea-sure the uniformity of and the degradation loss for object , respectively. They are defined as follows:

(36)

(37)

and are their associated weighting fac-tors.

c) Find the optimal solution ’s, to

NLP1 of (33). Thus, their code assignment is if

if . (38)

Step 3) Violation for the code number requirement for total users

a) If , then perform the following pro-cedure b) NLP2: (39) subject to i) (40) ii) (41) iii) Search range:

(42) where

and

denote the measure of uniformity and the degradation loss for total users, respec-tively.

c) Find the optimal solution ’s, to NLP2 of (39). Thus, their code assignment is

(43) d) Terminate the procedure, and output . NLP1 or NLP2, which are indeed quadratic programming problems, can be solved efficiently by a variety of techniques [12]. However, it is quite plausible that the solution to the afore-mentioned NLP1 or NLP2 is not feasible. If no solution ex-ists, then the reliability requirement of an object cannot be accommodated with the given maximum available number of WH codes . The feasible solution to NLP1 or NLP2 can be achieved by either decreasing the reliability requirement or increasing the value of . This implies that the search range of (35) or (42) becomes larger or the desired quality of BER is degraded. For the case of decreasing in the video application with a given , the maximum dropping ratio % of video frame is increasing. However, the quality of each video frame is still maintained with a constant performance of BER during the presentation period.

B. Dynamic Spreading Code Assignment

The aforementioned code assignment is valid for all the con-current objects that start their presentation at a time instant in a fixed time interval. However, for a general OCPN-based multi-media presentation, some objects may start their presentations at a time instant in the previous time interval. Therefore, the ear-lier code assignment should be modified to deal with the gen-eral multimedia presentation. Similarly, prior to describing the

Fig. 4. Illustration of the time relationship betweenO , O , and O : 9 and

9 ; O , O 2 9 , and O 2 9 .

dynamic code assignment, some notations and parameters used in the assignment are defined. Let denote the th interval as the current time interval. denotes the set of concurrent objects in , which is composed of two disjoint sub-sets, and . represents a set of new concurrent objects that start their presentation at . consists of objects that con-tinue their presentations during interval . denotes the set of objects belonging to for user . Similarly, and rep-resent the sets of objects belonging to and for user ,

re-spectively. Formally, , , and

can be defined as follows: if

if . (44)

Fig. 4 illustrates an example of , , and .

denotes the available number of WH codes assigned to all the users at time . Similarly, let be the available number of WH codes assigned to user at time , . represents the total number of WH codes assigned to object

belonging to in . and are the code number

requirements for new concurrent objects belonging to and in , respectively. Let denote the time just before the transition at time is fired. Thus, the available number of WH codes before firing the transition at is computed by

(total number of WH codes released by all the objects ended at a time instant between and

(45)

Note that the expression of (45) is also valid for , . The base station should update its available code number at the occurrence of each transition, and it is also able to assess the code number requirements for the next interval, based on the information given in OCPN’s of the connected users. In other words, the code assignment for the next interval can be determined continuously ahead of time by inspecting code number requirements specified in OCPN’s.

The procedure for dynamic code assignment is summarized as follows.

Step 0) Initialization: = maximum time index,

, , ,

, and .

Step 1) Test the termination:

If then terminate the procedure. Otherwise, go to Step 2.

Step 2) Code number requirements for new objects:

and

(46)

Step 3) Satisfaction for all the code assignment requirements:

if and

, then perform the following procedure. Otherwise, go to Step 4.

a) Code assignment: if

if . (47)

b) After code assignment for the new objects, the available code number at becomes

and then go to Step 6.

Step 4) Violation for code number requirement per user: a) If , and there exists a user, i.e.,

user , such that , then

per-form the following procedure. Otherwise, go to Step 5. b) NLP1: subject to (48) where and

c) Find the optimal solution ’s, to

NLP1 of (48), and their associated code assign-ment is given as

if

and user has violation if

and user has satisfaction

if .

(49)

d)

and then go to Step 6.

Step 5) Violation for code number requirement for total users.

a) If , then perform the following

procedure: b) NLP2: subject to (50) where and

c) Find the optimal solution ’s, to

NLP2 of (50), and their associated code assign-ment is given as

if

(51) d)

and then go to Step 6.

Step 6) Update the available code number for the next time interval.

a) b)

c) and then go to Step 1. (52)

V. ILLUSTRATEDEXAMPLE

Using the previous spreading code assignment procedure, the base station can assign the available WH codes to users dynam-ically based on information in users’ OCPN’s and the avail-ability of the station’s resources. The channel model described

Fig. 5. BER(p ) versus total number of WH codes assigned to all the K users, M. (a) [(b)] and (c) [(d)] denote curves obtained from the MRC and MRC plus BCH (15, 7) coding for the first (second) multimedia title, respectively, wherek = 2.

in this paper is identical to Model-I employed in [13]. In this model, the variances of the Rayleigh path gains are the same for all users and equals−14 dB (0.038). Additionally, the max-imum number of resolved multipaths for all users is equal to four, i.e., . The multicode CDMA system has a

trans-mission rate of kb/s, a processing gain of ,

and a bandwidth of 10 MHz. Its corresponding PN and orthog-onal spreading sequences are chosen as the m-sequence with a period equal to (242−1) and the WH code with a period of 128, respectively. For the CDMA receiver, it has a maximum ratio combiner (MRC) of order . To evaluate the per-formance of the multimedia transmission via multicode CDMA channels, two OCPN’s for users 1 and 2 are assumed to have two object types, i.e., voice and video objects. For the applica-tion of a wireless channel, Khansari et al. [14] showed that voice with source rate equal to 8 kb/s and low resolution video with source rate equal to 128 kb/s are suitable for such applications. The BER requirements for voice and video are 10−3and 10−4, respectively. From (28), the value of BER is found to be

10−3.5or –35 dBwhen . Moreover, two

different multimedia titles (programs) are conducted to evaluate our system. The first multimedia title has two video and voice object types of identical presentation activity factor. This yields the application of video telephone when

and . For simplicity, here, we assume

that is negligible compared to the multiuser and multi-path interference. Thus, from curve (a) of Fig. 5, the maximum

number of WH codes is found to be about for

the first title. The second multimedia title has a higher video

presentation activity factor , , and

. Curve (b) of Fig. 5 shows that becomes smaller and equals about 29for the second title. By employing the BCH (15, 7) error correction with a code rate

of , curves (c) and (d) show that for both

cases become extremely large, i.e., for title 1 (video phone) and for title 2. However, the source rates for both video and voice are also increased by a factor of . This would degrade the transmission efficiency. Meanwhile, it is able to offer multimedia services with lower transmission error via wireless channels.

It should be mentioned that the throughput requirement, of an object is always set to a multiple of its source rate of

, i.e., , , in order to shorten the

trans-mission delay. In other words, a larger number of WH codes are required to provide a sufficient number of virtual channels to meet the throughput requirement. Once the value of has been determined, Fig. 6 illustrates a result of the dynamic code assignment for the two OCPN’s in video phone application with BCH (15, 7) channel coding according to

of (30). Fig. 6(a) shows the timeline diagram of the OCPN representations for both users. The first OCPN has three objects , , and whereas the second OCPN has four objects , , , and . Note that each of those objects belongs to either video or voice type and has its corresponding throughput requirement. For example, the throughput require-ment of is almost twice more than that of or . Multi-media objects , , and start their presentation at time simultaneously. Prior to the presentation, the base station should perform the proposed code assignment procedure to determine the appropriate number of WH codes for each object. Fig. 6(b)

(a)

(b)

Fig. 6. (a) Example of OCPN’s for users 1 and 2. (b) Dynamic code assignment for OCPN-based multimedia objects, wheren denotes the WH code number assigned toO for the jth time interval [T ; T ) and M = 96.

shows the resulting code number assigned to for the th time

interval , denoted by . At time , it is found

that there are , and , for

where , ,

and . Thus, For the

next time interval , when a new call of two new objects and is received at time by base station, it tests the admissibility condition of the code number requirement for the call. At time , the code number assigned to is released since is ended at , and the available code number at that

time becomes . Since this

new call failed in the test, the base station proceeds for drop-ping uniformly some SIU’s of and by performing either NLP1 of (48) or NLP2 of (50). By (51), it leads to these two

new assigned code numbers and .

How-ever, the code numbers for old objects and during this

new time interval are given by and

. Finally, the appropriate code numbers for the remaining objects and during time intervals , , and can be obtained by the proposed assign-ment procedure and are shown in Fig. 6(b).

VI. CONCLUSION

In this paper, we have proposed a multicode SS-CDMA system to provide multiple access and retrieval of multimedia information via mobile radio channels interconnecting with the wireline B-ISDN networks. Since the maximum available number of spreading codes in the multicode CDMA system is limited, the objective is to appropriately manage the limited codes of the network for maximum utilization while guaran-teeing synchronized presentation of multimedia information within the quality constraints specified by a given BER. Some-times, since the total code number requirement may exceed the maximum available code number in a time interval, some SIU’s of multimedia information should be dropped within a reliability requirement in order to make the transmission rate of multimedia data stream over the mobile radio channel be matched with the playout rate at the mobile terminal. Toward this end, we present a dynamic code assignment scheme to determine both the total droppage of multimedia objects uniformly over all their SIU’s and an appropriate code number assigned to each of them.

REFERENCES

[1] M. Schwartz, “Network management and control issues in multimedia wireless networks,” IEEE Personal Commun. Mag., vol. 2, pp. 8–16, June 1995.

[2] D. Raychaudhuri and N. Wilson, “Multimedia transport in next-genera-tion personal communicanext-genera-tion networks,” in Proc. Int. Conf.

Communi-cations, May 1993, pp. 858–862.

[3] T. D. C. Little and A. Ghafoor, “Multimedia synchronization protocols for broadband integrated services,” IEEE J. Select. Areas Commun., vol. 9, pp. 1368–1382, Dec. 1991.

[4] M. Woo, N. U. Qazi, and A. Ghafoor, “A synchronization framework for communication of pre-orchestrated multimedia information,” IEEE

Network Mag., vol. 8, pp. 52–61, Jan./Feb. 1994.

[5] M. Woo, N. Prabhu, and A. Ghafoor, “Dynamic resource allocation for multimedia services in mobile communication environments,” IEEE J.

Select. Areas Commun., vol. 13, pp. 913–922, June 1995.

[6] A. J. Viterbi, CDMA: Principles of Spread Spectrum

Communica-tions. Reading, MA: Addison-Wesley , 1995.

[7] M. H. Fong, Q. Wang, and V. K. Bhargava, “Concatenated orthog-onal/PN codes for DS-CDMA systems in a multi-user and multipath fading environment,” in Proc. IEEE GLOBECOM’94, Nov. 1994, pp. 1642–1646.

[8] , “Concatenated orthogonal/PN spreading code scheme for cellular DS-CDMA system with integrated traffic,” in Proc. 1995 Int. Conf.

Communications, Seattle, WA, June 1995, pp. 905–909.

[9] C. L. I. et al., “Performance of multi-code CDMA wireless personal communication networks,” in Proc. 1995 IEEE 45th Vehicular

Tech-nology Conf., Chicago, IL, pp. 907–911.

[10] J. G. Proakis, Digital Communications. New York: McGraw-Hill, 1983.

[11] M. B. Pursley, “Spread spectrum multiple access communications,” in

Multi-User Communication Systems, New York: Springer-Verlag, 1981,

pp. 139–189.

[12] D. G. Luenberger, Linear and Nonlinear Programming. Reading, MA: Addison-Wesley, 1984.

[13] M. Kavehrad and B. Ramamurthi, “Direct-sequence spread spectrum with DPSK modulation and diversity for indoor wireless communica-tions,” IEEE Trans. Commun., vol. COM-35, pp. 224–236, Feb. 1987. [14] M. Khansari, A. Jalali, E. Dubois, and P. Mermelstein, “Robust low

bit-rate video transmission over wireless access systems,” in Proc. IEEE

ICC, May 1994, pp. 571–575.

[15] R. Wyrwas, M. J. Miller, R. Anjaria, and W. Zhang, “Multiple access op-tions for multi-media wireless systems,” in Proc. Third Workshop Third

Generation Wireless Information Networks, Apr. 1992, pp. 289–294.

Po-Rong Chang (M’87) received the B.S. degree in

electrical engineering from the National Tsing-Hua University, Hsinchu, Taiwan, R.O.C., in 1980, the M.S. degree in communication engineering from National Chiao-Tung University, Hsinchu, Taiwan, R.O.C., in 1982, and the Ph.D. degree in electrical engineering from Purdue University, West Lafayette, IN, 1988, respectively.

From 1982 to 1984, he was a Lecturer in the Chi-nese Air Force Telecommunication and Electronics School for his two-year military service. From 1984 to 1985, he was an Instructor of electrical engineering at National Taiwan Insti-tute of Technology, Taipei, Taiwan, R.O.C. From 1989 to 1990, he was a Project Leader in charge of SPARC chip design team at ERSO of Industrial Technology and Research Institute, Chu-Tung, Taiwan, R.O.C. Currently, he is a Professor of communication engineering at National Chiao-Tung University. His current interests include wideband CDMA systems, wireless multimedia tions, fuzzy neural networks, and low-power design for wireless communica-tions.

Dr. Chang was the recipient of the Best Paper Award in quality control for semiconductor manufacturing in 1990, Taiwan, R.O.C.

Chin-Feng Lin was born in Taiwan, in 1965. He

re-ceived the B.S. degree in electrical engineering from Chung-Yung University in 1996 and the M.S. degree in electrical engineering from Chung-Hua University in 1998. Currently, he is working toward the Ph.D. degree in communication engineering at the National Chiao-Tung University, Hsinchu, Taiwan, R.O.C.

His research interests include CDMA systems and wireless multimedia communications.