IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 47, NO. 3, AUGUST 1998 977 A Proposal for an Adaptive S-ALOHA Access System for a Mobile CDMA Environment Oriol Sallent, Student Member, IEEE, and Ramon Agust´ ı, Member, IEEE Abstract—This paper presents a novel adaptive direct-sequence code-division multiple-access (DS-CDMA) slotted-ALOHA (S- ALOHA) packet random-access scheme with transmitter-based spreading codes for mobiles. It is aimed at improving throughput and message delay delivery when traffic load values below the saturation point of the conventional DS-CDMA S-ALOHA system are sensed in the channel. For this purpose, one mobile-station (MS) and two base-station (BS)-assisted algorithms are envisaged to control the change of the transmission rate according to the traffic load. These algorithms revealed that the optimum behavior, obtained using a Markov chain model, may almost be reached at a low-complexity cost. A traffic model based on a realistic statistical length distribution of the messages illustrates how the delay delivery can be greatly reduced with the pro- posed algorithms. Finally, the impact of forward-error-correction (FEC) coding on the adaptive system is also studied. Index Terms— Access protocols, code-division multiaccess, packet radio. I. INTRODUCTION S LOTTED ALOHA (S-ALOHA) has been widely recog- nized for packet radio applications because of both its simplicity in managing bursty traffic and its ability to deliver a shorter delay than that of fixed multiple-access techniques in the presence of low traffic loads. The original version of this protocol assumes that whenever more than one packet is transmitted at the same time, the information contained in all the transmitted packets will be lost. This would not be the case if S-ALOHA was used with a multireceiver direct-sequence code-division multiple-access (DS-CDMA) technique. This CDMA S-ALOHA scheme allows several users to transmit at the same time using a different spreading code for each one [1], [2] so that unsuccessful transmissions are caused entirely by multiple-access interference, provided negligible thermal noise effects are assumed. Such an access technique could also benefit from the advantages of CDMA in operation flexibility and robustness to undesired interferences. In a CDMA-based wireless multimedia system, DS-CDMA S-ALOHA is cer- tainly a simple choice for bursty data transmission. This paper is intended to improve the throughput achieved with a multireceiver DS-CDMA S-ALOHA when a light load is offered to the system, and accordingly the delay decreases. For this purpose, we will focus on the transmission rate used Manuscript received April 7, 1996; revised November 18, 1996. This work was supported by CICYT (Spanish Education Ministry) under Contract TIC94- 0870-C02-01. The authors are with the Departamento de Teoria del Senyal i Comunica- cions, Universitat Polit` ecnica de Catalunya, 08034 Barcelona, Spain (e-mail: oriol@xaloc.upc.es). Publisher Item Identifier S 0018-9545(98)02474-8. instead of on the access protocol itself. In particular, different transmission rates are proposed in order to utilize the most suitable one according to the channel load at any time slot. Fast and simple algorithms that command the processing gain of the spread-spectrum DS scheme as a function of the channel load are envisaged. For low channel loads, a high CDMA processing gain is no longer necessary, and it can be reduced so as to increase the actual bit rate. Thus, an adaptive S- ALOHA in which the packet duration does not change is considered, though depending on the rate used, a different number of bits can be transmitted in a single time slot. In this way, system throughput increases and message transmission delay can be shortened to the extent that the packets resulting from the breaking down of these messages are able to increase the number of transported bits. This is also noticeable in a multireceiver CDMA, where the users are asked to contend for a lower number of channels when the user bit rate increases. This number of channels would coincide with the processing gain of an ideal CDMA, i.e., it would be able to operate with orthogonal spreading sequences. Therefore, in an extreme situation with a processing gain equal to one, all the users would access the same channel at their maximum bit rate according to a pure S-ALOHA. Certainly, the greatest access randomness, deriving from the greatest contention activity, would benefit the low traffic loads by shortening message delay delivery. The paper is organized as follows. In Section II, an analyti- cal model for the DS-CDMA S-ALOHA system is presented, which is used to evaluate the optimum achievable throughput with an adaptive change of transmission-rate algorithm. In Section III, several S-ALOHA adaptive algorithms are pro- posed and assessed depending on whether the algorithm is managed by a mobile station (MS) or by a base station (BS). Section IV considers a realistic situation where users generate messages that must be segmented into several packets to access the radio channel. Performance in this situation is also studied. Section V covers the impact of coding on the proposed schemes. Finally, some conclusions close the paper in Section VI. II. MODEL FOR AN S-ALOHA DS-CDMA ACCESS SYSTEM From now onwards, -registered users will be considered. These users may be in either of two different operation modes: “idle mode” (I mode) and “backlogged mode” (B mode). In the former, there is no packet to be retransmitted, and new packets are generated with probability . Terminals enter the backlogged mode when an attempt to transmit a new packet 0018–9545/98$10.00  1998 IEEE