A Joint MAC-PHY Approach for Medium Access Control in VBR MC-CDMA Broadband Indoor Connections

Abstract

Multicarrier Code Division Multiple Access (MC-CDMA) techniques were originally proposed at mid of 90's for wideband multi-user communications in wireless environments characterized by hostile propagation characteristics. In this work, 1 the design of a MC-CDMA-based infrastructure is considered for VBR broadband indoor connections with real-time asynchronous multiple access. At the present time, Broadband Fixed Wireless Access (BFWA) standards like IEEE 802.16 and HIPERMAN can bring broadband services inside buildings, but indoor access should be conveniently provided by a local area connection. The capability of MC-CDMA of supporting asynchronous multi-user variable-bit-rate (VBR) transmission is exploited jointly with an efficient and real-time Medium Access Control (MAC) strategy in order to allow a significant number of indoor VBR users to transmit information in CDMA modality with different quality of service (QoS) profiles. Different classes of users are defined at the MAC level. The available radio resources (i.e. the orthogonal subchannels) are selectively attributed to transmitting users depending on their performance achieved at MAC level and measured by an "intelligent" gateway. When the quality level is not satisfactory for one or more users, the AP issues a decrease of the data rate for such users while providing them with an increased number of subcarriers, guaranteeing a slower transmission fostered against frequency-selective channel distortions. The paper presents an overview of the system and tests its performance through extensive simulations. The proposed joint MAC-PHY approach demonstrates good performance in terms of achieved throughput and high flexibility in radio resource management.

Situation-aware radio resource management for multi-rate MC-CDMA wireless networks targeted at multimedia data exchanges in local areas

“…The MAC protocol presented in [9] provides a good efficiency in the use of the shared wireless medium, but, as drawback, BE users and highest-rate QG users are severely penalized by the heavy impact of multiple-access interference (MUI) on PHY-layer performances. The joint MAC-PHY approach proposed in [2] can be regarded as the natural evolution of [9] and the starting point for the novel MAC approach detailed in Section III. The "class downshift" discussed in [2] mechanism is a sort of "PHY-layer fostering" provided to "colliding" users, in order to make them more robust with respect to the MUI effects.…”

“…The adopted strategy is substantially a joint MAC-PHY layer approach derived by the one shown in [2] which relies on the variable-bit-rate (VBR) MC-CDMA physical layer design proposed in [3]. The overall transmitting users are partitioned into different VBR classes, each one characterized by a set of PHY-layer parameters consisting of: a) a variable-cardinality subcarrier set and b) a variable-length spreading code, each one depending on the bit-rate class.…”

“…When user performances in terms of FER are not satisfactory, the AP issues a "class downshift" for users penalized by heavy FERs. Such a downshift mechanism, introduced in [2], substantially consists in a decrease of the data rate, with a consequential increase of the processing gain and transmission SNR, achieved in the context of the VBR MC-CDMA PHY-layer design detailed in [3]. The effectiveness of the class downshift mechanism has been already proven in [2].…”

“…Such a downshift mechanism, introduced in [2], substantially consists in a decrease of the data rate, with a consequential increase of the processing gain and transmission SNR, achieved in the context of the VBR MC-CDMA PHY-layer design detailed in [3]. The effectiveness of the class downshift mechanism has been already proven in [2]. In fact, such procedure allows improving the aggregated throughput of a VBR MC-CDMA network, raising throughput curves of most penalized users from a cumbersome "zero-floor".…”

“…In fact, such procedure allows improving the aggregated throughput of a VBR MC-CDMA network, raising throughput curves of most penalized users from a cumbersome "zero-floor". In this work, we intend to complete the joint MAC-PHY layer approach of [2], considering also the possibility of a "class upshift" for those users previously downshifted or "on request" by users not yet downshifted. The "class upshift" is issued by the intelligent AP for previously downshifted users, only when the network situation allows it.…”