SLEEP MODE OPERATION IN WIMAX RELAYING NETWORKS

In the forthcoming IEEE 80216j standard, sleep mode operation has also been incorporated. In such relay networks, a BS coordinates all communications to and from subscriber stations. Although the initial standard provides a large coverage distance of up to 50 km under line-of-sight (LOS) conditions and typical cell radii of up to 8 km under non-line-of-sight (NLOS) conditions, there was still a huge demand for enhancing the network coverage without compromising system throughput. One of the possible options to achieve this was to introduce relay stations (RS) which bridge the communications between an SS out of coverage and the BS [9]. The great advantage of this multihop communication concept is that coverage is extended with cheap devices at a small cost of throughput degradation. RSs are divided into three categories, namely fixed RS (FRS) which are installed at fixed locations, nomadic RS (NRS) installed for a specific time duration, and mobile RS (MRS) which are mobile units operating with battery cells (Figure 1).

 

Figure 1: Sleep mode in IEEE 802.16j multihop networks.

Useful deployment of MRS with a battery power source or low-power fixed/NRS powered by solar power or battery require high-power efficiency. Moreover, this feature reduces the interference generated by RSs. Here, we briefly describe how sleep mode can function in relay scenarios.

In a centralized scheduling system, where the network is coordinated by the BS (named in Relaying Networks as MR-BS), the MR-BS informs all the stations of the network including both the RSs and the MSs, about listening and sleeping windows. In a second phase, the RS coordinates with the MSs the finding of common availability and unavailability intervals. If all the MSs connected to a single RS are sleeping then the RS can enter the sleep mode too. Similar to the initial IEEE 802.16e standard, the RS can request activation of RS sleep mode by sending RS_SLP-REQ message to the MR-BS. Note that now respective message types begin not with MOB as before but with RS.

In order for the RS to generate the listening and sleep windows, via a RS_SLP-REQ, the RS shall keep record of the information sleeping patterns of associated MSs. When an RS enters sleep mode it can be awakened by the serving MR-BS or by itself. The MR-BS can use the existing MOB_TRF-IND to awaken a sleeping RS.

Sleep Mode can be divided into

• Full RS Sleep Mode: This mode is entered if there is traffic at any relay and access link while the RS stays in sleep mode. All associated RSs and MSs connections are suspended RS sleeping.
  
  
• Partial RS Sleep Mode: This mode is similar to the previous case. The difference is that certain management messages are sent at predefined intervals to support MS network entry, re-entry, and handover.

A question remains whether these mobile relay stations can withstand the heavy usage. To what extent can such a terminal survive only on battery, and how energy efficient can IEEE 802.16e be in case of distributing information to many nodes?