JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Chapter: Cellular Networks
Section: Multiple Access, Random Access

Space Time Reservation Multiple Access

Space Time Reservation Multiple Access (STRMA) is a method to allow multiple portable terminals to access a communications infrastructure. It combines frequency reuse (in space or area) with multiple access. STRMA applies digital speech interpolation (DSI). The goal is to better support bursty multi-media traffic.
STRMA can be interpreted as a spatial extension of the Packet Reservation Multiple Access (PRMA). PRMA is a framed access scheme with frames of a fixed number of slots. If a terminal has a series of packets or speech segments to transmit, it competes for access in any free slot. If it successfully captures the base station, the terminal gains reservation in the corresponding slots of the next frames, until it releases the reservation.

In PRMA, adjacent cells use different carrier frequencies according to a cellular reuse plan, but in STRMA we synchronize base stations and terminals at slot level. Time slots are common to all cells, and all cells use the same carrier frequencies. However, if a terminal gains a reservation in one cell for certain time slots, the base stations of the first tier of surrounding cells inhibit all other terminals to use the same time slots. Hence, reservations occur not only in time domain, as in PRMA, but also in space.

In the STRMA protocol, the reservation of the time slot is not restricted to the cell in which the user is located. The timeslot will also be reserved in surrounding cells to avoid collisions of packets transmitted in those surrounding cells in the same timeslot.

For the STRMA system a frame consist of Ns timeslots. When a reservation is made only the cells in the first tier surrounding the user obtaining the reservation, are also reserved. So two active users are always separated by one cell. In a fixed reuse pattern such a separation compares to a 3-cell reuse pattern. So in every cell we have 1/3 of the available bandwidth which compares to having Ns/3 timeslots per frame.

Cellular Network. This figure illustrates a possible sample of an STRMA process, during one particular time slot, with idle (blue), busy (yellow) and inhibited cells (white).

Initial simulations revealed that for a uniform distribution of speech terminals, STRMA with frames of 21 slots outperforms conventional PRMA with 7 slots per frame and a three cell reuse pattern. We addressed a system with 5 erlang per cell and a speech activity of 0.4 (40 %) according to a Markovian on-off process with average duration of speech burst of 1 second and average length of a gap of 1.35 seconds. The speech clipping probability for PRMA was about 0.5% and 0.2% for STRMA, while both systems use the same spectrum bandwidth. With non-uniform distributions of users, we expect a further performance gain. The STRMA system remained very efficient, also at higher message traffic loads.

Figure 4

This figure illustrates a sample of an STRMA process with (very) heavy traffic, during one particular time slot, with idle (blue), busy (yellow) and inhibited cells (white).

More Samples from a Large STRMA network

Much to the surprise of the investigators, STRMA appears to have strong "self organizing" properties. Under very heavy traffic, the slot contents converge to a dense solid reuse pattern, much a "crystal" structure.

The simulation program is available on this CD ROM.

JPL's Wireless Communication Reference Website Jean-Paul M.G. Linnartz and C. van den Broek, 1993, 1995.