Advances in Packet Radio Technology

One-line summary: Basic concepts of packet radio technology are discussed, and an implementaton of a PR network (called PRnet) is described in detail.

Overview/Main Points

• Packet radio (PR) network capabilities: The PRnet was designed with the following capabilities in mind: Transparency - network operation is transparent to its users. Connectivity - total logical connectivity should be provided between all network nodes. Mobility - vehicular speeds should be supported, and PRs should have reasonable physical characteristics. Internetting - routing and gateway functionality to external networks should be present. Coexistence - the PR network should be allowed to coexist with other users of a frequency band. Throughput and low delay - variable length packet sizes, 0.1s latency, 100 mi sq coverage, 100-400 kb/s throughput. Rapid deployment - ad hoc self-organizing network deployment. Routing - broadcast and point-to-point. Addressing - multicast groups. Tactical apps - resist spoofing, jamming, detection, and direction finding.
• Radio characteristics: Throughput requirements dictate a minimum frequency of 100+ KHz, and absorptive path loss sets a maximum frequency of 10 GHz. Multipath (NLOS) effects, transmitter antenna height, and carrier frequency all affect the attenuation of a radio signal. Multipath effects cause signal fades, often many per packet, necessitating error correction and redundancy. Multipath also causes intersymbol interference (ISI) - adaptive equalization (RAKE) can mitigate this. Multipath also smears symbols - signal components arrive with delay spread; spread of signal in frequency domain is called coherence bandwidth. Spectral width of signal should not exceed coherence bandwidth, or else notches in symbol spectrum will occur. Intentional or non-intenional man-made interference cause bursty errors. Spread-spectrum techniques (frequency hopping (FH) or direct sequency pseudo-noise (PN)) mitigate multipath effects and interference, increase the signal-to-noise ratio by the chipping gain factor, lowers the signal's spectral density, increases the capture property (ability to receive one packet in the presence of other interfering packets), allows multiple users to coexist via CDMA, and suppresses ISI.
• System considerations:
  1. Multiple access channels: Signals can be made orthogonal in the frequency domain (FDMA) or the time domain (TDMA). FDMA is simpler to implement, but achieves lower channel efficiency since any given band may remain idle for a while. TDMA requires synchronization of transmissions.
  2. Controlling the MA channel: ALOHA (slotted vs. non-slotted) and CSMA are considered.
  3. System structure: network consists of terminal units with attached packet radios, repeaters (router infrastructure nodes), and stations (providing centralized administration). Stations know about all radios in the network, and compute network topology. Stations distribute routes from PRs to stations - this is called "labelling". PRs may have to serve as repeaters in ad hoc networks.
  4. Store-and-forward: repeaters perform store-and-forward style packet routing; half-duplex operation is typical. Packets contain a preamble for gain control and synchronization timing acquisition by receiver PRs.
  5. Point-to-point routing: stations compute point-to-point routes. Senders set up that route by transmitting route setup packets - &qoutsoft" virtual circuits are therefore established, with intermediate repeaters storing part of the route. (They are soft because the routes are flexible in case of node failures.)
  6. Broadcast routing: broadcast packets are flooded through the network, with nodes storing a history of broadcast packets so that flooding is controlled.
  7. Mobility: mobility implies rapid updates of point-to-point routes. If terminals move too quickly for routes to be updated, broadcast can be used (at a high cost, of course).
• Network initialization: Special "radio-on packets" (ROPs) are broadcast by all radios every 30s. ROP packets are used to establish peer-connectivity; this connectivity information is forwarded to stations so they can compute global topology. When a station is first connected, it begins the labelling process. Nodes N hops from the station are said to be at level N; labelling proceeds outwards from level 1 nodes to some maximum level nodes, using a special protocol called "SPP". A station has a maximum number of nodes that it can label, and labels time out and must be refreshed. Two stations that have labelled a common repeater are called neighbors. Multi-station design yields redundancy (hot-switching of stations), pulls complexity out of PRs. This may blow scalability, of course.
• Renewal points: A renewal point is a repeater at which a point-to-point route may be altered; headers of packets contain the next few repeaters in the end-to-end route, and this header is altered to modify the route.
• Multi-station operation: Multi-station networks provide scalability. Stations know routes to neighbor stations by the labelling process. When routing across station domains, route-finding packets are effectively broadcast across the logical network of stations; the destination PR's station receives these route-finding packets, and chooses among them for the most efficient route. It then transmits back a route-setup packet that sets up what is effectively the virtual circuit through the network for the end-to-end route.
• Stationless operation: Route-finding packet broadcast across all PRs (not just stations, as in the multi-station case) and route-setup packets are used. The lack of stations implies lower efficiency, a lack of ability to do global (centralized) congestion detection and control, and seamless route alteration in case of repeater failure.
• Spread spectrum details: if codes are dynamically calculated, network synchronization is required - preamble and synchronization headers on packets are used for this. Dynamic code calculation must be used for antijamming - codes are used within slots, and changed across slots. Slots are numbered sequentially to uniquely identify them. Slotted and non-slotted transmission within slots can be used. A code can be chosen given a (slot-number, receiver-id) pair - such a code is known as a receiver-addressed waveform, and allows channel sharing across receivers (CDMA). Explicit acknowledgements must then be used.
• Experimental PR setup: a test system was deployed in the bay area. Two different PR units were developed - the "experimental packet radio unit" (EPR) which implements the basic spread-spectrum techniques and network protocols, and an "upgraded packet radio unit" (UPR) which incorporates tactical functionality such as electronic counter counter-measures (ECCM). Details of these radios are interesting but archaic. 1000 16-bit words contain the entire PR OS. Wow!