QRZ! Ham Radio 8/QRZ Ham Radio Callsign Database - Volume 8.iso/pc/files/p_misc/netconf.arc/SPECTRUM.TXT

Text File | 1988-12-10 | 12KB | 265 lines

                   Digital Radio Networks and Spectrum Management

                               Paul A. Flaherty, N9FZX

                           Computer Systems Laboratory _a_n_d
               Space, Telecommunications, and Radioscience Laboratory
              Department of Electrical Engineering, Stanford University
                            ERL 408A,  Stanford, CA 94305




          Abstract

                  Spectrum Management is a vital  part  of  amateur  radio.
          Questions  of  where  to place services in the available spectrum
          continue to plague frequency coordinators.  This  paper  contends
          that  multiaccess  radio systems should be allocated in the spec-
          trum below one GigaHertz, and that monoaccess  or  link  oriented
          systems be placed above that frequency.



          Introduction

                  Electromagnetic Spectrum is a scarce, sometimes renewable
          resource.   Much of the research in radioscience today is devoted
          to spectrum - efficient methods of communication, including  such
          mechanisms  as  amplitude  -  compandored sideband telephony, and
          minimal shift keying data transmission.  Only recently,  however,
          has  research  touched on the area of spectrum reuse, and the im-
          pact of position within the radio spectrum considered.

                  Propagation characteristics of certain bands  make  those
          spectra  valuable  to  classes of users.  Ionospheric propagation
          below 30 MHz makes the High Frequency bands valuable to the world
          community.  Small component size and portability are important to
          mobile users, and so the Very High and Ultra High bands  play  an
          important part in mobile communications.

                  Beyond these characteristics, however, little can be gen-
          eralized about the appropriate spectra for certain classes of ap-
          plicants.  It is not readily apparent that  one  band  should  be
          preferred  for  multiaccess  applications, and another for link -
          oriented systems.

                  Packet Radio is considered to be a  spectrally  efficient
          mechanism  for  digital  communications.   Using  time - division
          techniques, several users may share  spectrum  without  interfer-
          ence, if certain traffic characteristics hold, and if the network
          load is limited.  Techniques for time - sharing spectrum  abound,
          but   all  require  some  degree  of  omnidirectionality  in  the
          transmission or reception system, which is characteristic of  all
          all multiaccess networks.

                  Using packet switching techniques, it is possible to con-
          struct  a  link - oriented, or monoaccess network, which is func-
          tionally equivalent to a multiaccess network.  This  duality  can
          be exploited for networks with fixed or portable stations.

                  In a hierarchal  networking  architecture,  the  Terminal
          Network is usually defined as that hierarchy or subnet which con-
          nects to end users.  The telephone local loop  plant,  and  radio
          repeaters  are  two examples of terminal networks.  This paper is
          primarily concerned with terminal networks, although many of  the
          principles may apply elsewhere.

          Synthesis

                  The forward gain of  a  parabolic  reflector  antenna  is
          given as:          G = eta pi sup 2 d sup 2 f sup 2 over C sup 2


                  It is of no small consequence that the gain of a  reason-
          ably  sized  antenna  increases dramatically with frequency; many
          digital satellite services exist explicitly because of this fact.

                  For the purposes of discussion, a "reasonably sized"  an-
          tenna  is  considered  to be unity, or one meter in diameter, for
          terrestrial applications.  "Reasonable size" is often a matter of
          community  tastes and economics; however, the one meter size cov-
          ers a large portion of of the contingencies.  Thus, the  gain  of
          reasonably sized antenna is:

          G sub 0 = eta pi sup 2 f sup 2 over C sup 2

                  The half power beamwidth of a typical parabolic reflector
          is:

          A = 139 over sqrt G



                  Digital  modulation  schemes  may  be  divided  into  two
          classes:  orthogonal  modulation  techniques, such as phase shift
          keying, and antipodal modulation, such as amplitude or  frequency
          shift  keying.  In order to add another bit per symbol in  a con-
          stant - bandwidth channel, an increase in the signal - to - noise
          ratio of 3 db is required for orthogonal modulation, and 6 db for
          antipodal systems.

          Frequency Division Tradeoff

                  The Frequency Division Tradeoff between  multiaccess  and
          monoaccess  networks  arises out of the increase in signal - to -
          noise ratio that occurs with the use  of  directional  radiators.
          With  the  increase  comes the ability to either multiply the bit
          rate, or divide the bandwidth to obtain equivalent service.   Be-
          cause antenna gain is tied integrally with frequency, the ability
          to fraction the bandwidth increases frequency, until a  point  is
          reached where each node occupies its own channel.  The transition
          from a multiaccess network to its monoaccess  dual  occurs  at  a
          certain  Critical Frequency, which is determined in turn by chan-
          nel access technique, and network size.

                  As an example,  consider  a  terminal  network  of  eight
          nodes,  using  a  Carrier  Sense - Multiple Access, and frequency
          shift keying, running at a rate of 19.2 Kbps.  Assuming the  best
          case for CSMA (no hidden nodes), the best aggregate throughput we
          can expect from such a network is about 10.6 Kbps.

                  The dual of this network is a set of eight links connect-
          ed  to  a  packet switch.  Again assuming the best case for CSMA,
          each user has access to a 19.2 Kbps data rate.  We wish to accom-
          plish  this  transition  using  equivalent  power  and bandwidth;
          therefore, we require an eightfold increase in the aggregate  bit
          rate.   Assuming the use of n-ary frequency shift keying, this in
          turn requires an increase of 42 db in the signal - to - noise ra-
          tio.   Such  an  increase  can be obtained by a pair of one meter
          aperture antennas, operating at 1.5 GHz, using  a  55%  efficient
          feed.   The  aggregate throughput for this network is 153.6 Kbps,
          in the same bandwidth.

                  In general, for a large class of terminal  networks,  the
          Critical  Frequency lies around one GigaHertz.  The extent of the
          tradeoff is limited in practice by packet switching  speeds,  and
          the extensibility of multilevel modulation schemes.

          Space Division Tradeoff

                  The propagation characteristics of radio limit  the  spa-
          tial  dimensions  of  any network.  However, it is often the case
          that the network itself covers far less territory than the  radio
          spectra  used to service it.  This is particularly true with mul-
          tiaccess networks which require omnidirectional radiators.

                  Radio propagation models are somewhat involved; the  more
          exacting  models have been implemented as computer simulations by
          researchers.  However, even a cursory analysis reveals that spec-
          trum reuse is much more practical at higher frequencies.  In par-
          ticular, path loss increases as the square of the  frequency,  as
          does  antenna  gain  (which  results  from a narrower beamwidth).
          Wave polarity separation also increases accordingly.  In general,
          it  should  be  possible  to  model  the multiaccess - monoaccess
          tradeoff, using the available computer tools.

                  As an example, consider the CSMA network  mentioned  ear-
          lier.   The  farthest  node  is at a distance R from the hub.  In
          order to preclude the "hidden station" problem, stations  on  the
          circle  described  by  R must have enough power for range 2R.  In
          the limit, as the number of stations grows, the area  covered  by
          the  radio network becomes four times as large as the area of the
          physical network.  The monoaccess dual is no larger than physical
          network  area  at some Critical Frequency, and can indeed be con-
          siderably smaller.


























          Towards a Spectrum Efficiency Quotient

                  Clearly, a combination  of  three  separation  techniques
          (spatial,  spectral,  and polar) can yield a spectrally efficient
          monoaccess network at higher frequencies.  At lower  frequencies,
          however, the multiaccess model predominates.

                  The term "spectrally efficient" has been used to describe
          multiaccess  networks,  without specificity.  What is needed is a
          "figure of merit" to describe a radio  network,  and  compare  it
          with  other  alternatives.   Propagation  characteristics  of the
          spectrum below one GigaHertz lend themselves to applications  re-
          quiring  a high degree of mobility and portability.  For fixed or
          semiportable operation, however, a monoaccess network provides  a
          spectrally  efficient alternative, when operated above the Criti-
          cal Frequency.

          Summary

                  The spectral efficiency  of  monoaccess  and  multiaccess
          networks  varies  with the frequency used.  The exact calculation
          of the Critical Frequency of the tradeoff is currently  the  sub-
          ject of research.  However, in general, multiaccess networks tend
          to be more spectrally efficient below one GigaHertz, and  monoac-
          cess networks predominate above.

          Implications for the Amateur Service

                  Coordination between different types of services  in  the
          Amateur Service at frequencies above 30 MHz has been accomplished
          fairly haphazardly and ad hoc.  With the advent of packet  radio,
          it  has  been difficult in major metropolitan areas to coordinate
          use of spectrum.  Repeater links have been  traditionally  placed
          in  bands  close  to  repeaters,  because  of the availability of
          equipment, and economy.

                  Ultimately, some changes need to be made in bandplans for
          the  Amateur Service.  In particular, it is recommended that sta-
          tions in Auxiliary Service (as defined in Part 97.86)  should  be
          relocated  to frequencies above one GigaHertz.  Terrestrial digi-
          tal links, used to interconnect multiaccess networks, should also
          be  placed in the microwave region.  In turn, multiaccess digital
          networks should be placed in the Amateur VHF and UHF allocations.



          References

          Wozencraft and Jacobs, _P_r_i_n_c_i_p_l_e_s _o_f _C_o_m_m_u_n_i_c_a_t_i_o_n_s  _E_n_g_i_n_e_e_r_i_n_g,
          1965, John Wiley and Sons, New York.  ISBN 0-471-96240-6

          William Stallings, _D_a_t_a _A_n_d _C_o_m_p_u_t_e_r _C_o_m_m_u_n_i_c_a_t_i_o_n_s,  1985,  Mac-
          millan Publishing, New York.  ISBN 0-02-415440-7