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					PERFORMANCE & IMPLEMENTATION OF
    DFH IN LIMITED BANDWIDTH
        CELLULAR SYSTEMS
              --Zoran Kostic & Nelson Sollenberger

                      S. GANGADHAR REDDY
                              NEHA KHERA


                                                1
TO BE DISCUSSED:
 Comparisons of frequency hops
 DFH (Dynamic Frequency Hopping)
 Requirements for DFH
 DFH implementation via NARA
 Simulations
 Conclusion


                                   2
COMPARISIONS:
 Fixed channel assignment
 Random frequency hopping
 frequency diversity is achieved.
 Dynamic frequency hopping
  combination of dynamic channel
     assignment, traditional frequency hopping
     & interference avoidance techniques.



                                                 3
WHY TO USE DFH?
System operators may initially deploy
  EDGE in small portions of bandwidth

To deploy an all IP network
  infrastructure & to avoid interference
  he can use this.
Slip over of frequency from other BS


                                           4
REQUIREMENTS:
 rapid signal quality, path loss
     measurements
     how these are made?
 each mobile continuously measures the
 quality of all channels available in the system
 Measurements are communicated over air to
 the base station
 Base station periodically updates frequency
 hop patterns to optimize overall performance
 BS sends info to MS about frequency hop.
SIMILAR PROCESS RUNS IN THE UPLINK
                                                   5
significant signaling overhead
     how to reduce
 System might restrict hopping pattern to
 change only one hop at a time.
 System might predefine a set of hopping
 patterns and restrict changes to one of those
 hopping patterns.
 Synchronization b/t BS is required ?
 to ensure different frequency use



                                             6
DFH IMPLEMENTATION VIA N/W –
ASSISTED RESOURCE ALLOCATION
   The signaling overhead required to
   communicate the measurement results
   to base are reduced by NARA.
   It takes the advantage of frame
   synchronization on system level &
   provides functionality identical to that of
   measurement based DFH.

                                             7
  BS monitors user performance & is
  reassigned another resource if it
  degrades below a threshold.
SIGNALLING:
  Signaling over air
  Inter base signaling
  it is observed that extra signaling
  bandwidth to the bandwidth is around
  5%.
                                         8
    A STAGGERED-FRAME resource management
  can be used to avoid the problem of 2 near by BS
  simultaneously choosing same resource.

Base stations are frame and super frame
 synchronized.
Control reuse pattern is defined in time, according
 to which only one base station is allowed to
 change frequency hop patterns for the duration of
 one frame.
All base stations get an opportunity to modify
 frequency hop patterns of their mobiles once per
 super frame.
                                                 9
        SIMULATION RESULTS

The  primary goal of the simulation study is to
establish relative performance improvements of
dynamic frequency hopping when compared to
random frequency hopping and fixed channel
assignment.
 The  simulation is done on the basis of cumulative
distribution functions(CDFs) indicating the
achievable word error rates ( WERs) as a function of
channel condition,loading, total no. of frequencies
and other factors.                                  10
A. Comparison of Channel Assignment Techniques
for Fixed Performance Point:
        One curve shows the performance of a 12-carrier
system that uses fixed channel allocation for three users
per base station.
        Another curve illustrates the performance of
random frequency hopping for four users per base station
(loading of 33%).

       A system using dynamic frequency hopping can
serve as many as six users per base station(50% loading) .
                                                      11
Fig. 1. FH versus fixed channel assignment   12
B. Effects of Voice Activity:
  The implementation of discontinuous transmission
(DTX) to exploit voice activity results in direct
interference reduction, and thereby in capacity
improvement.


  The capacity of the DFH system for satisfactory
performance falls from six to four users, while the
capacity of the fixed assignment system falls from three
to two. In both cases, DFH offers 100% improvement in
capacity.
                                                      13
Fig. 2. Voice activity effects   14
C. Uplink Performance Versus Downlink Performance
   Two of the main factors that contribute to the
performance difference between the downlink and uplink
are:
1)Different geographical distribution and mobility of
interferers
2) Presence of receiver antenna diversity on the uplink.
     Strong base station interferers will be easy to avoid by
means of DFH, and thereby improve the performance on
the downlink. On the other hand, the antenna diversity
helps the uplink performance.
                                                           15
Fig. 3. Comparative performance of uplink and downlink for DFH as a
function of occupancy for uncorrelated Rayleigh fading                16
               CONCLUSION

DFH   brings about 100% increase in capacity over
fixed channel allocation and 50% increase in capacity
over random frequency hopping.
Although    the performance of various systems is
sensitive to the number of available carriers, voice
activity, and channel coherence bandwidth, the system
using DFH offers the greatest overall robustness.
DFH   performs equally well for uplink and downlink.

                                                    17
References:
   R.L.Pickholtz et al.,”Theory of Spread Spectrum
   Communications-A tutorial,”—May 1982
   X.Wang and Z.Kostic,”Analysis of frequency-hopped
   cellular systems with dynamic FH pattern
   adaptation”1998

   “Performance and Implementation of Dynamic
   frequency hopping in limited bandwidth cellular
   systems,” Zoran Kostic & Nelson Sollenberger, jan
   2002.

                                                       18
ANY QUERIES ?



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