## JPL's Wireless Communication Reference Website## Chapter: Analog and Digital Transmission Section: Spread Spectrum , Frequency Hopping |

WISSCE (Wireless Indoor Spread Spectrum Communication Equipment) is an ad-hoc (nomadic) communication system meant to operate in the 2.4 GHz ISM-band providing random/multi-access wireless communication links. It is an example of a system exploiting the hybrid Direct Sequence / Frequency Hopping spread spectrum technique. This page covers the design considerations that are related to the this CDMA technique. WISSCE was a research project at T.U. Delft in The Netherlands.

Frequency regulation and WISSCE's full-duplex operation result in a
total available bandwidth of 10 MHz, a number that can be considered
as a boundary condition. Choices that will be motivated in this
section include: selecting the hopping rate (relative to the symbol
rate), the pn-code and FH sequence length (*N*_{DS} and *N*_{FH}) and
choosing a modulation format.

It is obvious that the user desires a high transmission capacity (overall data speed). However, an high data speed also leads to conflicts with user demands like ``low purchasing costs'', ``short access time'', ``meeting legal requirements'' and others. As a result a compromise should be found.

The relation between occupied bandwidth, spreading factor and data speed can be written as:

where *G*_{p} is the direct sequence spreading factor.
d_{MOD} expresses the extra
bandwidth usage due to the applied modulation type which is usually
small in comparison with the other bandwidth term. A plot
illustrating this trade-off is shown in figure 1.

The bit-rate (*r*_{d})can be expressed in terms of
the symbol-rate (*r*_{symb}):

From these formulas follows that the data speed can be increased independently of the occupied bandwidth by increasing the number of modulation levels (= number of bits per symbol). However, increasing this number of bits per symbol also increases both the implementation cost and the bit error probability.

Summarizing: increasing the data speed leads to either a larger bandwidth occupancy or higher hardware costs and a worse bit error rate.

Figure 2 shows the trade-off between data speed and the number of modulation levels. This plot makes clear that it is advantageous to have a rather small number of modulation levels: the data speed increases logarithmic with the number of levels while the data detection complexity increases linearly. Appropriate choices are 8, 16 or 32.

Secondly we focus on the trade-offs related to the hybrid DS/FH CDMA technique . Fast Frequency Hopping not only increases the cost of the frequency synthesizer, it also complicates data detection algorithm as an ``hop'' introduces a transient response that the data detector has to cope with. For this reasons we propose an hybrid system approach in which the ``hop rate'' is equal to the ``symbol rate''. Every data-symbol is spread with a complete pn-code while successive symbols are transmitted in different FH-channels as illustrated in figure 3. The actual spreading gain in such a system is equal to pn-code sequence length, while Frequency Hopping only provides protection against the Near-Far effect.

Following this concept, a user-address consists of a frequency-hopping
pattern of length *N*_{FH} and
*N*_{FH} (possibly different) pn-codes of length
*N*_{DS}. The start of a new set of pn-codes and
an FH-sequence are linked. This is important when regarding the
code-synchronization problem. An example user-address existing of an
FH-sequence of length 7 and 7 pn-codes is shown in figure 3. Every data-symbol is combined with
a pn-code causing the direct-sequence spreading. Subsequent
data-symbols are transmitted in different FH-channels according to a
certain sequence to perform FH-spreading. As a result we use neither
fast nor slow frequency-hopping. A frequency-synthesizer only has to
``hop'' at a speed equal to the symbol-rate.

For the relation between *N*_{DS}, *N*_{FH}, the symbol-rate and the occupied
bandwidth holds:

For constant *BW*_{total}/*r*_{symb} and a small d_{MOD}, the trade-off curves shown in figure
4 can be obtained for three values of
*BW*_{total}/*r*_{symb}. Concerning this trade-off the following
remarks can be made:

- It is likely that the DS-code will be made using
shift-registers (so-called Linear Feedback Shift Register
sequences
[Gol67]). The code-length is
then 2
^{n}-1 with*n*being an integer. For this reason, only the marked points in figure 4 are possible trade-off points. - Increasing
*N*_{DS}is advantageous (increasing of spreading gain) and can be realized at low implementation costs. - Increasing
*N*_{FH}results in the requirement for a more expensive FH-frequency-synthesizer and is for this reason undesired. A minimum number of FH-channels is however required to limit the influence of the near-far effect (near-interference).

To find a minimum value for *N*_{FH}, user demands
like ``reliable operation'' and ``large user capacity'' are translated
into a technical compromise that maximally 2 near users may be
present. As the frequency-hopping
sequences are chosen in such a way that they have at most 2
partial hits with another sequence [Gla96, p.63] [MT92], this compromise translates into
the worst case possibility of having at most 4 partial hits per
FH-sequence. Furthermore we need at least two frequency-hopping
channels without near-interference to allow proper synchronization [Gla96, p.86]. As a result the
minimum value of *N*_{FH} is 6, a vertical line
representing this value is also shown in figure 4.

After evaluating other trade-offs as well we will address the actual
choice of the values of *N*_{DS} and *N*_{FH}.

A well known disadvantage of FSK is that it requires a larger
bandwidth (higher value of
d_{MOD}) however this value will
still be small in comparison with the already occupied spread spectrum
bandwidth. Increasing the number of modulation levels is possible by
using more than two frequencies. The resulting modulation technique is
referred to as multiple frequency shift keying (MFSK). An additional
property of frequency modulation is the possibility of non-coherent
data detection. This saves hardware costs as a carrier tracking loop
is not required if frequency errors are small.

We will now address the actual parameter choices for WISSCE.

To obtain a communication system which is competitive in data speed
with currently available (wire line) modems, we select the gross
bit-rate to be 80 kbit/s at a BER of 10^{-3}. Depending on the
application (and the required BER) this will lead to different data
speeds after error correction.

To find the symbol-rate the number of bits per symbol has to be determined. A large number of bits per symbol leads to a more complex data detection circuit while it decreases the required bandwidth. By choosing the number of bits per symbol to be 4, the modulation format becomes 16-MFSK and the symbol-speed 20 ksymbol/s.

Evaluating the equation for the
total bandwidth now results in a spreading factor of about 500. This
number has to be shared between *N*_{DS} (length
of DS-code) and *N*_{FH} according to the formula that relates the total
bandwidth to these parameters. Increasing
*N*_{DS} is advantageous as it increases the spreading gain while
increasing *N*_{FH} leads to a higher reduction of
the near-far effect. This trade-off was shown in figure 4.

Two constraints exist: the minimum value of *N*_{FH} is 6 and for *N*_{DS}
only certain numbers are possible (2^{n}-1). From earlier discussions
followed that *N*_{DS} should be as high as possible while increasing *N*_{FH}
leads to higher implementation costs. From figure 4 we
conclude that a proper value for *N*_{DS} is 63, for the resulting value
of *N*_{FH} follows 7 and the chip-rate (*r*_{c}) is 1260 kHz (symbol rate
times *N*_{FH}).

The power density spectrum of a DS-spread signal has a
sync^{2} shape where the zero-values are the chip-rate apart.
For this reason the FH-spacing is fixed to be equal to the chip-rate
(D_{FH} = 1260 kHz). To obtain orthogonal frequency
shift keying, the frequency-distance between the MFSK-frequencies
should be an integer times the symbol-rate. To minimize
d_{MOD},
D_{FSK} is fixed to be 20 kHz.

**Gla96**-
Jack P.F. Glas.
*Non-Cellular Wireless Communication Systems*.

PhD thesis, Delft University of Technology, December 1996.

ISBN: 90-5326-024-2. **Gol67**-
S. W. Golomb.
*Shift Register Sequences*.

Holden-Day Inc., San Francisco, California, 1967. **MT92**-
Svetislav V. Maric and Edward L. Titlebaum.

A class of frequency hop codes with nearly ideal characteristics for use in multiple-access a spread-spectrum communications and radar and sonar systems.*IEEE Transactions on Communications*, COM-40(9):1442-1447, September 1992. **WIS**- WISSCE-team, Circuits and Systems group. WISSCE home-page.

- BER of a DS/FH CDMA System employing MFSK Modulation