Lecture 2 -- Wireless Communication

 

Text section 3.1-3.4

 

Cellular Systems

 

 

1. “Cell” and frequency reuse

 

Hexagonal Design…FIGURE 3.1

            Assume: omni at center, 1-5 mi radius, 100 W max BS, MS 600 mW, 20-30 users per channel.

 

System-level analysis of number of required subscribers in an area, how many frequencies will be needed (when and where) and how to meet that need with the most cost-effective solution.

 

S = # of available duplex (2-way) channels in a cluster

K = # of radio channels available

            B = total bandwidth (70 MHz GSM)

            CB = channel bandwidth (200 kHz simplex, 400 kHz duplex)

            K = B/CB = 70e6/200e3=350

N = cluster size = # of disjoint channel groups (# of cells before you encounter freq reuse)             4,7,12 are typical

S = kN = 350*4 = 1400 channels available in a cluster of 4 cells.

 

FIGURE 3.2

 

Cluster:  group of all cells that have different frequencies  (N cells in a cluster)

            N = I2 + IJ + J2

(1)   Move I cells to right

(2)   Turn 60 degrees counter-clockwise

(3)   Move J cells.

C = # of channels in a system = S*M

M = # of clusters in the system (number of times the disjoint groups are replicated).

 

Channel Assignment can be fixed (simple) or dynamic (fewer blockages)

 

 

2.  Handoff

           

Between basestations AND between MSCs.

If don’t make the transition… “DROPPED” call (verrrrry bad… user’s hate losing a call in the middle).

 

RSSI – Received signal strength indicator used to determine relative strength of call at different base stations.  Typically –90 to –100 dBm needed for good voice reception.

           

            ** -90 dB = 10-9 vs -90 dBm = 10-12 W!

            -90 dB = 10(-90/10) vs. -90 dBm = 10(-90/10)  milliWatts = 1 picoWatt

 

Typically

            AMPS – handoff if power too low ( < 10-12 dB above threshold) for 10 seconds

“Hard handoff” – must change frequency, channel at handoff.

Brief period when mobile is not connected to either (likely time to be dropped).

            GSM – handoff if power too low for 1-2 seconds

                        Handoff when RSSI is 0 to 6dB above threshold

CDMA -- “Soft handoff” – CDMA systems keep same frequency range, so there isn’t really a “handoff”… instead a different base station handles the call management tasks.  Multiple signals are received at different base stations, and the mobile or MSC can decide which is the best copy of the signal.

 

What can go wrong?

            Quick fluxuation in signal strength caused by nearby car/building/multipath/etc. getting in the way.

User moving too quickly, and call is lost before handed off

New cell is too busy.

“Cell Dragging” > Slow moving mobiles (pedestrians) may have very slow decay in RSSI, and may stay with one base station until it has moved deep within another cell.  This can result in channel interference, and is certainly non-optimal.

 

 

Methods:

(1)   Guard Channel:  Hold a few channels free just for handoff requests

(2)   Umbrella cell:  Use one cell tower with high antennas (for large umbrella cells) and lower antennas (for smaller cells under this umbrella).  Use the umbrella cells for fast-moving (cars) mobiles and smaller cells for slow-moving (pedestrians) mobiles.

 

 

3.  Interference

 

Co-channel interference intro

·         “co-channels” are nearby channels with the same frequency:

o       Voice Channels:  Loss of quality

o       Control Channels: Dropped calls

·         Increasing SNR does NOT solve co-channel interference (in fact, it can make it worse)

·         Reduce co-channel interference by increasing distance between co-channels

o       R = radius of each hexagonal cell

o       D = distance between centers of cells with co-channel interference

o       Q = co-channel reuse ratio = D/R = sqrt(3N) for hexagonal cells

§         Small Q increases system capacity (N is small)

§         Small Q increases co-channel interference (less distance between cells)

Do problem 3.1 to prove the ratio D/R = sqrt(3N)

 

Adjacent channel interference:

Channels that are adjacent in frequency are supposed to be unable to interfere with each other.  In practice, electronics are imperfect, and adjacent channels may have sidebands that interfere.   This is why FCC regulates the “out of band” noise that communication transmitters can have.  It is also why engineers design “tight” input filters so that their systems do not pick up out of band noise.

 

Near-Far Effect

If a nearby transmitter has just a little bit of out-of-band noise, it might swamp out the desired signal transmitted by a transmitter far away.  This near-far problem is reduced by controlling the power level that is transmitted by the mobiles to keep everyone on as close to the same power level as possible at the receiver base station.  This means that antennas far away must transmit larger power than those nearby. This saves on battery life, as well as reducing adjacent channel interference.

 

Interesting note:  When you are using your cell phone inside your car, it is partially blocked by the metal car structure.  It must send much higher power levels in order to get the power to the base station.  This is the “worst-case” scenario for power deposition in the head and for interference and lack of adequate power to the base station.

 

 

Signal-to-interference ratio

 

SIR or SINR is Similar to SNR.

Redraw BER curve.

 

S = Signal Strength (power)

I = co-channel interference strength (power)

Ii = power of co-channel interference from ith cell

 

To find total interference, sum up interference power from all cells:

 

Typically S/I must be 15-18 dB for good reception.

 

Signal strength dependence on distance

 

Pr = Power received

Po  = Power at a nearby point (do) in the far field of the transmitter

do =  near distance in the far field of the transmitter

d = far away distance (also in the far field of the transmitter)

 

n = path loss exponent, depends on environment

 

Table 3.2 says n ranges from 1.6 to 6.

 

Converting this to the form above:

S = Po = worst-case signal is measured at the outer edge of the cell

do  = distance to where S is measured = R

Ii = Pr =Power of interference from the ith cell (received power from co-channel cells is not desired, and is therefore interference)

d = distance to the ith cell Di

 

Substituting into the S/I equation:

 

 

The cells that are the farthest away have much less interference.  For nearest-neighbors only (io = 1)

 

 

 

What is the optimal value of N for omni-directional antennas?

 

Assume path loss = 4, TDMA/FDMA can tolerate S/I = 15 dB

 

Work Example 3.2

Have student re-work for N = 4.  (S/I = 13.8 dB)  Conclude that N large helps against co-chan interference.

Have student re-work for n = 3.  (S/I = 12.1 dB)  Conclude that n large helps against co-chan intereference.

 

 

 

 

Equation

Variable

 

 

 

 

 

cluster size

 

N

4

(choices 4,7,12)

 

path loss exponent (meas)

 

n

4

 

3.4

co-channel reuse ratio

Q

sqrt(3N)

3.464102

 

 

distance between co-channels

 

D

 

meter

 

radius of cells

 

R

 

meter

3.4

Ratio of distance to radius

Q

D/R

3.464102

 

 

number of neighboring cells

io

 

6

# of sides of hexagon

3.9

signal to interference ratio

S/I

(D/R)^n / io

24

 

 

convert to dB

S/I

10log(S/I)

13.80211

dB

 

 

 

 

 

 

 

If S/I is greater than required, it will work:  NO!

 

 

 

 

Equation

Variable

 

 

 

 

 

cluster size

 

N

7

(choices 4,7,12)

 

path loss exponent (meas)

 

n

3

 

3.4

co-channel reuse ratio

Q

sqrt(3N)

4.582576

 

 

distance between co-channels

 

D

 

meter

 

radius of cells

 

R

 

meter

3.4

Ratio of distance to radius

Q

D/R

4.582576

 

 

number of neighboring cells

io

 

6

# of sides of hexagon

3.9

signal to interference ratio

S/I

(D/R)^n / io

16.03901

 

 

convert to dB

S/I

10log(S/I)

12.05178

dB

 

 

 

 

 

 

 

If S/I is greater than required, it will work:  NO!