2. Diversity - · PDF file5 2: Diversity Wireless Communication Systems Rayleigh Flat Fading...
Transcript of 2. Diversity - · PDF file5 2: Diversity Wireless Communication Systems Rayleigh Flat Fading...
2
2: Diversity
Wireless Communication Systems
Diversity In Wireless Radio
• Communication over a flat fading channel has poor performance due to significant probability that channel is in a deep fade.
•• PerformancePerformance is increased by providing more independent looksindependent looks at the signal paths that fade independently.
• Diversity can be provided across timetime, frequencyfrequency and spacespace (Relate this to the three spreadsthree spreads that we talked about in the channel modeling section)
• Our goal here is how to exploit the added diversity in an efficient manner.
3
2: Diversity
Wireless Communication Systems
Baseline: AWGN Channel
BPSK modulation: x = ± a
Error probability decays exponentially with SNR.
y x w= +
( )0
2
0
2/ 2
SNR
SNR =
eap Q Q
N
aN
⎡ ⎤= = ⋅⎢ ⎥
⎢ ⎥⎣ ⎦
Received Signal Transmitted Symbol Noise
5
2: Diversity
Wireless Communication Systems
Rayleigh Flat Fading Channel
BPSK: x = ± a Coherent detection.
Conditional on h,
Averaged over h,
at high SNR.
(0,1)y h x wh= ⋅ +
CN:
( )2 2 SNRQ h⋅
1 112 4
SNR1+SNR SNRep
⎛ ⎞= − ≈⎜ ⎟⎜ ⎟ ⋅⎝ ⎠
7
2: Diversity
Wireless Communication Systems
Conditional on h,
When error probability is very small.When error probability is large:
Typical error event is due to channel being in deep faderather than noise being large.
Typical Error Event
( )2 2 SNRQ h⋅
1/2 SNRh ?1/2 SNRh <
2
2
2
1 1
~ exp(1), . . ( )
SNR SNRe
xh
p P h
h i e f x e−
⎛ ⎞≈ < ≈⎜ ⎟⎝ ⎠
=
8
2: Diversity
Wireless Communication Systems
BPSK, QPSK and 4-PAM
• BPSK uses only the I-phase.The Q-phase is wasted.• QPSK delivers 2 bits per complex symbol.• To deliver the same 2 bits, 4-PAM requires 4 dB more transmit power.• QPSK exploits the available degrees of freedom in the channel better.
• A good communication scheme exploits all the available d.o.f. in the channel.
9
2: Diversity
Wireless Communication Systems
Time Diversity• Time diversity can be obtained by interleavinginterleaving and codingcoding over
symbols across different coherent timecoherent time periods.
Coding alone is not sufficient!. Not effective over slow fading channel
10
2: Diversity
Wireless Communication Systems
Example: GSM
• A frequency division duplex system with 25 MHz for the UL and 25MHz for the DL– Bands are divided into 200 KHz sub-channel– Each sub channel is shared by 8 users in a time division
fashion.
• • • 125 sub-channels • • •
25 MHz
200 KHz
TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7
8 users per sub-channel
4.615 ms
577 µs
11
2: Diversity
Wireless Communication Systems
Example: GSM ….
• Voice is divided into frames of length 20 ms – Each frame is compressed into 228 bits.– A rate ½ convolutional code (generator polynomials are D4+D3+1 and
D4+D3+D+1) is used to encode the data → 456 bits.• To achieve time diversity, the coded bits in two such 20 ms speech frame are
interleaved across 8 consecutive time slots assigned to a specific user (114 bits per slot)
– A delay of roughly 40 ms • Amount of time diversity limited by delay constraint and how fast channel
varies.• To get full diversity of 8, needs v > 30 km/hr at fc = 900Mhz.
13
2: Diversity
Wireless Communication Systems
Simplest Code: Repetition
After interleaving over L coherence time periods,
Repetition coding:Repetition coding: xl = x for all l
This is classic vector detection in white Gaussian noise.
1, ,l l l ly h x w l L= ⋅ + = K
1 2 1 2 1 2[ , , , ] , [ , , , ] , [ , , , ]T T TL L L
xy y y h h h w w w
= ⋅ +
= = =
y h wy h wL L L
14
2: Diversity
Wireless Communication Systems
Geometry
For BPSK: x = ± a
Is a sufficient statistic (match filtering).
Reduces to scalar detection problem:
*
,A Ba a
y
= + = −
=
u h u h
h yh
%
y x w= ⋅ +h% %
15
2: Diversity
Wireless Communication Systems
Deep Fades Become Rarer
( )2 1!
LPL
ε ε< ≈h
2 1
1 1! L
SNR
SNR
ep P
L
⎛ ⎞≈ <⎜ ⎟⎝ ⎠
≈
h
17
2: Diversity
Wireless Communication Systems
Beyond Repetition Coding
• Repetition coding gets full diversity, but sends only one symbol every L symbol times.
• Does not exploit fully the degrees of freedom in the channel. (analogy: PAM vs QAM)
• How to do better?
18
2: Diversity
Wireless Communication Systems
Example: Rotation code (L=2)
where d1 and d2 are the distances between the codewords in the two directions.
x1, x2 are two BPSK symbols before rotation.
19
2: Diversity
Wireless Communication Systems
Rotation vs Repetition Coding
Rotation code uses the degrees of freedom better!
20
2: Diversity
Wireless Communication Systems
Product Distance
product distanceChoose the rotation angle to maximize the worst-case product distance to all the other codewords:
21
2: Diversity
Wireless Communication Systems
Antenna Diversity
Receive Transmit Both
Multiple antennas are used for transmission and or reception of the signal.Key parameter here is the separation between the antennas (coherencecoherencedistancedistance).
22
2: Diversity
Wireless Communication Systems
Receive Diversity
• Use a number of receive antennas that are well separate (> coherence distancecoherence distance ) to generate independent receptions of the transmitted signal
• Same as repetition coding in time diversity, except that there is a further power gain.
• Optimal reception is via match filtering ( also known optimal optimal beamformingbeamforming).
x= ⋅ +y h w
23
2: Diversity
Wireless Communication Systems
Receive Diversity ….
•• Selection Diversity:Selection Diversity: choose received signal with the largest received power, SNR, etc.
•• Switched Diversity:Switched Diversity: choose an alternate receive antenna if the signal level falls below a certain threshold.
•• Linear Combining:Linear Combining: linearly combine a weighted copy of all received siganls
• There is a dramatic improvement even with just two branchs
24
2: Diversity
Wireless Communication Systems
DataDetector 1
DataDetector 1
Rx: Selection and Switched Diversity
FrontEnd
FrontEnd
SelectorData
FrontEnd
FrontEnd
DataDetector
DataSwitch
SelectionDiversity
SwitchedDiversity
25
2: Diversity
Wireless Communication Systems
Rx Diversity: Linear Combining
FrontEnd
FrontEnd
∑DataData
Detector
y1
α2
α1
y2
1
2
1 1 1 1 1
2 2 2 2 2
j
j
y h a n h A e
y h a n h A e
φ
φ
= ⋅ + = ⋅
= ⋅ + = ⋅
1 2
1 2
1 2
1 2
1 1 2 22
1 2 , 1 1 2 2
1 1 2 2
,,
( , ) arg min | |
( )
Equal Gain:Maximal Ratio:
MMSE Training:Decoding:
j j
j j
e eA e A e
y y a
a y y
φ φ
φ φ
α α
α αα αα α α α
α α
− −
− −
= == ⋅ = ⋅
= + −
= +S
26
2: Diversity
Wireless Communication Systems
Transmit Diversity
• Provide a diversity benefit to a receiver without having multiple receive antennas by using multiple transmit antennasmultiple transmit antennas.
• If the transmitter know the channel then transmit x = x. h /||h|| . This will maximizes the received SNRmaximizes the received SNR by in-phase addition of signals at the receiver (transmit beamforming).
• Reduces to scalar channel: y = ||h|| x + w, same as Rx beamforming.• What happens if transmitter does not have explicit knowledge of the
channel? Two kind of transmit diversity techniques: – Transmit diversity with feedback from the receiver– Transmit diversity without feedback from the receiver
Blind, i.e. no trainingWith feedforward information, i.e. with training.
•• Note:Note: transmitting the same symbol from all antennas does not work! Why?.
*y = ⋅ +h x w
27
2: Diversity
Wireless Communication Systems
Transmit Diversity with Feedback
• w1 and w2 are varied such that |y(t)|2 is maximizedmaximized.• w1 and w2 are adapted based on feedback information from the
receiver.
Mod
w1
w2
S1(t)
S2(t)
Tx 1
Tx 2 y(t)Demod
y1(t)
y2(t)
y(t) y1(t)
y2(t)
2 21 2 1w w+ =
1 2,variations
w w
28
2: Diversity
Wireless Communication Systems
Transmit Diversity with Frequency Weighting
• Use frequency weighting to mitigate the harm of scenario B.• Simulate fast fadingfast fading → can use conventional channel coding and channel coding and
intereleavingintereleaving techniques.• Suitable for slow fading or staticslow fading or static channels.
ChannelEncoder
ejθ(t)
S1(t)
S2(t)
Tx 1
Tx 2 y(t) ChannelDecoder
y1(t)
y2(t)
( ) 2 mkT f kTθ π=
Rx
y1(t)y2(t)
(A) Constructive Interference
(B) Destructive Interference
29
2: Diversity
Wireless Communication Systems
Tx Diversity with Antenna Hopping
• At time i, 1≤ i ≤ N, transmit s from antenna i.• Achieves a diversity orderdiversity order of N.•• Bandwidth efficiencyBandwidth efficiency is 1/N.
RepetitionCode
Tx 1
MLDetection
Rx
s 1 2, ,..., Ns s s
Tx 2
Tx N
T
30
2: Diversity
Wireless Communication Systems
Tx Diversity with Channel coding
• The channel code has a minimum Hamming distanceHamming distance dmin≤ N.• At time i, transmit code symbol ci from antenna i.• After receiving the N code symbols, the receiver performance ML
decoding to decode the received code word.
Channel Code
Tx 1
MLDetection
Rx
1 2, ,..., ks s s 1 2, ,..., Nc c c
Tx 2
Tx N
T
31
2: Diversity
Wireless Communication Systems
Tx Diversity via Delay Diversity
• Provides a diversity benefit by introducing intentional multipath (simulating a multipath channel).
• Receiver uses an equalizer or MLSEequalizer or MLSE for detection.• Provides diversity order of N, no loss in BW efficiency• Provides very little diversity benefit, if the channel if multipath to begin with.
Tx 1
Equalizer
Rx
( )s tTx 2
Tx N
DelayT
Delay(N-1)T
( )s t
( )s t T−
( ( 1) )s t N T− −
32
2: Diversity
Wireless Communication Systems
Space-time Codes
• Space-time codes are designed specifically for the transmit diversity scenario.
• For each input symbol, the space-time encoder chooses the constellation points to simultaneouslysimultaneously transmit from each antenna so that codingcoding and diversitydiversity gains are maximizedmaximized.
•• No cheating!No cheating! . Total transmitted powertransmitted power is still the samesame as single transmit antennas case.
• Both flavors: trellistrellis codes & blockblock codes.
‘Space-Time Codeword’
QAMSymbol
Multi-Element Receiver
TxEn
code
r...
.
.
.
.
.
1
NT
A
BC
MIMOChannel Matrix, H
t0 t1 t2
Rx
Dec
oder
.
.
.
1
NR
A’
B’C’
.
.
.
.
.
TxA
nten
na
Space-TimeSymbol
Multi-Element Transmitter
‘Space-Time Codeword’
-
TxEn
code
r...
.
.
.
.
.
.
.
.
1
T
MIMOChannel Matrix, H
MIMOChannel Matrix, H
t0 1 t2
Rx
Dec
oder
.
.
.
.
.
.
R
’
’’
.
.
.
.
.
--
2: Diversity
Wireless Communication Systems
Space-Time Block Codes: Alamouti Scheme
•• Idea:Idea:
•• Assumption:Assumption: channel is quasiquasi--staticstatic.
c cc cc c1 2
1 2
2 1
→−L
NMOQP
∗
∗
Burst 1
Burst 2
ConstellationMapperInformation
Source
ST Block Code
[ ]*
1 21 2 *
2 1
c cc c
c c⎡ ⎤−
→ ⎢ ⎥⎣ ⎦
timetime
Ant
enna
34
2: Diversity
Wireless Communication Systems
Decoding of STBC
• Received Signal:
• H is orthogonal:orthogonal:
– Projecting onto the two columns of the H matrix yields:
r h c h c n
r h c h c n1 1 1 2 2 1
2 1 2 2 1 2
= + +
= − + +* *
r H c n=LNM
OQP= −
LNM
OQPLNM
OQP+
LNM
OQP= ⋅ +
rr
h hh h
cc
nn
1
2
1 2
2 1
1
2
1
2* * * *
H H I h I* h h12
22 2e j
~ ~*r H r h c n2
35
2: Diversity
Wireless Communication Systems
Decoding of STBC ….
• The noise term is still whitewhite c1 and c2 are detected independentlyindependentlylower complexitylower complexity.
• Double the symbol rate of repetition coding
• 3dB loss of received SNR compared to transmit beamforming.
• With M receive antennas:
• With M receive antennas, a diversity order of 2M is achieved.• Only simple linear processingsimple linear processing at the receiver is required.• Complete CSICSI is required at the receiver. In practice channel channel
estimationestimation is used to obtain CSI.
~ ~*r H r h c nFHG
IKJi i
i
M
ii
M
1
2
1
~n
2: Diversity
Wireless Communication Systems
Decoding of STBC ….
Linear Combiner
soft decision for c2
soft decision for c1
c1
ML Decision
c2
ML Decision
H2
H1
r1 r1 c1
c2r2r2
~ * *r H Hrr
= M P1 21
2
37
2: Diversity
Wireless Communication Systems
Space-time Code Design
A space-time code is a set of matrices
Full diversity is achieved if all pairwise differences have full rank.
Coding gain determined by the determinants of
Time-diversity codes have diagonal matrices and the determinant reduces to squared product distances.
38
2: Diversity
Wireless Communication Systems
Frequency Diversity
•• Resolution of Resolution of multipathsmultipaths provides diversity.• Full diversity is achieved by sending one symbol every
L symbol times.– Sounds like repetition coding →this is inefficient
• Sending symbols more frequently may result in intersymbol interference (ISI).
• Challenge is how to mitigate the ISI while extracting the inherent diversity in the frequency-selective channel.
1
0[ ] [ ] [ ]
L
ll
y m h x m l w m−
=
= − +∑
39
2: Diversity
Wireless Communication Systems
Approaches
• Time-domain equalization (eg. GSM)
• Direct-sequence spread spectrum (eg. IS-95 CDMA)
• Orthogonal frequency-division multiplexing OFDM (eg. IEEE 802.11a/g, Flash-OFDM, IEEE 802.16, IEEE 802.20)
40
2: Diversity
Wireless Communication Systems
ISI Equalization
• Suppose a sequence of uncoded symbols are transmitted. Can full diversity be achieved?
• Answer is YES!YES!• Maximum likelihood sequence detection (MLSD) is the
optimal solution:– Performed using the Viterbi algorithm.– Complexity might be quite high
• Other suboptimal equalization techniques:– Linear equalizers: zero-forcing and MMSE– Decision feedback equalizers (DFE)
41
2: Diversity
Wireless Communication Systems
MLSD: Reduction to Transmit Diversity
0[1] [1]y x h= ⋅
0 1[2] [2] [1]y x h x h= ⋅ + ⋅
0 1 2[3] [3] [2] [3]y x h x h x h= ⋅ + ⋅ + ⋅
0 1 2[4] [4] [3] [2]y x h x h x h= ⋅ + ⋅ + ⋅
42
2: Diversity
Wireless Communication Systems
MLSD: Reduction to Transmit Diversity …
• Consider the transmitted sequence
[ ][ ] [ ]0 1
[1], [2], , [ 1]
, , , , [1], [2], , [ 1]
[1] [2] [ ] [ 1]0 [1] [2] [ ] [ 2]0 0 [1] [2]
0 0 [1] [2] [ ]
T T T
T
T TL
y y y N L
h h h w w w N L
x x x N x N Lx x x N x N L
x x
x x x N
= ⋅ +
= + −
= = + −
⋅ ⋅ ⋅ ⋅ ⋅ + −⎡ ⎤⎢ ⎥⋅ ⋅ ⋅ + −⎢ ⎥⎢ ⎥= ⋅ ⋅ ⋅ ⋅ ⋅⎢ ⎥⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅⎢ ⎥⎢ ⎥⋅ ⋅ ⋅ ⋅⎣ ⎦
y h X wy
h w
X
L
L L
[ ][1], [2], , [ 1]T x x x N L= + −x L
43
2: Diversity
Wireless Communication Systems
MLSD: Reduction to Transmit Diversity …• Maximum Likelihood detection of the sequence x based on the
received sequence y (MLSD). The probability of confusing xA with xB , when xA is transmitted is:
• The probability of error decays like SNR whenever the differenceXA-XB is of rank L (i.e. full rank).
• Can show that is XA-XB indeed of full rank (see details in text). Hence:
{ } ( )( )* *A B A B
A B
21 l
SNRPr
2
11 SNR / 4
L
l
E Q
λ=
⎧ ⎫⎛ ⎞⋅ − −⎪ ⎪⎜ ⎟→ = ⎨ ⎬⎜ ⎟⎪ ⎪⎝ ⎠⎩ ⎭
≤+ ⋅∏
h X X X X hx x
MLSD achieves full diversity on symbol x[N] using the observations up to time N+L-1, i.e. a delay of L-1 symbol times.
44
2: Diversity
Wireless Communication Systems
Direct Sequence Spread Spectrum
• Data Rate R is than the transmission bandwidth W (also known as chip rate) .
Processing Gain: Processing Gain: n n == W/RW/R• Signal-to-noise ratio per chip is low.• Example: IS-95 CDMA system (used by Verizon, Sprint):
ChannelEncoder
MP channel
Pseudo RandomPattern Generator
Pseudo RandomPattern Generator
ChannelDecoder
Modulator DemodulatorInformationbits
Outputbits
W = 1.288 MHz, R=9.6 kbits/sec, G =128
45
2: Diversity
Wireless Communication Systems
Direct Sequence Spread Spectrum ….
• Symbol duration Tb >> delay spread Td → ISI is not a ISI is not a problemproblem (as compared to interference from other users).
• Channel is constant over one symbol. i.e. symbol duration Tb=n/W << coherence time Tc.
[ ]s m
[ ]u i
Data
PNSequence
1τ 2τ 3τ 4τ
ChannelIR
[ ]h m
[ ]0h m[ ]1h m
[ ]2h m [ ]3h m
1sT
W=
bTSymbol duration
Chip duration
dTDelay Spread
46
2: Diversity
Wireless Communication Systems
Direct Sequence Spread Spectrum ….
• PN Sequences are chosen such that shifted versions of the same sequence are nearly orthogonal, i.e. :
( ) ( ) ( ) ( )* * 2
1
, , [ ]n
l k l l
i
u i l k=
= ≠∑u u u u=
[ ]uR m
m
n
1−
PN Sequence Autocorrelation
47
2: Diversity
Wireless Communication Systems
Direct Sequence Spread Spectrum ….
Σi=1
n
( )*0u
*0h
( )*1u
*1h
Σi=1
n
( )*Lu
*1Lh −
Σi=1
n
Σ Decision
correlator
correlator
correlator
y
0y
1y
1Ly −
0,..., 1l l ly h s w l L= ⋅ ⋅ + = −u
RAKE Receiver
48
2: Diversity
Wireless Communication Systems
Direct Sequence Spread Spectrum ….
• Signal at the output of the correlators:• The orthogonality ul of implies that wl are i.id ΧΝ(0,N0) . • This looks exactly the same as the L-branch diversity model for the
repetition code interleaved over time that we have seen before. Hence, the RAKE receiver in this case is nothing more a ML ratio combinerML ratio combiner of the signals from the L multipath components (LL RAKE fingersRAKE fingers)
• Probability of error:
• Assume a Rayleigh fading model such that hl are i.i.d ΧΝ(0,1/L), i.e. the energy is split among the L taps (and normalizing such that E{||hl||2}=1). The SNR per branch is SNR= ||u||2/(N0L)=(1/L). Es /N0
• As L→∞, ||hl||2 → 1 and the probability of error becomes:
0,..., 1l l ly h s w l L= ⋅ ⋅ + = −u
12 2
00
2 /L
e ll
p E Q h N−
=
⎧ ⎫⎛ ⎞⎪ ⎪= ⎜ ⎟⎨ ⎬⎜ ⎟⎪ ⎪⎝ ⎠⎩ ⎭∑u
( )02 /e sp Q E N→
i.e. the performance of AWGN with the same i.e. the performance of AWGN with the same EEss /N/N00 is asymptotically achievedis asymptotically achieved
49
2: Diversity
Wireless Communication Systems
ISI and Frequency Diversity
• In narrowband systems, ISI is mitigated using a complex receiver.
• In asynchronous CDMA uplink, ISI is there but small (compared to interference from other users).
• But ISI is not intrinsic in a channel with frequency diversity.
• The transmitter needs to do some work too!
50
2: Diversity
Wireless Communication Systems
OFDM: Basic Concept
• Most wireless channels are underunder--spreadspread (Td << Tc) .• Can be approximated by a linear time invariantlinear time invariant channel
over a long time scale.• Complex sinusoidssinusoids are the only eigenfunctions of linear
time-invariant channels.• Should signal in the frequency domainfrequency domain and then transform
to the time domain.
Frequency f
( )H f
1f 2f 3f 4f
51
2: Diversity
Wireless Communication Systems
OFDM Modulation ….1
0
, , ,
1
1
1
1 1 1
1
1
( ) ( ) ( - ) ( )
0 00 00 00 0
0
0 0
L
l
k k k
o L
o L L
k o L
L o L
L
N N N N N
o
y i h l x i l w i
h h hh h h
h h hh h h
h h h
+
=
−
×
−
× × ×
= +
= ⋅ +
⎡ ⎤⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥= ⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎣ ⎦
∑y H X n
H
L LO L
O O O O OL L
L L LM O O O O O M
L L
• The channel Matrix H is circulantcirculant, hence it will have the eigenvaluedecomposition Hk=Q*ΛkQ.
• FFT of received signal*
k k k
k k k k
⋅ = ⋅ ⋅ + ⋅
= ⋅ +
Q y Q Q Λ Q X Q wY Λ S N
IFFT P/S CyclicPrefixS/P
( )ks i
kS kX
( )kx i ( )kx i%
52
2: Diversity
Wireless Communication Systems
OFDM Modulation ….
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 01
2
3
4
5
6
7
8
9
's sT T Tν= +
TνOFDM symbol
Cyclic prefix
Data Symbols
'1 1/ sf T=
'2 2 / sf T=
'/N sf N T=
'sT
55
2: Diversity
Wireless Communication Systems
OFDM Modulation ….
OFDM transforms the communication problem into the frequency domain:
a bunch of nonnon--interfering interfering (parallel)(parallel) sub-channels, one for each sub-carrier.
Can apply time-diversity techniques.
0,..., 1i i i iy h s w i N= ⋅ + = −%
( ) 0,..., 1ii Wh H H i f i NN⋅⎛ ⎞= = ⋅Δ = −⎜ ⎟
⎝ ⎠%
56
2: Diversity
Wireless Communication Systems
Cyclic Prefix Overhead
• OFDM overhead = length of cyclic prefix / OFDM symbol time
• Cyclic prefix dictated by delay spreaddelay spread.• OFDM symbol time limited by channel coherence timecoherence time.• Equivalently, the inter-carrier spacing should be much
larger than the Doppler spread.• Since most channels are underspread, the overhead can
be made small.
57
2: Diversity
Wireless Communication Systems
Diversity Summary
• Fading makes wireless channels unreliable.• Diversity increases reliability and makes the channel
more consistent.• Smart codes yields a coding gain in addition to the
diversity gain.• Different diversity schemes for different channel
conditions:–– Time Diversity:Time Diversity: exploit the diversity inherent in the
time varying nature of the channel–– Frequency Diversity:Frequency Diversity: exploit the multipath
(frequency) in the channel–– Space Diversity:Space Diversity: exploits the spatial variation in the
channel
58
2: Diversity
Wireless Communication Systems
The Big Picture So Far
• In wireless communications, information are sent over wireless channels
• Impairments of Wireless channels:
–– Path Loss and Shadowing:Path Loss and Shadowing:
Link budget and cell planningLink budget and cell planning
–– MultipathMultipath Fast Fading:Fast Fading:
Use diversity (time, frequency, space) techniquesUse diversity (time, frequency, space) techniques.