Chapter6. Modulation Techniques for Mobile Radio To encode information from a message source

in a manner suitable for transmission. To translate a baseband message signal to a

bandpass signal at very high frequencies. Modulated signal (bandpass signal),

modulating signal (baseband message signal ). To vary the amplitude, phase, or frequency of

a high frequency carrier. Analog modulation (1G mobile radio systems) Digital modulation (current and future systems)

常用调制方式 常规双边带调幅（AM） 抑制载波双边带调幅（DSB） 单边带调幅（SSB）

线性调制

残留边带调幅（VSB） 频率调制（FM）

模

拟

调

制 非线性调制

相位调制（PM） 线性 相移键控（BPSK，DPSK，QPSK 等）

频移键控（BFSK） 非线性

最小频移键控（MSK） 多进制相移键控（MPSK） 多进制正交幅度调制（QAM）

数 字 调 制 线性和非线性联合

多进制频移键控（MFSK）

6.1 Analog modulation techniques FM: frequency is varied by modulating

message signals.

( ) ( )

( )

( )

cos 2 cos 2 2

cos 2

,

2 1 2

t

FM c c c c f

f

PM c c

f mm

T f m T

S A f t t A f t k m d

k

S A f t k m t

kk A f k AW W

Carson B f B f

θ

θ

θ

π θ π π η η

π

β β θ

β

∞

= + = +

= +

∆= = = = ∆

= + = ∆

∫－

f p

频偏常数（Hz/V）

相移常数（rad/V）

调频指数： 调相指数：

公式： （上边带）， （下边带）

FM detection: frequency discriminators.

AM: the amplitude of a high frequency carrier signals is varied with the instantaneous amplitude of the modulating message signal.

[ ] ( )2 2

1 ( ) cos 2 ,12 , 1 2 ( ) ( )2

AM c c

AM m AM c

S A m t f t

B f P A m t m t

π= +

= = + +

Demodulation of AM: non-coherent envelope detector and

coherent product detector.

FM vs. AM FM has better noise immunity. An FM signal is a constant envelop signal,

FM exhibits a capture effect characteristic. Constant envelop allows efficient Class C

power amplifiers for RF power amplifica. To require a wider frequency band to obtain

the advantages of reduced noise and capture effect. FM transmitter and receiver equipment is

also more complex.

6.2 Digital Modulation-An Overview A desirable modulation scheme provides

low BER at low received SIR, performs well in multipath and fading conditions, occupies a minimum of bandwidth, and is easy and cost-effective to implement.

The performance of a modulation scheme is often measured by power efficiency and bandw idth efficiency.

Power efficiency: ability to preserve the fidelity of the digital message at low power levels, ηp (Eb/N0).

Bandwidth efficiency: ability to accommodate data within a limited bandwidth, ηb=R/B bps/Hz.

The system capacity is directly to bandwidth efficiency of the modulation.

Shannon’s bound applies for AWGN .

max 2log 1BC SB N

η = = +

the power spectral density（PSD）is defined as

( ){ }

( ) ( )

2( )( ) lim

( ), -T/2 t T/2 ( ) ( ) ( )

0,

if bandpass signal is : ( ) Re ( )exp 2

1( )4

T

T

T T T

c

s g c g c

W fP f

T

w tW f w t w t

s t g t j f t

P t P f f P f f

ω

π

→∞

=

< <=

=

= − + +

是 的傅立叶变换,其它

The absolute bandwidth of a signal: has a non-zero power spectral density.

The null-to-null bandwidth: is equal to the width of main spectral lobe.

The half-power （3dB ） bandwidth: PSD has drop to half power, or 3dB below the peak value.

Another commonly used method to specify bandwidth is to state that everywhere outside the specified band, the PSD is below a certain stated level.（45dB to 60dB attenuation）.

6.3 Line coding Digital baseband signals often use line

codes to provide particular spectral characteristics of a pulse train. The most common codes for mobile

communication are RZ, NRZ, and Manchester codes. RZ codes: pulse returns to zero within

every bit period. This leads to spectral widening, but improves timing sync.

NRZ codes: do not return to zero during a bit period. They are more spectrally efficient than RZ codes, but offer poorer synchronization capability.

The Manchester codes is a special type of NRZ code that is ideally suited for signaling that must pass through phone line and other dc blocking circuits. Manchester codes use two pulses to represent each binary symbol and thereby provide easy clock recovery.

Unipolar NRZ

Bipolar RZ

Manchester NRZ

6.4 Pulse Shaping Techniques ISI: rectangular pulses are passed

through a band-limited channel, pulses will spread in time, and the pulse for each symbol will smear into the time intervals of succeeding symbols. To minimize ISI: increase the channel

bandwidth and reduce the modulation bandwidth. Out-of-band radiation in the adjacent

channel in a mobile radio system should generally be 40 dB to 80 dB below that in the desired pass-band.

Since it is difficult to directly manipulate the transmitter spectrum at RF frequencies, spectral shaping is done through base-band or IF processing.

There are a number of well know pulse shaping techniques which are used to simultaneously reduce the ISI and the spectral width of a modulated digital signal.

6.4.1 Nyquist Criterion for ISI Cancellation Nyquist was the first to solve the problem

of overcoming ISI while keeping the transmission bandwidth low. The effect of ISI could be completely

nullified if the overall response of the communication system is designed so that at every sampling instant at the receiver, the response due to all symbol except the current symbol is equal to zero.

0( )

0 0Nyquist condition for ISI cancellation

( ) ( )* ( )* ( )* ( )

( )is the pluse sharp of a symbol( )is the channel impluse response( )is the receiver implus

eff s

eff c r

c

r

K nh nT

n

h t t p t h t h tp th th t

δ

== ≠

=

整个通信系统的冲击响应：

e response

There are two important considerations in selecting Heff(f) which satisfy heff(nTs) .

(1)heff(t) should have a fast decay with a small magnitude near the sample values for n≠0.

(2)If hc(t)=δ(t), then it should be possible to realize or closely approximate shaping filters at both the transmitter and receiver to produce the desired Heff(f) .

If communication system can be modeled as a filter with the impulse response

the transfer function of the filter which

satisfies the zero ISI can be expressed as:

0

0

sin( ) ( ), ( ) ( )

1( ) (- ), ( ) 0 2

seff eff

s

tTfH f rect Z f h t z t

f t

Z f Z f Z f for f f T

π

π

= ⊗ =

= = ≥ ≥

sin 1( ) , ( ) completely eliminate ISIseff eff

s ss

tT fh t H f rectt f fT

π

π

= =

6.4.2 Raised Cosine Rolloff Filter

It is the most popular pulse shaping filter. ( )

( ) ( )

( )

11, 0

2

2 1 1 11( ) 1 cos , 2 2 2 2

10,

2

s

sRC

s s

s

fT

f TH f f

T T

fT

α

π α α αα

α

−≤ ≤

⋅ − + − += + ≤ ≤

+ >

( ) ( )( )2

sin cos( )

1 4 2

1 2 , 1 1

s sRC

S

s ss

t T t Th t

t t T

B BR RF passband RT

π παα

π α

α α

= ↑→ ↑ ↓ −

= = =+ +

，带宽，时间旁瓣

0.0

0.2

0.4

0.6

0.8

1.0

|HRC(f)|

0-R/2 -R R R/2

α = 1α = 0.5α = 0

频 率

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

4Ts3Ts2TsTs0-Ts-2Ts-3Ts-4Ts

hRC(t)

α = 0 α = 0.5 α = 1

时间

6.4.3 Gaussian Pulse-Shaping Filter（ISI≠0）

It is also possible to use non-Nyquist techniques for pulse shaping. Gaussian pulse-shaping filter is

particularly effective when used in conjunction with MSK modulation, or other modulations which are well suited for power efficient nonlinear amplifiers.

( )2 2

22

2

33

( ) exp

( ) exp

ln 2 0.58872

G

G

dBdB

H f f

h t t

BB

α

π πα α

α

= −

= −

= =

时间3Ts/2 -Ts/2 3Ts/2Ts/2

α=2

α=1

α=0.75

α=0.5

6.5 Linear Modulation Techniques The amplitude of s(t), varies linearly with

the modulating digital signal m(t).

bandwidth efficient and are very attractive for use in wireless communication system.

[ ][ ]

( ) Re ( )exp( 2 )

( ) cos(2 ) ( )sin(2 )

( ) ( ) ( )

c

R c I c

R I

s t Am t j f t

A m t f t m t f t

m t m t jm t

π

π π

=

= −

= + 为复数形式的已调信号的复包络

linear modulation schemes have very good spectral efficiency, they must be transmitted using linear RF amplifiers which have poor power efficiency.

The most popular linear modulation techniques include pulse-shaped QPSk、OQPSK, and π/4 QPSK.

6.5.1 Binary Phase Shift Keying（BPSK）

The phase of a carrier signal is switched between m1 and m2 corresponding to 1 and 0.

BPSK uses coherent or synchronous demodulation.

( )

{ }

( )( )

( )( )

2

2

2 2

2( ) ( ) cos 2

1Re ( )exp( 2 ) ,2

( )

2 sin( ) ( ) , ( ) 2

sin sin( )

2

c

BPSK

BPSK

bBPSK c c

b

BPSK c b c b

BPSK

jb bBPSK g b

b b

c b c bb

c b c b

ES t m t f tT

g t j f t E A T

g t

E fTg t m t e P f ET fT

f f T f f TEP ff f T f f T

θ

π θ

π

ππ

π ππ π

= +

= =

= =

− − −

= + − − −

是信号的复包络，

b

22

90% energy BW=1.6R for rectangular pulseall of energy within 1.5 for pulse with =0.5 raised cosine filter

bb

b

BW R T

R α

= =

bR

Q

I

bE− bE0

2 bEQN

=

e,BPSKP

6.5.2 Differential Phase Shift Keying（DPSK） DPSK is a noncoherent form of PSK, and

avoids the need for a coherent reference signal at the receiver.

Input binary sequence is differentially encoded and modulated using a BPSK mod.

The {dk}is generated : { } 1: −⊕= kkkk dmdd

{mk} 1 0 0 1 0 1 1 0 {dk-1} 1 1 0 1 1 0 0 0 {dk} 1 1 0 1 1 0 0 0 1

It has the advantage of reduced receiver complexity.

Its energy efficiency is inferior to that of coherent PSK by about 3dB.

6.5.3 Quadrature Phase Shift Keying (QPSK） QPSK has twice the bandwidth efficiency of

BPSK, since two bits are transmitted in a single modulation symbol.

A striking result that the bit error probability of QPSK is identical to BPSK.

( )( )

( )( )

s

2 2

2( ) cos 2 ( 1) 0 t T , i 1,2,3,42

sin 2 sin 2( )

2 2

bQPSK c

s

c b c bQPSK b

c b c b

ES t f t iT

f f T f f TP f E

f f T f f T

ππ

π ππ π

= + − ≤ ≤ =

− − − = + − − −

I

Q

sE

I

Q

sE2

(a) (b)

,0

2 be QPSK

EP QN

=

6.5.4 Offset QPSK (OQPSK) When QPSK signals are pulse shaped,

they lose the constant envelop property. Due to the time alignment of mI(t) and

mQ(t) in stand QPSK, phase transitions occur only every Ts=2Tb and will be a maximum of 1800 if there is a change in the value of both mI(t) and mQ(t) .

The occasional phase shift of 1800 can cause the signal envelop to pass zero for just instant.

By switching phases more frequently Ts=Tb , OQPSK signaling eliminates 1800 phase transitions.

The maximum phase shift is only±900 . Thus, band-limiting of OQPSK signals

does not cause the signal envelope to go to zero.

The spectrum of an OQPSK signal is identical to that of a QPSK signal, hence both signals occupy the same bandwidth.

)(tmI

2m 14m

12m

10m8m6m

4m0m

3m 15m

13m11m9m

7m

5m1m

)(tmQ

6.5.5 π/4QPSK π/4QPSK modulation is a QPSK technique

which offers a compromise between OQPSK and QPSK in terms of the allowed maximum phase transitions.

maximum phase change is limited ±1350.

Ik

Qk

It may be demodulated in a coherent or non-coherent fashion.

When differentially encoded, π /4 QPSK is called π /4 DQPSK.

In AWGN channel, the BER performance of differentially detection is lower 3dB than QPSK, while coherent detection is used.

The BER performance is the same as QPSK.

6.6 Constant Envelop Modulation Advantages: 1）Power efficient Class C amplifiers can be

used without introducing degradation in the spectrum occupancy of transmitted signal.

2）Low out-of -band radiation of the order of -60dB to -70dB can be achieved.

3）Limiter-discriminator detection can be used, which simplifies receiver design and provides high immunity against random FM noise and signal fluctuations due to Rayleigh fading.

Disadvantage: They occupy a large bandwidth than

linear modulation schemes.

6.6.1 Binary Frequency Shift Keying（BFSK）

The frequency of carrier signal is switched between two values according to the two possible message states,1 or 0.

The complex envelope of an FSK signal is a nonlinear function of the m(t), evaluation of the spectra is very different.

( )2( ) cos 2 ( )

2 cos 2 2 ( )

bFSK c

s

tb

c fs

ES t f t tT

E f t k m dT

π θ

π π η η−∞

= +

= +

∫

FSK transmission bandwidth is given by Carson’s rule as：

Both coherent or Non-coherent detection can be used for BFSK signal’s demodu.

( )2 2 2 2 2 1TB f B f R f Rα= ∆ + = ∆ + = ∆ + +

, ,0

1 exp2 2

be FSK NC

EPN

= −

6.6.2 Minimum Shift Keying（MSK）

MSK is a special type of CPFSK wherein the peak freq. deviation equals to 1/4 Rb .

In other words, MSK is continuous phase FSK with a modulation index of 0.5.

The modul. index is similar to FM modu. index, and is defined as: kFSK=(2Δf)/Rb.

the minimum frequency spacing that allows two FSK signals to be orthogonal.

0( ) ( ) 0

T

H Lt tν ν =∫

MSK is sometimes referred to as fast FSK. MSK is a spectrally efficient modulation

scheme and is particularly attractive for use in mobile radio systems.

It possesses properties such as constant envelop, spectral efficiency, good BER performance, and self-synchronizing capability.

An MSK signal can be thought of as a special from of OQPSK where the baseband rectangular pulses are replaced with half-sinusoidal pulses.

Thus, the normalized PSD for MSK is：

1 1

0 0( ) ( 2 )cos 2 ( 2 )sin 2

cos , t T( ) 2

0,

i i

N N

MSK I b c Q b ci i

S t m p t iT f m p t iT f

tp t T

π π

π

− −

= =

= − + −

< =

∑ ∑

其他

2 2

2 2 2 2 2 2

cos 2 ( ) cos 2 ( )16 16( )1.16 1.16

c cMSK

f f T f f TP ff T f T

π ππ π

+ −= +

99% power within B=1.2/T for MSK, B=8/T for QPSK, OQPSK

6.6.3 Gaussian MSK（GMSK） GMSK is a simple binary modulation

scheme which may be viewed as a derivative of MSK.

In GMSK, the sidelobe levels of the spectrum are further reduced by passing the modulating NRZ data waveform through a pre-modulation Gaussian pulse-shaping filter.

Baseband Gaussian pulse shaping smoothes the phase trajectory of the MSK signal and hence stabilizes the frequency variations over time .

The pre-modulation Gaussian filtering introduces ISI in the transmitted signal.

But the degradation is not severe if the 3dB-bandwidth and bit duration product (BT) of the filter is greater than 0.5.

The BER degradation due to ISI caused by filtering is minimum for a BT=0.5887, where the degradation in the require Eb/N0 is only 0.14dB from the case of no ISI.

GMSK scarifies the irreducible error rate caused by partial response signaling in exchange for extremely good spectral efficiency and constant envelop properties. GMSK is most attractive for its excellent

power efficiency and its excellent spectral efficiency (2G GSM, 3G TD-CDMA systems). GMSK pre-modulation filter:

( )2

2 2 22( ) exp ( ) exp

ln 2 0.58872

G Gh t t H f f

BB

π π αα α

α

= − ⇒ =

= =

6.7 Combined Linear and Constant Envelope Modulation Techniques

Modern modulation techniques exploit the fact that digital baseband data may be sent by varying both the envelope and phase of an RF carrier.

Because the envelope and phase offer two degrees of freedom, they can represent more signals than if just the amplitude or phase were varied alone.

In an M-ary signaling scheme, two or more bits are grouped together to form symbols and one of M (M=2n )possible signals is transmitted during each Ts. M-ary signaling is particularly attractive

for use in band-limited channels, but are limited in their applications due to sensitivity to timing jitter. M-ary modulation schemes achieve

better bandwidth efficiency at the expense of power efficiency.

6.7.1 M-ary Phase Shift Keying (MPSK)

The modulated MPSK waveform

Constellation diagram of an MPSK (M=8)

( ) ( )

1 2

1 2

2 2

( ) cos ( 1) ( ) sin ( 1) ( ) , 1, 2,... ,2 2

( ) 2 cos(2 ) ( ) 2 sin(2 )

log , log

MPSK s s

s c s c

s b s b

S t E i t E i t i M

t T f t t T f t

E M E T M T

π πφ φ

φ π φ π

= − − − =

= =

= =

和 是基本正交信号

1( )tφ

2 ( )tφ

the distance between adjacent symbols is equal to 2(Es)1/2 sin(π/M). The average SER of an MPSK system is

The PSD of the MPSK signal with

rectangular pulse is:

( )( )

( )( )

−−−−

+

−−

=22

sinsin2

)(sc

sc

sc

scsMPSK Tff

TffTff

TffEfPππ

ππ

( )2

0 0

2 log 42 sin 2 sin 4b se

E M EP Q Q MN M N M

π π ≤ ≈ ≥

The first null bandwidth decreases as M increases while Rb is held constant. Therefore, the bandwidth efficiency also increases.

Increasing M implies that the constellation is more densely packed, and hence the power efficiency is decreases.

6.7.2 M-ary Quadrature Amplitude Modulation（QAM）

By allowing the amplitude to also vary with the phase, a new modulation scheme QAM is obtained.

)(1 tφ

)(2 tφ

)(1 tφ

)(2 tφ

The general form of an MQAM signal be :

The power spectrum and bandwidth efficiency of QAM is identical to MPSK. The power efficiency is superior to MPSK.

Pilot tones or equalization must be used for QAM in mobile systems.

min min2 2( ) cos(2 ) sin(2 ), 1, 2,...i i c i cs s

E ES t a f t b f t i MT T

π π= + =

( )min

0 0

31 2 14 1 4 11av

eEEP Q Q

N M NM M

≅ − ≅ − −

6.7.3 M-ary Frequency Shift Keying (MFSK), OFDM

MFSK transmitted signals are defined by:

M transmitted signals are of equal energy and duration, and the signal frequencies are separated by 1/2Ts Hz, making the signals orthogonal to one another.

mins

2( ) cos ( ) , 0 t T , i 1,2,...M

, / 2

i cs s

c c c s

ES t n i tT Tn f n T

π = + ≤ ≤ = =对于某些固定的

Both coherent and non-coherent detectors can be used to MFSK receiver.

For coherent MFSK, the optimum receiver is Match Filters, which are tuned to the M distinct carriers. For non-coherent detection using MFs

followed by envelop detectors.

( ) ( )2

0 2

3log1 ,2 logbb

eR ME MP M Q B

N M +

≤ − =

1

1 0 2

1( 1) exp ,1 ( 1) 2 log

kMs b

ek

M kE R MP Bkk k N M

+

=

− −−= = + + ∑

The bandwidth efficiency of an MFSK signal decreases with increasing M. The power efficiency increase with M. MFSK can be amplified using nonlinear

amplifiers with no performance degradation. The orthogonality of MFSK has led to

explore OFDM as a means of providing power efficient signaling for a large number of users on the same channel. MFSK and OFDM modulation methods are

being explored for part of the IEEE 802.11a standards to provide 54Mbps WLAN.

6.8 Spread Spectrum Modulation Techniques

many users can simultaneously use the same bandwidth without significantly interfering.

In a multi-user( MAI), it is very band efficient. SS signals occupy a very large bandwidth, and

have pseudorandom and noise-like properties. The advantages are inherent interference

rejection, simple to frequency planning, resistance to multipath fading, and so on.

The characteristics of SS modulation: pseudorandom sequence modulation and correlation processes.

6.8.1 PN sequence It is a binary sequence. Its autocorrelation resembles of a random

binary sequence, and also roughly resembles of band-limited white noise.

It has many characteristics that are similar to those of random binary sequence.

a nearly equal number of 0 and 1 very low correl. between shift of the sequence very low cross-correlation between two

sequences, etc.

)2(mod1∑=

n

iii XC

1 2 3 4 •••••• n

X1 X2 X3 X4 •••••• Xn

C1 C2 C3 C4 •••••• Cn

The PN sequence is usually generated using sequential logic circuits.

A sequence of period 2n-1 generated by a linear feedback register is called a maximal length sequence.

6.8.2 Direct Sequence SS（DS-SS）

It spreads the baseband data by directly multiplying with a PN sequence. 数

据

PN码

发生器振荡器

码片时钟

基带BPF 发射信号( )ssS t

cf

IF宽带滤波

器

IF接收

DS-SS信号

PN码

发生器同步系统

相干相移键控或差分相移键

控解调器

接收数据

发射机

接收机

The waveform of DSSS signal with BPSK modu.

时间

数据信号

编码信号

数据信号×编码信号

BPSK 调制信号

( )2( ) ( ) ( ) cos 2 ,

( ) ( )

2

sss c

s

s c ss

c s s

ES t m t p t f tT

m t p t PNT R WPGT R R

π θ= +

= = =

和 分别是数据和 扩频序列

扩频处理增益：

频率

谱密度

信号

干扰

宽带滤波器输出

频率

谱密度

干扰

信号

解扩相关器输出

处理增益

6.8.3 Frequency Hopped SS(FH-SS）

a period change of transmission frequency 数据

PN码发生器

频率合成器

码片时钟

调制器 跳频信号

宽带滤波器

同步系统

解调器接收数据

振荡器

跳频信号

PN码发生器

频率合成器

6.8.4 Performance of DSSS

1( )m t 1( )S t

1cos(2 )cf tπ φ+1( )PN t

1τ

( )Km t ( )KS t

cos(2 )c Kf tπ φ+( )KPN t

Kτ

( )r t∑

( )r t 1( )S t

1cos(2 )cf tπ φ+1( )PN t

( )0

Tdt•∫

(1)iZ >

<

' ( )m t

用户1的接收机结构

K个用户的发射机结构

( )2( ) ( ) ( ) cos 2 ,sk k k c k

s

Ek S t m t p t f tT

π φ= +第 个用户的发射信号：

( )1

1

11 1 1 1( 1)

12

( ) ( ) cos 2iT

i ci T

K

kk

Z r t p t f t dt

I I

τ

ττ π τ φ

ξ

+

− +

=

= − − +

+ +

∫

∑

第1个用户的判决变量：

=

( )

( )

( )

1 1 10

10

10

0

( ) ( ) cos 22

( ) ( ) cos 2

( ) ( ) cos 2

1 , 11

3 2

T sc

T

c

T

k k k c

e

b

E TI S t p t f t dt

n t p t f tdt

I S t p t f t dt

P Q K BPSKNK

N E

π

ξ π

τ π

= =

=

= −

= = −

+

∫

∫∫

时，与 调制相同

6.8.5 Performance of FHSS

( )0

1

0

1

1

0

21 1exp 1 ,2 2

11 1

21 1 1 1exp 12 2

1 11 1 1

21 1 1exp 1 12

be h h h

K

h

be

K

hb

K

be

b

EP p p pN

pM

E K KPN M M

pM N

EPN M N

−

−

−

= − − +

− − = − − +

= −

是一次碰撞的概率

同步跳频系统中， ＝－ －

异步跳频系统，有 ＝－ － ＋

－ ＋

11 1 11 1 12

K

bM N

− + － － ＋

6.9 Modu. performance in fading and multipath channels

BER gives a good indication of the performance of a particular modulation scheme, it does not provide information about the type of errors.

Probability of outage.

6.9.1 Performance in slow flat-fading channel

0

2

0

2 2

2

0

( ) ( ) exp( ( )) ( ) ( )

( ) ( )

( ) ,

( )2

1( ) exp ,

e e

be

b

r t t j t s t n t

P P X p X dX

EP X X BER X Np X X PDF Rayleigh

X

EXp XN

α θ

α

α χ

α

∞

= − +

=

=

= − Γ = Γ Γ

∫是信噪比为的任意调制的，

是衰落信道中的。对衰落信道，

和具有自由度为的分布,

( )

,

,

,

,

,

1 1 ( )2 1

1 1 ( )2 2

1 ( )2 1

1 ( )2

0.68, 0.251 1 ( ),0.85, 2 1

e PSK

e FSK

e DPSK

e NCFSK

e GMSK

P PSK

P FSK

P PSK

P FSK

BTP GMSK

BTδ δδ

Γ= − + Γ

Γ= − + Γ

=+ Γ

=+Γ =Γ

= − = = ∞+ Γ

相干二进制

相干二进制

差分二进制

正交非相干

相干

6.9.2 Digital modulation in frequency selective mobile channels

Frequency selective fading causes ISI, which results in an irreducible BER floor.

time-varying Doppler spread due to motion creates an irreducible BER floor.

Simulation is the major tool used for analyzing frequency selective fading.

The BER performance of BPSK is the best. 4-level modulations (QPSK, OQPSK and

MSK) are more resistant to delay spread than BPSK for constant infor. throughput.