Why Digital radio is less susceptible to noise and interference.

If the path from the transmitter to the receiver either has reflections or obstructions, we can get fading effects,it is doppler effect. In this case, the signal reaches the receiver from many different routes, each a copy of the original. Each of these rays has a slightly different delay and slightly different gain. The time delays result in phase shifts which added to main signal component (assuming there is one.) causes the signal to be degraded.Fading is big problem for signals. The signal is lost and demodulation must have a way of dealing with it. Fading is particular problem when the link path is changing, such as for a moving car or inside a building or in a populated urban area with tall building.

Digital radio is less susceptible to noise and interference, which can manifest itself as fade, hiss or crackle on FM radio. Digital radios don’t suffer in this way because they maintain reception quality up until the signal becomes unsustainably weak, whereas the reception quality of FM radios gets steadily worse as soon as the signal begins to weaken. This is particularly relevant to people using radios in cars because constant movement means signal strength is likely to vary it is doppler effect.

Coded orthogonal frequency division multiplexing Modulation is an essential component of terrestrial Broadcasting.COFDM signals are comprised of large no of closely spaced carriers ie frequency division multiplexing .These carriers have to be closely spaced for bandwidth efficiency but must be prevented from interfering with each other so it is important they are orthogonal .In this typical multipath condition a high proportion of these carriers may be notched out . Resulting in transmission errors so the transmitted signal must include powerful error coding codecs

COFDM Modulation was specifically designed to reduce the problems caused by multipath. The process takes I/P bit stream,divides it up into multiple parallel lower rate bit stream ,and modulates these on to a large number of individual carriers.The number of carriers can vary between few hundred to several thousand,depending on the transmission specification and design parameters of broadcast signal.These carriers are closely spaced in frequency and this would normally mean that adjacent carriers would interfere with each other .

COFDM system avoids self interference by making the use of fact that if carriers spacing-is reciprocal of symbol length it is possible to demodulate each carrier individually and ignore the contribution of its neighbor.Under this condition the carriers are said to be orthogonal to each other.Demodulation process is typically accomplished using fast Fourier transform.In order to minimize the determined effect of delayed echoes,each symbol is transmitted for a length of time equal to useful symbol period plus an additional Guard interval .

COFDM demodulator can work effectively with out being subject to inter symbol interference in both the frequency domain and the time domain. COFDM works well with echoes .This fact allows single frequency network.The final mechanism of multipath interference to be discussed is the Doppler shift.If receiver is moving itself or is in the presence of the moving object ,then reflected signal will have frequency shift on them . The amount of frequency shift that is acceptable is governed by the carriers spacing . systems which closely spaced carriers are more susceptible to problems caused by Doppler shift , which is requirement for in car receivers .

The modulation used for DRM is coded orthogonal frequency division multiplexing (COFDM), where every carrier is modulated with quadrature amplitude modulation (QAM) with a selectable error coding.The choice of transmission parameters depends on signal robustness wanted and propagation conditions. Transmission signal is affected by noise, interference, multipath wave propagation and Doppler effect.

It is possible to choose among several error coding schemes and several modulation patterns: 64-QAM, 16-QAM and 4-QAM. OFDM modulation has some parameters that must be adjusted depending on propagation conditions. This is the carrier spacing which will determine the robustness against Doppler effect (which cause frequencies offsets, spread: Doppler spread) and OFDM guard interval which determine robustness against multipath propagation (which cause delay offsets, spread: delay spread). The DRM consortium has determined four different profiles corresponding to typical propagation conditions:

• A: Gaussian channel with very little multipath propagation and Doppler effect. This profile is suited for local or regional broadcasting.
• B: multipath propagation channel. This mode is suited for medium range transmission. It is nowadays frequently used.
• C: similar to mode B, but with better robustness to Doppler (more carrier spacing). This mode is suited for long distance transmission.
• D: similar to mode B, but with a resistance to large delay spread and Doppler spread. This case exists with adverse propagation conditions on very long distance transmissions. The useful bit rate for this profile is decreased.

The trade-off between these profiles stands between robustness, resistance in regards to propagation conditions and useful bit rates for the service. This table presents some values depending on these profiles. The larger the carrier spacing, the more the system is resistant to Doppler effect (Doppler spread). The larger the guard interval, the greater the resistance to long multipath propagation errors (delay spread).The resulting low-bit rate digital information is modulated using COFDM. It can run in simulcast mode by switching between DRM and AM, and it is also prepared for linking to other alternatives

DRM has been tested successfully on shortwave, mediumwave (with 9 as well as 10 kHz channel spacing) and longwave.

Mode	OFDM carrier spacing (Hz)	Number of carriers				Symbol length (ms)	Guard interval length (ms)	Nb symbols per frame
		9 kHz	10 kHz	18 kHz	20 kHz
A	41.66	204	228	412	460	26.66	2.66	15
B	46.88	182	206	366	410	26.66	5.33	15
C	68.18	-	138	-	280	20.00	5.33	20
D	107.14	-	88	-	178	16.66	7.33	24

Error coding

Error coding can be chosen to be more or less robust.

This table shows an example of useful bitrates depending on protection classes

• OFDM propagation profiles (A or B)
• carrier modulation (16QAM or 64QAM)
• and channel bandwidth (9 or 10 kHz)

Bitrates, kbit/s

Protection class	A (9 kHz)	B (9 kHz)	B (10 kHz)		C (10 kHz)		D (10 kHz)
	64-QAM	16-QAM	16-QAM	64-QAM	16-QAM	64-QAM	16-QAM	64-QAM
0	19.6	7.6	8.7	17.4	6.8	13.7	4.5	9.1
1	23.5	10.2	11.6	20.9	9.1	16.4	6.0	10.9
2	27.8	-	-	24.7	-	19.4	-	12.9
3	30.8	-	-	27.4	-	21.5	-	14.3

The lower the protection class the higher the level of error correction.