MobileWorld

Spectrum Efficiency

The frequency bands allocated are 890-915MHz and 935-960MHz. Half is used for transmitting and the other half is used for receiving. To allow maximum number of users access, each band is subdivided into 124 carrier frequencies spaced 200KHz apart, using FDMA techniques. By applying TDMA techniques, each of these carrier frequencies is further subdivided into time slots which provide each user with the carrier frequency for approximately 0.577ms. This equates to approx. 217 jumps per second, but amongst a very small frequency range so encryption is a must for proper security of calls. In fact it is not exactly that, it is hopping 13 times every 60 ms, which gives 13/0.06 per second. 0.577ms = 13 frames/60 ms /8 time slots There is also an extension band of 15 MHz in both directions. There is also DCS 1800 which is equivalent to GSM but at 1800 MHz and the USA will use the 1900 MHz band for what they call the PCS (which is either CDMA or GSM like).

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The application of speech coding, frequency hopping, channel coding, power control, discontinuous transmission and modulation scheme assists the high level of spectrum Efficiency.

Speech Coding - The algorithm used is RPELPC which converts the digitally converted analog speech. The LPC (long term prediction), is very important as this allows an efficient speech extrapolation when a speech block has not been received properly, the system can accept up to 2% of missing speech block without audible effect.
Frequency Hopping - The transmit and receive carrier frequencies are dynamically assigned. This avoids collisions between frequencies. What avoids the collision is the fact that all the mobiles of a cell, even when they use the 'random' frequency hopping, do it in such a way that they never use the same frequency at the same time. In fact, the Frequency hopping gives both the frequency diversity (protection against the fading on one frequency, better effect with cyclic hopping) and the interfere diversity (protection against user mobiles using the same frequency, better effect with random hopping).
Channel Coding - Channel coding is used to achieve very reliable communication with error correction. A unique feature of the GSM is to provide a different protection of the speech bits, according to their importance, most important bits are protected, less important bits are not protected at all. This is a difference with the Qualcom CDMA which offers the same protection of all the bits, it is simpler to realise but it is a waste of bandwidth. This does not apply to Data Calls which offers protection to all the bits.
Power Control - Power output is controlled dependent upon signal strength so that is good signal areas the phone uses less power.
Discontinuous Transmission - During transmission silence which occurs 60% of the time during a conversation the transmitter is turned off. This allows reducing the radiated power, hence reducing interference's and of course saves the batteries.
Modulation Scheme - GMSK is used to modulate the digital signal to minimise co-channel interference. (GMSK has probably be chosen because it did not require linear amplifier (one of the most beautiful error of GSM, the amplifier *must* be linear), and because of its spectral efficiency (2.70 bit/Hertz))

Power Class	GSM 900 Maximum output power	DCS 1800 Maximum output power	Tolerance (dB) normal	Tolerance (dB) extreme
1	-	1 (30 dBM)	2	2.5
2	8 (39 dBM)	.25W (24 dBM)	2	2.5
3	5 (37 dBM)	4W (36 dBM)	2	2.5
4	2 (33 dBM)	-	2	2.5
5	.8 (29 dBM)	-	2	2.5

NOTE: The lowest power control level for all classes of GSM 900 MS is 19 (5 dBm) and for all classes of DCS 1800 MS is 15 (0 dBm)

GSM 900	Power Output	Tolerance (dB) normal	Tolerance (db) extreme	DCS 1800	Power Output	Tolerance (dB) normal	Tolerance (dB) extreme
0	-	-	-	29	36	2	2.5
1	-	-	-	30	34	3	4
2	39	2	2.5	31	32	3	4
3	37	3	4	0	30	3	4
4	35	3	4	1	28	3	4
5	33	3	4	2	26	3	4
6	31	3	4	3	24	3	4
7	29	3	4	4	22	3	4
8	27	3	4	5	20	3	4
9	25	3	4	6	18	3	4
10	23	3	4	7	16	3	4
11	21	3	4	8	14	3	4
12	19	3	4	9	12	4	5
13	17	3	4	10	10	4	5
14	15	3	4	11	8	4	5
15	13	3	4	12	6	4	5
16	11	5	6	13	4	4	5
17	9	5	6	14	2	5	6
18	7	5	6	15	0	5	6
19	5	5	6