edge_2_v1-5e

2 Air Interface

EDGE Basics, Version 1.5

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2.1 EDGE Advantage ......................................................................... 3 2.2 GMSK Modulation (1/5)................................................................ 4 2.2 GMSK Modulation (2/5)................................................................ 5 2.2 GMSK Modulation (3/5)................................................................ 6 2.2 GMSK Modulation (4/5)................................................................ 7 2.2 GMSK Modulation (5/5)................................................................ 8 2.3 8-PSK Modulation (1/7)................................................................. 9 2.3 8-PSK Modulation (2/7)............................................................... 10 2.3 8-PSK Modulation (3/7)............................................................... 11 2.3 8-PSK Modulation (4/7)............................................................... 12 2.3 8-PSK Modulation (5/7)............................................................... 13 2.3 8-PSK Modulation (6/7)............................................................... 14 2.3 8-PSK Modulation (7/7)............................................................... 15 2.4 GSM Burst ................................................................................... 16 2.5 EDGE Burst ................................................................................. 17 2.6 EDGE / GSM Comparison........................................................... 18

EDGE Basics, Version 1.5

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2.1 EDGE Advantage

The most important advantage of EDGE is that it is no longer necessary to make any changes to the present GSM air interface structure. EDGE uses the same 200 kHz FDMA scheme to provide a higher data rate using an 8-PSK modulation that enables a 3 times higher user bit rate per radio time slot. 8-PSK modulation always maps 3 bits of the user signal onto one modulation symbol, i.e., a bit within the modulated burst. Thus, burst lengths and the current symbol rate of 270.833 ksps stay the same. In addition, 8-PSK modulators are also able to handle the standard GMSK modulation that is used today, making EDGE systems downwardly compatible with GSM phase 2 systems. The combination of up to 8 timeslots with 8-PSK modulated signals each lead to the tremendous increase of available data rates up to 474 kbps. Let us now get to know the new modulation scheme by comparing it with the key features of the present modulation type.

EDGE Basics, Version 1.5

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2.2 GMSK Modulation (1/5)

To assure high speech quality and a maximum spectral efficiency, standard GSM uses a phase-continuous type of modulation named: Gaussian Minimum Shift Keying. It is a phase modulation that represents a serial bit stream as a sliding phase shift of the RF carrier.

EDGE Basics, Version 1.5

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2.2 GMSK Modulation (2/5)

The main processing steps in the modulator / demodulator are: • • •

Differential coding of the user signal by a chained Exclusive OR-gates chain to avoid the need for a reference signal for demodulation Base-band filtering by a Gaussian low pass filter to delimited the spectrum of the modulation signal An I / Q phase modulation performed by a Minimum Shift Keying modulator

EDGE Basics, Version 1.5

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2.2 GMSK Modulation (3/5)

Shown in the mathematic complex plane, GMSK modulated signals have four different phase states, n*90° (n = 1...4). The resulting vector of the modulated signal slides from one position to another avoiding hard jumps that would increase the modulation spectrum. The length of the vector representing the amplitude of the carrier remains constant which is a big advantage requiring less accuracy from the subsequent power amplifier.

EDGE Basics, Version 1.5

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2.2 GMSK Modulation (4/5)

This example shows the RF phase behavior over time for a dedicated user bit stream if a GMSK modulator is used. Note the relationship between the input and the phase of the RF carrier.

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2.2 GMSK Modulation (5/5)

The big disadvantage of this modulation type is the non-efficient use of the phase modulator. GMSK modulation was developed for speech transmission 20 years ago and is adequate for this purpose. But in order to transmit higher data rates, it is necessary to have a more efficient modulator. 8-PSK fulfils this need with regard to Hardware implementation efforts and compatibility with existing GMSK modulators. An 8-PSK signal is able to carry three bits per modulated symbol over the radio path. These symbols are represented as eight different positions in the mathematic complex plane, each symbol showing a possible combination of three bits.

EDGE Basics, Version 1.5

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2.3 8-PSK Modulation (1/7)

An 8-PSK signal is able to carry three bits per modulated symbol over the radio path. These symbols are represented as eight different positions in the mathematic complex plane, each symbol showing a possible combination of three bits.

EDGE Basics, Version 1.5

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2.3 8-PSK Modulation (2/7)

Let us now consider what happens to the phase of the modulated RF carrier when a user bit stream is processed. The initial symbol representing a 011 bit combination changes to 000. With 8-PSK, the top of the amplitude - the blue vector in the complex plane - will not move along the circle to find its actual position. Instead, it follows the red line getting smaller and bigger again when reaching its 000 destination.

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2.3 8-PSK Modulation (3/7)

The result of the phase transition from one state to the other is an additional amplitude modulation - a side effect that is due to this particular type of modulation. There is no useful information within this amplitude variation, it only consumes energy (e.g., battery capacity of an EDGE mobile), reducing the standby and talk time of the device and producing unnecessary heat that requires better cooling mechanisms for the electronic elements involved. And even worse - this non-linear behavior of the RF carrier requires higher performance from the amplifier used.

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2.3 8-PSK Modulation (4/7)

A comparison of the two different modulator output signals points out the problem. A GMSK modulated signal performs with a nearly constant amplitude whereas an 8PSK signal produces amplitude variations of +/- 8dB. A carefully selected bit stream with a minimum of additional amplitude modulation is used for the training sequence in the middle of the 8-PSK burst.

EDGE Basics, Version 1.5

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2.3 8-PSK Modulation (5/7)

But there is another problem that has to be solved with 8-PSK. What happens if 011 changes to 101? The top of the amplitude vector again follows the red line, thus passing through the zero point. This would lead to a disappearance of the carrier resulting in problems during communication between the EDGE mobile and BTS.

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2.3 8-PSK Modulation (6/7)

In order to avoid this, an additional shift of the bit positions of 3p/8 = 67.5º per TDMA frame period of 4.615 ms is applied. 101 - that must be modulated one burst later then resides at a position where no trespassing of the zero point occurs.

EDGE Basics, Version 1.5

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2.3 8-PSK Modulation (7/7)

3GPP specifications require EDGE transceivers and mobile devices to handle GMSK and 8-PSK modulated signals in parallel. The screen shot in the animation shows an uplink signal in a TRX for one TDMA frame. Uplink CCH information, e.g., RACH, has to be processed in TS0 whereas TS1 carries 8-PSK uplink inforamtion, e.g., packet user data and TS2 caters for a legacy (GMSK-modulated) speech service.

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2.4 GSM Burst

Let us now have a closer look at the burst structures. The GMSK modulated normal burst is familiar to us. CS or PS user data is included in the 2*57 bits of payload. Tail bits, stealing flags and training sequence complete this structure.

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2.5 EDGE Burst

An 8-PSK modulated normal burst must have the same structure, fulfilling the requirements for an unchanged air interface structure. But now each bit within the burst represents 3 user data bits going into the new modulator. This is also valid for the stealing flags associated with the training sequence. With 8-PSK we now have signaling symbols, i.e., 2*3 bits for stealing flag functionality (FACCH) and additional signaling tasks, e.g., intra burst power control. With EDGE, these bits are also counted as payload.

EDGE Basics, Version 1.5

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2.6 EDGE / GSM Comparison

We will finish our comparison of 8-PSK and GMSK modulated air interface characteristics with a summarizing table. EDGE maps 3 bits onto one symbol of the modulated carrier whereas GSM presents a 1-to-1 relation. In both cases, the symbol rate must be the same, i.e., 270.833 ksymbols per second or 270.833 kbps. EDGE payload consists of 116 symbols per burst, GSM provides 114. The air Interface gross bit rate is increased to 69.2 kbps due to an additional interleaving of 4 information blocks of 346 bits per 20ms. GSM gross bit rate is 22.8 kbps.

EDGE Basics, Version 1.5

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